http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/io/LoadPartitionIOCommand.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/LoadPartitionIOCommand.java
 
b/giraph-core/src/main/java/org/apache/giraph/ooc/io/LoadPartitionIOCommand.java
deleted file mode 100644
index ce24fe2..0000000
--- 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/LoadPartitionIOCommand.java
+++ /dev/null
@@ -1,102 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one
- * or more contributor license agreements.  See the NOTICE file
- * distributed with this work for additional information
- * regarding copyright ownership.  The ASF licenses this file
- * to you under the Apache License, Version 2.0 (the
- * "License"); you may not use this file except in compliance
- * with the License.  You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.apache.giraph.ooc.io;
-
-import com.google.common.base.Preconditions;
-import org.apache.giraph.bsp.BspService;
-import org.apache.giraph.comm.messages.MessageStore;
-import org.apache.giraph.ooc.OutOfCoreEngine;
-import org.apache.giraph.ooc.data.DiskBackedEdgeStore;
-import org.apache.giraph.ooc.data.DiskBackedMessageStore;
-import org.apache.giraph.ooc.data.DiskBackedPartitionStore;
-
-import java.io.IOException;
-
-/**
- * IOCommand to load partition data, edge data (if in INPUT_SUPERSTEP), and
- * message data (if in compute supersteps). Also, this command can be used to
- * prefetch a partition to be processed in the next superstep.
- */
-public class LoadPartitionIOCommand extends IOCommand {
-  /**
-   * Which superstep this partition should be loaded for? (can be current
-   * superstep or next superstep -- in case of prefetching).
-   */
-  private final long superstep;
-
-  /**
-   * Constructor
-   *
-   * @param oocEngine out-of-core engine
-   * @param partitionId id of the partition to be loaded
-   * @param superstep superstep to load the partition for
-   */
-  public LoadPartitionIOCommand(OutOfCoreEngine oocEngine, int partitionId,
-                                long superstep) {
-    super(oocEngine, partitionId);
-    this.superstep = superstep;
-  }
-
-  @Override
-  public boolean execute(String basePath) throws IOException {
-    boolean executed = false;
-    if (oocEngine.getMetaPartitionManager()
-        .startLoadingPartition(partitionId, superstep)) {
-      long currentSuperstep = oocEngine.getSuperstep();
-      DiskBackedPartitionStore partitionStore =
-          (DiskBackedPartitionStore)
-              oocEngine.getServerData().getPartitionStore();
-      numBytesTransferred +=
-          partitionStore.loadPartitionData(partitionId, basePath);
-      if (currentSuperstep == BspService.INPUT_SUPERSTEP &&
-          superstep == currentSuperstep) {
-        DiskBackedEdgeStore edgeStore =
-            (DiskBackedEdgeStore) oocEngine.getServerData().getEdgeStore();
-        numBytesTransferred +=
-            edgeStore.loadPartitionData(partitionId, basePath);
-      }
-      MessageStore messageStore;
-      if (currentSuperstep == superstep) {
-        messageStore = oocEngine.getServerData().getCurrentMessageStore();
-      } else {
-        Preconditions.checkState(superstep == currentSuperstep + 1);
-        messageStore = oocEngine.getServerData().getIncomingMessageStore();
-      }
-      if (messageStore != null) {
-        numBytesTransferred += ((DiskBackedMessageStore) messageStore)
-            .loadPartitionData(partitionId, basePath);
-      }
-      oocEngine.getMetaPartitionManager()
-          .doneLoadingPartition(partitionId, superstep);
-      executed = true;
-    }
-    return executed;
-  }
-
-  @Override
-  public IOCommandType getType() {
-    return IOCommandType.LOAD_PARTITION;
-  }
-
-  @Override
-  public String toString() {
-    return "LoadPartitionIOCommand: (partitionId = " + partitionId + ", " +
-        "superstep = " + superstep + ")";
-  }
-}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/io/StoreDataBufferIOCommand.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/StoreDataBufferIOCommand.java
 
b/giraph-core/src/main/java/org/apache/giraph/ooc/io/StoreDataBufferIOCommand.java
deleted file mode 100644
index f1769dd..0000000
--- 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/StoreDataBufferIOCommand.java
+++ /dev/null
@@ -1,99 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one
- * or more contributor license agreements.  See the NOTICE file
- * distributed with this work for additional information
- * regarding copyright ownership.  The ASF licenses this file
- * to you under the Apache License, Version 2.0 (the
- * "License"); you may not use this file except in compliance
- * with the License.  You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.apache.giraph.ooc.io;
-
-import org.apache.giraph.ooc.OutOfCoreEngine;
-import org.apache.giraph.ooc.data.DiskBackedEdgeStore;
-import org.apache.giraph.ooc.data.DiskBackedMessageStore;
-import org.apache.giraph.ooc.data.DiskBackedPartitionStore;
-
-import java.io.IOException;
-
-/**
- * IOCommand to store raw data buffers on disk.
- */
-public class StoreDataBufferIOCommand extends IOCommand {
-  /**
-   * Types of raw data buffer to offload to disk (either vertices/edges buffer
-   * in INPUT_SUPERSTEP or incoming message buffer).
-   */
-  public enum DataBufferType { PARTITION, MESSAGE };
-  /**
-   * Type of the buffer to store on disk.
-   */
-  private final DataBufferType type;
-
-  /**
-   * Constructor
-   *
-   * @param oocEngine out-of-core engine
-   * @param partitionId id of the partition to offload its buffers
-   * @param type type of the buffer to store on disk
-   */
-  public StoreDataBufferIOCommand(OutOfCoreEngine oocEngine,
-                                  int partitionId,
-                                  DataBufferType type) {
-    super(oocEngine, partitionId);
-    this.type = type;
-  }
-
-  @Override
-  public boolean execute(String basePath) throws IOException {
-    boolean executed = false;
-    if (oocEngine.getMetaPartitionManager()
-        .startOffloadingBuffer(partitionId)) {
-      switch (type) {
-      case PARTITION:
-        DiskBackedPartitionStore partitionStore =
-            (DiskBackedPartitionStore)
-                oocEngine.getServerData().getPartitionStore();
-        numBytesTransferred +=
-            partitionStore.offloadBuffers(partitionId, basePath);
-        DiskBackedEdgeStore edgeStore =
-            (DiskBackedEdgeStore) oocEngine.getServerData().getEdgeStore();
-        numBytesTransferred += edgeStore.offloadBuffers(partitionId, basePath);
-        break;
-      case MESSAGE:
-        DiskBackedMessageStore messageStore =
-            (DiskBackedMessageStore)
-                oocEngine.getServerData().getIncomingMessageStore();
-        numBytesTransferred +=
-            messageStore.offloadBuffers(partitionId, basePath);
-        break;
-      default:
-        throw new IllegalStateException("execute: requested data buffer type " 
+
-            "does not exist!");
-      }
-      oocEngine.getMetaPartitionManager().doneOffloadingBuffer(partitionId);
-      executed = true;
-    }
-    return executed;
-  }
-
-  @Override
-  public IOCommandType getType() {
-    return IOCommandType.STORE_BUFFER;
-  }
-
-  @Override
-  public String toString() {
-    return "StoreDataBufferIOCommand: (partitionId = " + partitionId + ", " +
-        "type = " + type.name() + ")";
-  }
-}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/io/StoreIncomingMessageIOCommand.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/StoreIncomingMessageIOCommand.java
 
b/giraph-core/src/main/java/org/apache/giraph/ooc/io/StoreIncomingMessageIOCommand.java
deleted file mode 100644
index c9d8829..0000000
--- 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/StoreIncomingMessageIOCommand.java
+++ /dev/null
@@ -1,69 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one
- * or more contributor license agreements.  See the NOTICE file
- * distributed with this work for additional information
- * regarding copyright ownership.  The ASF licenses this file
- * to you under the Apache License, Version 2.0 (the
- * "License"); you may not use this file except in compliance
- * with the License.  You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.apache.giraph.ooc.io;
-
-import org.apache.giraph.ooc.OutOfCoreEngine;
-import org.apache.giraph.ooc.data.DiskBackedMessageStore;
-
-import java.io.IOException;
-
-import static com.google.common.base.Preconditions.checkState;
-
-/**
- * IOCommand to store incoming message of a particular partition.
- */
-public class StoreIncomingMessageIOCommand extends IOCommand {
-  /**
-   * Constructor
-   *
-   * @param oocEngine out-of-core engine
-   * @param partitionId id of the partition to store its incoming messages
-   */
-  public StoreIncomingMessageIOCommand(OutOfCoreEngine oocEngine,
-                                       int partitionId) {
-    super(oocEngine, partitionId);
-  }
-
-  @Override
-  public boolean execute(String basePath) throws IOException {
-    boolean executed = false;
-    if (oocEngine.getMetaPartitionManager()
-        .startOffloadingMessages(partitionId)) {
-      DiskBackedMessageStore messageStore =
-          (DiskBackedMessageStore)
-              oocEngine.getServerData().getIncomingMessageStore();
-      checkState(messageStore != null);
-      numBytesTransferred +=
-          messageStore.offloadPartitionData(partitionId, basePath);
-      oocEngine.getMetaPartitionManager().doneOffloadingMessages(partitionId);
-      executed = true;
-    }
-    return executed;
-  }
-
-  @Override
-  public IOCommandType getType() {
-    return IOCommandType.STORE_MESSAGE;
-  }
-
-  @Override
-  public String toString() {
-    return "StoreIncomingMessageIOCommand: (partitionId = " + partitionId + 
")";
-  }
-}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/io/StorePartitionIOCommand.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/StorePartitionIOCommand.java
 
b/giraph-core/src/main/java/org/apache/giraph/ooc/io/StorePartitionIOCommand.java
deleted file mode 100644
index 797ac9d..0000000
--- 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/StorePartitionIOCommand.java
+++ /dev/null
@@ -1,85 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one
- * or more contributor license agreements.  See the NOTICE file
- * distributed with this work for additional information
- * regarding copyright ownership.  The ASF licenses this file
- * to you under the Apache License, Version 2.0 (the
- * "License"); you may not use this file except in compliance
- * with the License.  You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.apache.giraph.ooc.io;
-
-import org.apache.giraph.bsp.BspService;
-import org.apache.giraph.comm.messages.MessageStore;
-import org.apache.giraph.ooc.data.DiskBackedEdgeStore;
-import org.apache.giraph.ooc.data.DiskBackedMessageStore;
-import org.apache.giraph.ooc.data.DiskBackedPartitionStore;
-import org.apache.giraph.ooc.OutOfCoreEngine;
-
-import java.io.IOException;
-
-/**
- * IOCommand to store partition data, edge data (if in INPUT_SUPERSTEP), and
- * message data (if in compute supersteps).
- */
-public class StorePartitionIOCommand extends IOCommand {
-  /**
-   * Constructor
-   *
-   * @param oocEngine out-of-core engine
-   * @param partitionId id of the partition to store its data
-   */
-  public StorePartitionIOCommand(OutOfCoreEngine oocEngine,
-                                 int partitionId) {
-    super(oocEngine, partitionId);
-  }
-
-  @Override
-  public boolean execute(String basePath) throws IOException {
-    boolean executed = false;
-    if (oocEngine.getMetaPartitionManager()
-        .startOffloadingPartition(partitionId)) {
-      DiskBackedPartitionStore partitionStore =
-          (DiskBackedPartitionStore)
-              oocEngine.getServerData().getPartitionStore();
-      numBytesTransferred +=
-          partitionStore.offloadPartitionData(partitionId, basePath);
-      if (oocEngine.getSuperstep() != BspService.INPUT_SUPERSTEP) {
-        MessageStore messageStore =
-            oocEngine.getServerData().getCurrentMessageStore();
-        if (messageStore != null) {
-          numBytesTransferred += ((DiskBackedMessageStore) messageStore)
-              .offloadPartitionData(partitionId, basePath);
-        }
-      } else {
-        DiskBackedEdgeStore edgeStore =
-            (DiskBackedEdgeStore)
-                oocEngine.getServerData().getEdgeStore();
-        numBytesTransferred +=
-            edgeStore.offloadPartitionData(partitionId, basePath);
-      }
-      oocEngine.getMetaPartitionManager().doneOffloadingPartition(partitionId);
-      executed = true;
-    }
-    return executed;
-  }
-
-  @Override
-  public IOCommandType getType() {
-    return IOCommandType.STORE_PARTITION;
-  }
-
-  @Override
-  public String toString() {
-    return "StorePartitionIOCommand: (partitionId = " + partitionId + ")";
-  }
-}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/io/WaitIOCommand.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/WaitIOCommand.java 
b/giraph-core/src/main/java/org/apache/giraph/ooc/io/WaitIOCommand.java
deleted file mode 100644
index 74e72eb..0000000
--- a/giraph-core/src/main/java/org/apache/giraph/ooc/io/WaitIOCommand.java
+++ /dev/null
@@ -1,64 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one
- * or more contributor license agreements.  See the NOTICE file
- * distributed with this work for additional information
- * regarding copyright ownership.  The ASF licenses this file
- * to you under the Apache License, Version 2.0 (the
- * "License"); you may not use this file except in compliance
- * with the License.  You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.apache.giraph.ooc.io;
-
-import org.apache.giraph.ooc.OutOfCoreEngine;
-
-import java.io.IOException;
-import java.util.concurrent.TimeUnit;
-
-/**
- * IOCommand to do nothing regarding moving data to/from disk.
- */
-public class WaitIOCommand extends IOCommand {
-  /** How long should the disk be idle? (in milliseconds) */
-  private final long waitDuration;
-
-  /**
-   * Constructor
-   *
-   * @param oocEngine out-of-core engine
-   * @param waitDuration duration of wait
-   */
-  public WaitIOCommand(OutOfCoreEngine oocEngine, long waitDuration) {
-    super(oocEngine, -1);
-    this.waitDuration = waitDuration;
-  }
-
-  @Override
-  public boolean execute(String basePath) throws IOException {
-    try {
-      TimeUnit.MILLISECONDS.sleep(waitDuration);
-    } catch (InterruptedException e) {
-      throw new IllegalStateException("execute: caught InterruptedException " +
-          "while IO thread is waiting!");
-    }
-    return true;
-  }
-
-  @Override
-  public IOCommandType getType() {
-    return IOCommandType.WAIT;
-  }
-
-  @Override
-  public String toString() {
-    return "WaitIOCommand: (duration = " + waitDuration + "ms)";
-  }
-}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/io/package-info.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/io/package-info.java 
b/giraph-core/src/main/java/org/apache/giraph/ooc/io/package-info.java
deleted file mode 100644
index 2230ec4..0000000
--- a/giraph-core/src/main/java/org/apache/giraph/ooc/io/package-info.java
+++ /dev/null
@@ -1,21 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one
- * or more contributor license agreements.  See the NOTICE file
- * distributed with this work for additional information
- * regarding copyright ownership.  The ASF licenses this file
- * to you under the Apache License, Version 2.0 (the
- * "License"); you may not use this file except in compliance
- * with the License.  You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-/**
- * Package of classes related to IO operations in out-of-core mechanism
- */
-package org.apache.giraph.ooc.io;

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/DataIndex.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/DataIndex.java 
b/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/DataIndex.java
new file mode 100644
index 0000000..d44204b
--- /dev/null
+++ b/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/DataIndex.java
@@ -0,0 +1,198 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.giraph.ooc.persistence;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Index chain used in out-of-core data accessor object (DAO) to access
+ * serialized data.
+ */
+public class DataIndex {
+  /** Chain of data indices */
+  private final List<DataIndexEntry> indexList = new ArrayList<>(5);
+
+  /**
+   * Add an index to the index chain
+   *
+   * @param entry the entry to add to the chain
+   * @return the index chain itself
+   */
+  public DataIndex addIndex(DataIndexEntry entry) {
+    indexList.add(entry);
+    return this;
+  }
+
+  /**
+   * Remove/Pop the last index in the index chain
+   *
+   * @return the index chain itself
+   */
+  public DataIndex removeLastIndex() {
+    indexList.remove(indexList.size() - 1);
+    return this;
+  }
+
+  /**
+   * Create a copy of the existing DataIndex
+   *
+   * @return a copy of the existing index chain
+   */
+  public DataIndex copy() {
+    DataIndex index = new DataIndex();
+    for (DataIndexEntry entry : indexList) {
+      index.indexList.add(entry);
+    }
+    return index;
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    if (!(obj instanceof DataIndex)) {
+      return false;
+    }
+    DataIndex dataIndex = (DataIndex) obj;
+    return indexList.equals(dataIndex.indexList);
+  }
+
+  @Override
+  public int hashCode() {
+    return indexList.hashCode();
+  }
+
+  @Override
+  public String toString() {
+    StringBuffer sb = new StringBuffer();
+    for (DataIndexEntry entry : indexList) {
+      sb.append(entry);
+    }
+    return sb.toString();
+  }
+
+  /** Interface to unify different types of entries used as index chain */
+  public interface DataIndexEntry { }
+
+  /**
+   * Different static types of index chain entry
+   */
+  public enum TypeIndexEntry implements DataIndexEntry {
+    /** The whole partition */
+    PARTITION("_partition"),
+    /** Partition vertices */
+    PARTITION_VERTICES("_vertices"),
+    /** Partition edges */
+    PARTITION_EDGES("_edges"),
+    /** Partition messages */
+    MESSAGE("_messages"),
+    /** Edges stored in edge store for a partition */
+    EDGE_STORE("_edge_store"),
+    /** Raw data buffer (refer to DiskBackedDataStore) */
+    BUFFER("_buffer");
+
+    /** String realization of entry type */
+    private final String name;
+
+    /**
+     * Constructor
+     *
+     * @param name name of the type
+     */
+    TypeIndexEntry(String name) {
+      this.name = name;
+    }
+
+    public String getName() {
+      return name;
+    }
+
+    @Override
+    public String toString() {
+      return name;
+    }
+  }
+
+  /**
+   * Class representing any index chain that depends on something with id.
+   * Generally this is used for identifying indices in two types:
+   *  - Index entry based on superstep id ('S' and the superstep number)
+   *  - Index entry based on partition id ('P' and the partition id)
+   */
+  public static final class NumericIndexEntry implements DataIndexEntry {
+    /** Type of index */
+    private final char type;
+    /** Id of the index associated with the specified type */
+    private final long id;
+
+    /**
+     * Constructor
+     *
+     * @param type type of index (for now 'S' for superstep, or 'P' for
+     *             partition)
+     * @param id id of the index associated with the given type
+     */
+    private NumericIndexEntry(char type, long id) {
+      this.type = type;
+      this.id = id;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+      if (!(obj instanceof NumericIndexEntry)) {
+        return false;
+      }
+      NumericIndexEntry index = (NumericIndexEntry) obj;
+      return index.type == type && index.id == id;
+    }
+
+    @Override
+    public int hashCode() {
+      int result = 17;
+      result = result * 37 + type;
+      result = result * 37 + (int) id;
+      result = result * 37 + (int) (id >> 32);
+      return result;
+    }
+
+    @Override
+    public String toString() {
+      return String.format("_%c%d", type, id);
+    }
+
+    /**
+     * Create a data index entry for a given partition
+     *
+     * @param partitionId id of the partition
+     * @return data index entry for a given partition
+     */
+    public static NumericIndexEntry createPartitionEntry(int partitionId) {
+      return new NumericIndexEntry('P', partitionId);
+    }
+
+    /**
+     * Create a data index entry for a given superstep
+     *
+     * @param superstep the superstep number
+     * @return data index entry for a given superstep
+     */
+    public static NumericIndexEntry createSuperstepEntry(long superstep) {
+      return new NumericIndexEntry('S', superstep);
+    }
+  }
+}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/LocalDiskDataAccessor.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/LocalDiskDataAccessor.java
 
b/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/LocalDiskDataAccessor.java
new file mode 100644
index 0000000..2e42906
--- /dev/null
+++ 
b/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/LocalDiskDataAccessor.java
@@ -0,0 +1,252 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.giraph.ooc.persistence;
+
+import com.esotericsoftware.kryo.io.Input;
+import com.esotericsoftware.kryo.io.KryoDataInput;
+import com.esotericsoftware.kryo.io.KryoDataOutput;
+import com.esotericsoftware.kryo.io.Output;
+import com.esotericsoftware.kryo.io.UnsafeInput;
+import com.esotericsoftware.kryo.io.UnsafeOutput;
+import org.apache.giraph.conf.GiraphConstants;
+import org.apache.giraph.conf.ImmutableClassesGiraphConfiguration;
+import org.apache.giraph.conf.IntConfOption;
+import org.apache.log4j.Logger;
+
+import java.io.DataInput;
+import java.io.DataOutput;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.RandomAccessFile;
+
+import static com.google.common.base.Preconditions.checkState;
+import static org.apache.giraph.conf.GiraphConstants.ONE_MB;
+
+/**
+ * Data accessor object to read/write data in local disk.
+ * Note: This class assumes that the data are partitioned across IO threads,
+ *       i.e. each part of data can be accessed by one and only one IO thread
+ *       throughout the execution. Also, each IO thread reads a particular
+ *       type of data completely and, only then, it can read other type of 
data;
+ *       i.e. an IO thread cannot be used to read two different files at the
+ *       same time. These assumptions are based on the assumptions that the
+ *       current out-of-core mechanism is designed for.
+ */
+public class LocalDiskDataAccessor implements OutOfCoreDataAccessor {
+  /**
+   * Size of the buffer used for (de)serializing data when reading/writing
+   * from/to disk
+   */
+  public static final IntConfOption OOC_DISK_BUFFER_SIZE =
+      new IntConfOption("graph.oocDiskBufferSize", 4 * ONE_MB,
+          "size of the buffer when (de)serializing data for reading/writing " +
+              "from/to disk");
+
+  /** Class logger */
+  private static final Logger LOG =
+      Logger.getLogger(LocalDiskDataAccessor.class);
+  /**
+   * In-memory buffer used for (de)serializing data when reading/writing
+   * from/to disk using Kryo
+   */
+  private final byte[][] perThreadBuffers;
+  /** Path prefix for different disks */
+  private final String[] basePaths;
+  /** How many disks (i.e. IO threads) do we have? */
+  private final int numDisks;
+
+  /**
+   * Constructor
+   *
+   * @param conf Configuration
+   */
+  public LocalDiskDataAccessor(
+      ImmutableClassesGiraphConfiguration<?, ?, ?> conf) {
+    // Take advantage of multiple disks
+    String[] userPaths = GiraphConstants.PARTITIONS_DIRECTORY.getArray(conf);
+    this.numDisks = userPaths.length;
+    if (!GiraphConstants.NUM_OUT_OF_CORE_THREADS.isDefaultValue(conf) ||
+        GiraphConstants.NUM_OUT_OF_CORE_THREADS.get(conf) != numDisks) {
+      LOG.warn("LocalDiskDataAccessor: with this data accessor, number of " +
+          "out-of-core threads is only specified by the number of " +
+          "directories given by 'giraph.partitionsDirectory' flag! Now using " 
+
+          numDisks + " IO threads!");
+    }
+    this.basePaths = new String[numDisks];
+    int ptr = 0;
+    String jobId = conf.getJobId();
+    for (String path : userPaths) {
+      File file = new File(path);
+      if (!file.exists()) {
+        checkState(file.mkdirs(), "LocalDiskDataAccessor: cannot create " +
+            "directory " + file.getAbsolutePath());
+      }
+      basePaths[ptr] = path + "/" + jobId;
+      ptr++;
+    }
+    final int diskBufferSize = OOC_DISK_BUFFER_SIZE.get(conf);
+    this.perThreadBuffers = new byte[numDisks][diskBufferSize];
+  }
+
+  @Override
+  public void initialize() { }
+
+  @Override
+  public void shutdown() {
+    for (String path : basePaths) {
+      File file = new File(path).getParentFile();
+      for (String subFileName : file.list()) {
+        File subFile = new File(file.getPath(), subFileName);
+        checkState(subFile.delete(), "shutdown: cannot delete file %s",
+            subFile.getAbsoluteFile());
+      }
+      checkState(file.delete(), "shutdown: cannot delete directory %s",
+          file.getAbsoluteFile());
+    }
+  }
+
+  @Override
+  public int getNumAccessorThreads() {
+    return numDisks;
+  }
+
+  @Override
+  public DataInputWrapper prepareInput(int threadId, DataIndex index)
+      throws IOException {
+    return new LocalDiskDataInputWrapper(basePaths[threadId] + 
index.toString(),
+        perThreadBuffers[threadId]);
+  }
+
+  @Override
+  public DataOutputWrapper prepareOutput(
+      int threadId, DataIndex index, boolean shouldAppend) throws IOException {
+    return new LocalDiskDataOutputWrapper(
+        basePaths[threadId] + index.toString(), shouldAppend,
+        perThreadBuffers[threadId]);
+  }
+
+  @Override
+  public boolean dataExist(int threadId, DataIndex index) {
+    return new File(basePaths[threadId] + index.toString()).exists();
+  }
+
+  /** Implementation of <code>DataInput</code> wrapper for local disk reader */
+  private static class LocalDiskDataInputWrapper implements DataInputWrapper {
+    /** File used to read the data from */
+    private final File file;
+    /** Kryo's handle to read the data */
+    private final Input input;
+
+    /**
+     * Constructor
+     *
+     * @param fileName file name
+     * @param buffer reusable byte buffer that will be used in Kryo's Input
+     *               reader
+     * @throws IOException
+     */
+    @edu.umd.cs.findbugs.annotations.SuppressWarnings(
+        "OBL_UNSATISFIED_OBLIGATION")
+    LocalDiskDataInputWrapper(String fileName, byte[] buffer)
+        throws IOException {
+      file = new File(fileName);
+      LOG.info("LocalDiskDataInputWrapper: obtaining a data input from local " 
+
+          "file " + file.getAbsolutePath());
+      if (LOG.isDebugEnabled()) {
+        LOG.debug("LocalDiskDataInputWrapper: obtaining a data input from " +
+            "local file " + file.getAbsolutePath());
+      }
+      input = new UnsafeInput(buffer);
+      input.setInputStream(new FileInputStream(
+          new RandomAccessFile(file, "r").getFD()));
+    }
+
+    @Override
+    public DataInput getDataInput() {
+      return new KryoDataInput(input);
+    }
+
+    @Override
+    public long finalizeInput(boolean deleteOnClose) {
+      input.close();
+      long count = input.total();
+      checkState(!deleteOnClose || file.delete(),
+          "finalizeInput: failed to delete %s.", file.getAbsoluteFile());
+      return count;
+    }
+  }
+
+  /** Implementation of <code>DataOutput</code> wrapper for local disk writer 
*/
+  private static class LocalDiskDataOutputWrapper implements DataOutputWrapper 
{
+    /** File used to write the data to */
+    private final File file;
+    /** Kryo's handle to write the date */
+    private final Output output;
+
+    /**
+     * Constructor
+     *
+     * @param fileName file name
+     * @param shouldAppend whether the <code>DataOutput</code> should be used
+     *                     for appending to already existing files
+     * @param buffer reusable byte buffer that will be used in Kryo's Output
+     *               writer
+     * @throws IOException
+     */
+    @edu.umd.cs.findbugs.annotations.SuppressWarnings(
+        "OBL_UNSATISFIED_OBLIGATION")
+    LocalDiskDataOutputWrapper(String fileName, boolean shouldAppend,
+                               byte[] buffer) throws IOException {
+      file = new File(fileName);
+      LOG.info("LocalDiskDataOutputWrapper: obtaining a data output from " +
+          "local file " + file.getAbsolutePath());
+      if (LOG.isDebugEnabled()) {
+        LOG.debug("LocalDiskDataOutputWrapper: obtaining a data output from " +
+            "local file " + file.getAbsolutePath());
+        if (!shouldAppend) {
+          checkState(!file.exists(), "LocalDiskDataOutputWrapper: file %s " +
+              "already exist", file.getAbsoluteFile());
+          checkState(file.createNewFile(), "LocalDiskDataOutputWrapper: " +
+              "cannot create file %s", file.getAbsolutePath());
+        }
+      }
+      output = new UnsafeOutput(buffer);
+      RandomAccessFile raf = new RandomAccessFile(file, "rw");
+      if (shouldAppend) {
+        raf.seek(file.length());
+      }
+      output.setOutputStream(new FileOutputStream(raf.getFD()));
+    }
+
+    @Override
+    public DataOutput getDataOutput() {
+      return new KryoDataOutput(output);
+    }
+
+
+    @Override
+    public long finalizeOutput() {
+      output.close();
+      long count = output.total();
+      return count;
+    }
+  }
+}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/OutOfCoreDataAccessor.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/OutOfCoreDataAccessor.java
 
b/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/OutOfCoreDataAccessor.java
new file mode 100644
index 0000000..d4ddc62
--- /dev/null
+++ 
b/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/OutOfCoreDataAccessor.java
@@ -0,0 +1,115 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.giraph.ooc.persistence;
+
+import java.io.DataInput;
+import java.io.DataOutput;
+import java.io.IOException;
+
+/**
+ * Interface representing data accessor object (DAO) used as persistence layer
+ * in out-of-core mechanism.
+ * Note: any class implementing this interface should have one and only one
+ *       constructor taking one and only one argument of type
+ *       <code>ImmutableClassesGiraphConfiguration</code>
+ */
+public interface OutOfCoreDataAccessor {
+  /** Initialize the DAO */
+  void initialize();
+
+  /** Shut down the DAO */
+  void shutdown();
+
+  /**
+   * @return the number of threads involved in data persistence
+   */
+  int getNumAccessorThreads();
+
+  /**
+   * Prepare a wrapper containing <code>DataInput</code> representation for a
+   * given thread involved in persistence for a given index chain for data.
+   *
+   * @param threadId id of the thread involved in persistence
+   * @param index index chain of the data to access the serialized data form
+   * @return the wrapper for <code>DataInput</code> representation of data
+   * @throws IOException
+   */
+  DataInputWrapper prepareInput(int threadId, DataIndex index)
+      throws IOException;
+
+  /**
+   * Prepare a wrapper containing <code>DataOutput</code> representation for a
+   * given thread involved in persistence for a given index chain for data.
+   *
+   * @param threadId id of the thread involved in persistence
+   * @param index index chain of the data to access the serialized data form
+   * @param shouldAppend whether the <code>DataOutput</code> should be used for
+   *                     appending to already existing data for the given index
+   *                     or the <code>DataOutput</code> should create new
+   *                     instance to store serialized data
+   * @return the wrapper for <code>DataOutput</code> representation of data
+   * @throws IOException
+   */
+  DataOutputWrapper prepareOutput(int threadId, DataIndex index,
+                                  boolean shouldAppend) throws IOException;
+
+  /**
+   * Whether the data for the given thread and index chain exists?
+   *
+   * @param threadId id of the thread involved in persistence
+   * @param index index chain used to access the data
+   * @return True if the data exists for the given index chain for the given
+   *         thread, False otherwise
+   */
+  boolean dataExist(int threadId, DataIndex index);
+
+  /** Interface to wrap <code>DataInput</code> */
+  interface DataInputWrapper {
+    /**
+     * @return the <code>DataInput</code>
+     */
+    DataInput getDataInput();
+
+    /**
+     * Finalize and close the <code>DataInput</code> used for persistence.
+     *
+     * @param deleteOnClose whether the source of <code>DataInput</code>
+     *                      should be deleted on closing/finalizing
+     * @return number of bytes read from <code>DataInput</code> since it was
+     *         opened
+     */
+    long finalizeInput(boolean deleteOnClose);
+  }
+
+  /** Interface to warp <code>DataOutput</code> */
+  interface DataOutputWrapper {
+    /**
+     * @return the <code>DataOutput</code>
+     */
+    DataOutput getDataOutput();
+
+    /**
+     * Finalize and close the <code>DataOutput</code> used for persistence.
+     *
+     * @return number of bytes written to <code>DataOutput</code> since it was
+     *         opened
+     */
+    long finalizeOutput();
+  }
+}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/package-info.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/package-info.java 
b/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/package-info.java
new file mode 100644
index 0000000..adf8dba
--- /dev/null
+++ 
b/giraph-core/src/main/java/org/apache/giraph/ooc/persistence/package-info.java
@@ -0,0 +1,22 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+/**
+ * Package of classes related to IO abstraction or persistence layer used for
+ * out-of-core mechanism
+ */
+package org.apache.giraph.ooc.persistence;

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/policy/FixedPartitionsOracle.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/policy/FixedPartitionsOracle.java
 
b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/FixedPartitionsOracle.java
new file mode 100644
index 0000000..ffc5f7f
--- /dev/null
+++ 
b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/FixedPartitionsOracle.java
@@ -0,0 +1,140 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.giraph.ooc.policy;
+
+import com.sun.management.GarbageCollectionNotificationInfo;
+import org.apache.giraph.conf.GiraphConstants;
+import org.apache.giraph.conf.ImmutableClassesGiraphConfiguration;
+import org.apache.giraph.ooc.OutOfCoreEngine;
+import org.apache.giraph.ooc.command.IOCommand;
+import org.apache.giraph.ooc.command.LoadPartitionIOCommand;
+import org.apache.giraph.ooc.command.StorePartitionIOCommand;
+import org.apache.log4j.Logger;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+/** Oracle for fixed out-of-core mechanism */
+public class FixedPartitionsOracle implements OutOfCoreOracle {
+  /** Class logger */
+  private static final Logger LOG =
+      Logger.getLogger(FixedPartitionsOracle.class);
+  /** Maximum number of partitions to be kept in memory */
+  private final int maxPartitionsInMemory;
+  /**
+   * Number of partitions to be added (loaded) or removed (stored) to/from
+   * memory. Each outstanding load partition counts +1 and each outstanding
+   * store partition counts -1 toward this counter.
+   */
+  private final AtomicInteger deltaNumPartitionsInMemory =
+      new AtomicInteger(0);
+  /** Out-of-core engine */
+  private final OutOfCoreEngine oocEngine;
+
+  /**
+   * Constructor
+   *
+   * @param conf configuration
+   * @param oocEngine out-of-core engine
+   */
+  public FixedPartitionsOracle(ImmutableClassesGiraphConfiguration conf,
+                               OutOfCoreEngine oocEngine) {
+    this.maxPartitionsInMemory =
+        GiraphConstants.MAX_PARTITIONS_IN_MEMORY.get(conf);
+    this.oocEngine = oocEngine;
+  }
+
+  @Override
+  public IOAction[] getNextIOActions() {
+    int numPartitionsInMemory =
+        oocEngine.getMetaPartitionManager().getNumInMemoryPartitions();
+    if (LOG.isInfoEnabled()) {
+      LOG.info("getNextIOActions: calling with " + numPartitionsInMemory +
+          " partitions in memory, " + deltaNumPartitionsInMemory.get() +
+          " to be loaded");
+    }
+    int numPartitions =
+        numPartitionsInMemory + deltaNumPartitionsInMemory.get();
+    // Fixed out-of-core policy:
+    //   - if the number of partitions in memory is less than the max number of
+    //     partitions in memory, we should load a partition to memory. This
+    //     basically means we are prefetching partition's data either for the
+    //     current superstep, or for the next superstep.
+    //   - if the number of partitions in memory is equal to the the max number
+    //     of partitions in memory, we do a 'soft store', meaning, we store
+    //     processed partition to disk only if there is an unprocessed 
partition
+    //     on disk. This basically makes room for unprocessed partitions on 
disk
+    //     to be prefetched.
+    //   - if the number of partitions in memory is more than the max number of
+    //     partitions in memory, we do a 'hard store', meaning we store a
+    //     partition to disk, regardless of its processing state.
+    if (numPartitions < maxPartitionsInMemory) {
+      return new IOAction[]{
+        IOAction.LOAD_PARTITION,
+        IOAction.STORE_MESSAGES_AND_BUFFERS};
+    } else if (numPartitions > maxPartitionsInMemory) {
+      LOG.warn("getNextIOActions: number of partitions in memory passed the " +
+          "specified threshold!");
+      return new IOAction[]{
+        IOAction.STORE_PARTITION,
+        IOAction.STORE_MESSAGES_AND_BUFFERS};
+    } else {
+      return new IOAction[]{
+        IOAction.STORE_MESSAGES_AND_BUFFERS,
+        IOAction.LOAD_TO_SWAP_PARTITION};
+    }
+  }
+
+  @Override
+  public boolean approve(IOCommand command) {
+    int numPartitionsInMemory = oocEngine.getMetaPartitionManager()
+        .getNumInMemoryPartitions();
+    // If loading a partition result in having more partition in memory, the
+    // command should be denied. Also, if number of partitions in memory is
+    // already less than the max number of partitions, any command for storing
+    // a partition should be denied.
+    if (command instanceof LoadPartitionIOCommand &&
+        numPartitionsInMemory + deltaNumPartitionsInMemory.getAndIncrement() >
+            maxPartitionsInMemory) {
+      deltaNumPartitionsInMemory.getAndDecrement();
+      return false;
+
+    } else if (command instanceof StorePartitionIOCommand &&
+        numPartitionsInMemory + deltaNumPartitionsInMemory.getAndDecrement() <
+            maxPartitionsInMemory) {
+      deltaNumPartitionsInMemory.getAndIncrement();
+      return false;
+    }
+    return true;
+  }
+
+  @Override
+  public void commandCompleted(IOCommand command) {
+    if (command instanceof LoadPartitionIOCommand) {
+      deltaNumPartitionsInMemory.getAndDecrement();
+    } else if (command instanceof StorePartitionIOCommand) {
+      deltaNumPartitionsInMemory.getAndIncrement();
+    }
+  }
+
+  @Override
+  public void gcCompleted(GarbageCollectionNotificationInfo gcInfo) { }
+
+  @Override
+  public void shutdown() { }
+}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/policy/OutOfCoreOracle.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/policy/OutOfCoreOracle.java 
b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/OutOfCoreOracle.java
new file mode 100644
index 0000000..45b9914
--- /dev/null
+++ 
b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/OutOfCoreOracle.java
@@ -0,0 +1,135 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.giraph.ooc.policy;
+
+import com.sun.management.GarbageCollectionNotificationInfo;
+import org.apache.giraph.ooc.command.IOCommand;
+
+/**
+ * Interface for any out-of-core oracle. An out-of-core oracle is the brain of
+ * the out-of-core mechanism, determining/deciding on out-of-core actions (load
+ * or store) that should happen.
+ * Note: any class implementing this interface should have one and only one
+ *       constructor taking only two arguments of types
+ *       <code>ImmutableClassesGiraphConfiguration</code> and
+ *       <code>OutOfCoreEngine</code>
+ */
+public interface OutOfCoreOracle {
+  /**
+   * Different types of IO actions that can potentially lead to a more desired
+   * state of computation for out-of-core mechanism. These actions are issued
+   * based on the status of the memory (memory pressure, rate of data transfer
+   * to memory, etc.)
+   */
+  enum IOAction {
+    /**
+     * Either of:
+     *    - storing incoming messages of any partition currently on disk, or
+     *    - storing incoming messages' raw data buffer of any partition
+     *      currently on disk, or
+     *    - storing partitions' raw data buffer for those partitions that are
+     *      currently on disk.
+     */
+    STORE_MESSAGES_AND_BUFFERS,
+    /**
+     * Storing a partition that is *processed* in the current iteration cycle.
+     * This action is also known as "soft store"
+     */
+    STORE_PROCESSED_PARTITION,
+    /**
+     * Storing a partition from memory on disk, prioritizing to *processed*
+     * partitions on memory. However, if there is no *processed* partition,
+     * store should happen at any cost, even if an *unprocessed* partition has
+     * to be stored. This action is also know as "hard store".
+     */
+    STORE_PARTITION,
+    /**
+     * Loading an *unprocessed* partition from disk to memory, only if there 
are
+     * *processed* partitions in memory. This action basically initiates a swap
+     * operation.
+     */
+    LOAD_TO_SWAP_PARTITION,
+    /**
+     * Loading an *unprocessed* partition from disk to memory. This action is
+     * also known as "soft load".
+     */
+    LOAD_UNPROCESSED_PARTITION,
+    /**
+     * Loading a partition (prioritizing *unprocessed* over *processed*) from
+     * disk to memory. Loading a *processed* partition to memory is a prefetch
+     * of that partition to be processed in the next superstep. This action is
+     * also known as "hard load".
+     */
+    LOAD_PARTITION,
+    /**
+     * Loading a partition regardless of the memory situation. An out-of-core
+     * mechanism may use this action to signal IO threads that it is allowed to
+     * load a partition that is specifically requested.
+     */
+    URGENT_LOAD_PARTITION
+  }
+
+  /**
+   * Get the next set of viable IO actions to help bring memory to a more
+   * desired state.
+   *
+   * @return an array of viable IO actions, sorted from highest priority to
+   *         lowest priority
+   */
+  IOAction[] getNextIOActions();
+
+  /**
+   * Whether a command is appropriate to bring the memory to a more desired
+   * state. A command is not executed unless it is approved by the oracle. This
+   * method is specially important where there are multiple IO threads
+   * performing IO operations for the out-of-core mechanism. The approval
+   * becomes significantly important to prevent all IO threads from performing
+   * identical command type, if that is a necessity. For instance, execution of
+   * a particular command type by only one thread may bring the memory to a
+   * desired state, and the rest of IO threads may perform other types of
+   * commands.
+   *
+   * @param command the IO command that is about to execute
+   * @return 'true' if the command is approved for execution. 'false' if the
+   *         command should not be executed
+   */
+  boolean approve(IOCommand command);
+
+  /**
+   * Notification of command completion. Oracle may update its status and 
commit
+   * the changes a command may cause.
+   *
+   * @param command the IO command that is completed
+   */
+  void commandCompleted(IOCommand command);
+
+  /**
+   * Notification of GC completion. Oracle may take certain decisions based on
+   * GC information (such as amount of time it took, memory it reclaimed, etc.)
+   *
+   * @param gcInfo GC information
+   */
+  void gcCompleted(GarbageCollectionNotificationInfo gcInfo);
+
+  /**
+   * Shut down the out-of-core oracle. Necessary specifically for cases where
+   * out-of-core oracle is using additional monitoring threads.
+   */
+  void shutdown();
+}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/policy/SimpleGCMonitoringOracle.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/policy/SimpleGCMonitoringOracle.java
 
b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/SimpleGCMonitoringOracle.java
new file mode 100644
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--- /dev/null
+++ 
b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/SimpleGCMonitoringOracle.java
@@ -0,0 +1,357 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.giraph.ooc.policy;
+
+import com.google.common.collect.Maps;
+import com.sun.management.GarbageCollectionNotificationInfo;
+import org.apache.giraph.conf.FloatConfOption;
+import org.apache.giraph.conf.ImmutableClassesGiraphConfiguration;
+import org.apache.giraph.ooc.OutOfCoreEngine;
+import org.apache.giraph.ooc.OutOfCoreIOStatistics;
+import org.apache.giraph.ooc.command.IOCommand;
+import org.apache.giraph.ooc.command.LoadPartitionIOCommand;
+import org.apache.giraph.ooc.command.WaitIOCommand;
+import org.apache.log4j.Logger;
+
+import java.lang.management.MemoryUsage;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicLong;
+
+/**
+ * Out-of-core oracle to adaptively control data kept in memory, with the goal
+ * of keeping the memory state constantly at a desired state. This oracle
+ * monitors GC behavior to keep track of memory pressure.
+ *
+ * After each GC is done, this oracle retrieve statistics about the memory
+ * pressure (memory used, max memory, and how far away memory is compared to a
+ * max optimal pressure). Based on the the past 2 recent memory statistics,
+ * the oracle predicts the status of the memory, and sets the rate of 
load/store
+ * of data from/to disk. If the rate of loading data from disk is 'l', and the
+ * rate of storing data to disk is 's', the rate of data injection to memory
+ * from disk can be denoted as 'l-s'. This oracle determines what 'l-s' should
+ * be based on the prediction of memory status.
+ *
+ * Assume that based on the previous GC call the memory usage at time t_0 is
+ * m_0, and based on the most recent GC call the memory usage at time t_1 is
+ * m_1. So, the rate of memory increase is alpha = (m_1 - m_0) / (t_1 - t_0).
+ * Assume that the ideal memory pressure happens when the memory usage is
+ * m_ideal. So, at time 't_2 = t_1 + (t_1 - t_0)', we want m_ideal. That means
+ * the ideal rate would be beta = (m_ideal - m_1) / (t_2 - t_1). If the date
+ * injection rate to memory so far was i, the new injection rate should be:
+ * i_new = i - (alpha - beta)
+ */
+public class SimpleGCMonitoringOracle implements OutOfCoreOracle {
+  /**
+   * The optimal memory pressure at which GC behavior is close to ideal. This
+   * fraction may be dependant on the GC strategy used for running a job, but
+   * generally should not be dependent on the graph processing application.
+   */
+  public static final FloatConfOption OPTIMAL_MEMORY_PRESSURE =
+      new FloatConfOption("giraph.optimalMemoryPressure", 0.8f,
+          "The memory pressure (fraction of used memory) at which the job " +
+              "shows the optimal GC behavior. This fraction may be dependent " 
+
+              "on the GC strategy used in running the job.");
+
+  /** Class logger */
+  private static final Logger LOG =
+      Logger.getLogger(SimpleGCMonitoringOracle.class);
+  /** Cached value for OPTIMAL_MEMORY_PRESSURE */
+  private final float optimalMemoryPressure;
+  /** Out-of-core engine */
+  private final OutOfCoreEngine oocEngine;
+  /** Status of memory from the last GC call */
+  private GCObservation lastGCObservation;
+  /** Desired rate of data injection to memory */
+  private final AtomicLong desiredDiskToMemoryDataRate =
+      new AtomicLong(0);
+  /** Number of on the fly (outstanding) IO commands for each command type */
+  private final Map<IOCommand.IOCommandType, AtomicInteger> commandOccurrences 
=
+      Maps.newConcurrentMap();
+
+  /**
+   * Constructor
+   *
+   * @param conf configuration
+   * @param oocEngine out-of-core engine
+   */
+  public SimpleGCMonitoringOracle(ImmutableClassesGiraphConfiguration conf,
+                                  OutOfCoreEngine oocEngine) {
+    this.optimalMemoryPressure = OPTIMAL_MEMORY_PRESSURE.get(conf);
+    this.oocEngine = oocEngine;
+    this.lastGCObservation = new GCObservation(-1, 0, 0);
+    for (IOCommand.IOCommandType type : IOCommand.IOCommandType.values()) {
+      commandOccurrences.put(type, new AtomicInteger(0));
+    }
+  }
+
+  @Override
+  public synchronized void gcCompleted(GarbageCollectionNotificationInfo
+                                             gcInfo) {
+    long time = System.currentTimeMillis();
+    Map<String, MemoryUsage> memAfter = gcInfo.getGcInfo()
+        .getMemoryUsageAfterGc();
+    long usedMemory = 0;
+    long maxMemory = 0;
+    for (MemoryUsage memDetail : memAfter.values()) {
+      usedMemory += memDetail.getUsed();
+      maxMemory += memDetail.getMax();
+    }
+    GCObservation observation = new GCObservation(time, usedMemory, maxMemory);
+    if (LOG.isInfoEnabled()) {
+      LOG.info("gcCompleted: GC completed with: " + observation);
+    }
+    // Whether this is not the first GC call in the application
+    if (lastGCObservation.isValid()) {
+      long deltaDataRate =
+          lastGCObservation.getDesiredDeltaDataRate(observation);
+      long diskBandwidthEstimate =
+          oocEngine.getIOStatistics().getDiskBandwidth();
+      // Update the desired data injection rate to memory. The data injection
+      // rate cannot be less than -disk_bandwidth (the extreme case happens if
+      // we only do 'store'), and cannot be more than disk_bandwidth (the
+      // extreme case happens if we only do 'load').
+      long dataInjectionRate = desiredDiskToMemoryDataRate.get();
+      desiredDiskToMemoryDataRate.set(Math.max(
+          Math.min(desiredDiskToMemoryDataRate.get() - deltaDataRate,
+              diskBandwidthEstimate), -diskBandwidthEstimate));
+      if (LOG.isInfoEnabled()) {
+        LOG.info("gcCompleted: changing data injection rate from " +
+            String.format("%.2f", dataInjectionRate / 1024.0 / 1024.0) +
+            " to " + String.format("%.2f", desiredDiskToMemoryDataRate.get() /
+            1024.0 / 1024.0));
+      }
+    }
+    lastGCObservation = observation;
+  }
+
+  /**
+   * Get the current data injection rate to memory based on the commands ran
+   * in the history (retrieved from statistics collector), and outstanding
+   * commands issued by the IO scheduler.
+   *
+   * @return the current data injection rate to memory
+   */
+  private long getCurrentDataInjectionRate() {
+    long effectiveBytesTransferred = 0;
+    long effectiveDuration = 0;
+    for (IOCommand.IOCommandType type : IOCommand.IOCommandType.values()) {
+      OutOfCoreIOStatistics.BytesDuration stats =
+          oocEngine.getIOStatistics().getCommandTypeStats(type);
+      int occurrence = commandOccurrences.get(type).get();
+      long typeBytesTransferred = stats.getBytes();
+      long typeDuration = stats.getDuration();
+      // If there is an outstanding command, we still do not know how many 
bytes
+      // it will transfer, and how long it will take. So, we guesstimate these
+      // numbers based on other similar commands happened in the history. We
+      // simply take the average number of bytes transferred for the particular
+      // command, and we take average duration for the particular command. We
+      // should multiply these numbers by the number of outstanding commands of
+      // this particular command type.
+      if (stats.getOccurrence() != 0) {
+        typeBytesTransferred += stats.getBytes() / stats.getOccurrence() *
+            occurrence;
+        typeDuration += stats.getDuration() / stats.getOccurrence() *
+            occurrence;
+      }
+      if (type == IOCommand.IOCommandType.LOAD_PARTITION) {
+        effectiveBytesTransferred += typeBytesTransferred;
+      } else {
+        // Store (data going out of memory), or wait (no data transferred)
+        effectiveBytesTransferred -= typeBytesTransferred;
+      }
+      effectiveDuration += typeDuration;
+    }
+    if (effectiveDuration == 0) {
+      return 0;
+    } else {
+      return effectiveBytesTransferred / effectiveDuration;
+    }
+  }
+
+  @Override
+  public IOAction[] getNextIOActions() {
+    long error = (long) (oocEngine.getIOStatistics().getDiskBandwidth() * 
0.05);
+    long desiredRate = desiredDiskToMemoryDataRate.get();
+    long currentRate = getCurrentDataInjectionRate();
+    if (desiredRate > error) {
+      // 'l-s' is positive, we should do more load than store.
+      if (currentRate > desiredRate + error) {
+        // We should decrease 'l-s'. This can be done either by increasing 's'
+        // or issuing wait command. We prioritize wait over hard store.
+        return new IOAction[]{
+          IOAction.STORE_MESSAGES_AND_BUFFERS,
+          IOAction.STORE_PROCESSED_PARTITION};
+      } else if (currentRate < desiredRate - error) {
+        // We should increase 'l-s'. We can simply load partitions/data.
+        return new IOAction[]{IOAction.LOAD_PARTITION};
+      } else {
+        // We are in a proper state and we should keep up with the rate. We can
+        // either soft store data or load data (hard load, since we desired 
rate
+        // is positive).
+        return new IOAction[]{
+          IOAction.STORE_MESSAGES_AND_BUFFERS,
+          IOAction.STORE_PROCESSED_PARTITION,
+          IOAction.LOAD_PARTITION};
+      }
+    } else if (desiredRate < -error) {
+      // 'l-s' is negative, we should do more store than load.
+      if (currentRate < desiredRate - error) {
+        // We should increase 'l-s', but we should be cautious. We only do soft
+        // load, or wait.
+        return new IOAction[]{IOAction.LOAD_UNPROCESSED_PARTITION};
+      } else if (currentRate > desiredRate + error) {
+        // We should reduce 'l-s', we do hard store.
+        return new IOAction[]{
+          IOAction.STORE_MESSAGES_AND_BUFFERS,
+          IOAction.STORE_PARTITION};
+      } else {
+        // We should keep up with the rate. We can either soft store data, or
+        // soft load data.
+        return new IOAction[]{
+          IOAction.STORE_MESSAGES_AND_BUFFERS,
+          IOAction.STORE_PROCESSED_PARTITION,
+          IOAction.LOAD_UNPROCESSED_PARTITION};
+      }
+    } else {
+      // 'l-s' is almost zero. If current rate is over the desired rate, we do
+      // soft store. If the current rate is below the desired rate, we do soft
+      // load.
+      if (currentRate > desiredRate + error) {
+        return new IOAction[]{
+          IOAction.STORE_MESSAGES_AND_BUFFERS,
+          IOAction.STORE_PROCESSED_PARTITION};
+      } else if (currentRate < desiredRate - error) {
+        return new IOAction[]{IOAction.LOAD_UNPROCESSED_PARTITION};
+      } else {
+        return new IOAction[]{
+          IOAction.STORE_MESSAGES_AND_BUFFERS,
+          IOAction.STORE_PROCESSED_PARTITION,
+          IOAction.LOAD_UNPROCESSED_PARTITION};
+      }
+    }
+  }
+
+  @Override
+  public synchronized boolean approve(IOCommand command) {
+    long error = (long) (oocEngine.getIOStatistics().getDiskBandwidth() * 
0.05);
+    long desiredRate = desiredDiskToMemoryDataRate.get();
+    long currentRate = getCurrentDataInjectionRate();
+    // The command is denied iff the current rate is above the desired rate and
+    // we are doing load (instead of store), or the current rate is below the
+    // desired rate and we are doing store (instead of loading).
+    if (currentRate > desiredRate + error &&
+        command instanceof LoadPartitionIOCommand) {
+      return false;
+    }
+    if (currentRate < desiredRate - error &&
+        !(command instanceof LoadPartitionIOCommand) &&
+        !(command instanceof WaitIOCommand)) {
+      return false;
+    }
+    commandOccurrences.get(command.getType()).getAndIncrement();
+    return true;
+  }
+
+  @Override
+  public void commandCompleted(IOCommand command) {
+    commandOccurrences.get(command.getType()).getAndDecrement();
+  }
+
+  @Override
+  public void shutdown() { }
+
+  /** Helper class to record memory status after GC calls */
+  private class GCObservation {
+    /** The time at which the GC happened (in milliseconds) */
+    private long time;
+    /** Amount of memory used after the GC call */
+    private long usedMemory;
+    /** Maximum amounts of memory reported by GC listener */
+    private long maxMemory;
+
+    /**
+     * Constructor
+     *
+     * @param time time of GC
+     * @param usedMemory amount of used memory after GC
+     * @param maxMemory amount of all available memory based on GC observation
+     */
+    public GCObservation(long time, long usedMemory, long maxMemory) {
+      this.time = time;
+      this.usedMemory = usedMemory;
+      this.maxMemory = maxMemory;
+    }
+
+    /**
+     * Is this a valid observation?
+     *
+     * @return true iff it is a valid observation
+     */
+    public boolean isValid() {
+      return time > 0;
+    }
+
+    /**
+     * Considering a new observation of memory status after the most recent GC,
+     * what is the desired rate for data injection to memory.
+     *
+     * @param newObservation the most recent GC observation
+     * @return desired rate of data injection to memory
+     */
+    public long getDesiredDeltaDataRate(GCObservation newObservation) {
+      long newUsedMemory = newObservation.usedMemory;
+      long newMaxMemory = newObservation.maxMemory;
+      long lastUsedMemory = usedMemory;
+      long lastMaxMemory = maxMemory;
+      // Scale the memory status of two GC observation to be the same
+      long scaledMaxMemory = Math.min(lastMaxMemory, newMaxMemory);
+      newUsedMemory =
+          (long) (((double) scaledMaxMemory / newMaxMemory) * newUsedMemory);
+      lastUsedMemory =
+          (long) (((double) scaledMaxMemory / lastMaxMemory) * lastUsedMemory);
+      long desiredUsedMemory = (long) (optimalMemoryPressure * 
scaledMaxMemory);
+      if (LOG.isInfoEnabled()) {
+        LOG.info("getDesiredDeltaDataRate: " + String.format("previous usage " 
+
+            "= %.2f MB, ", lastUsedMemory / 1024.0 / 1024.0) + String.format(
+            "current usage = %.2f MB, ", newUsedMemory / 1024.0 / 1024.0) +
+            String.format("ideal usage = %.2f MB", desiredUsedMemory / 1024.0 /
+                1024.0));
+      }
+      long interval = newObservation.time - time;
+      if (interval == 0) {
+        interval = 1;
+        LOG.warn("getDesiredDeltaRate: two GC happened almost at the same " +
+            "time!");
+      }
+      long currentDataRate = (long) ((double) (newUsedMemory -
+          lastUsedMemory) / interval * 1000);
+      long desiredDataRate = (long) ((double) (desiredUsedMemory -
+          newUsedMemory) / interval * 1000);
+      return currentDataRate - desiredDataRate;
+    }
+
+    @Override
+    public String toString() {
+      return String.format("(usedMemory: %.2f MB, maxMemory: %.2f MB at " +
+          "time: %d ms)", usedMemory / 1024.0 / 1024.0,
+          maxMemory / 1024.0 / 1024.0, time);
+    }
+  }
+}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/policy/ThresholdBasedOracle.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/policy/ThresholdBasedOracle.java
 
b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/ThresholdBasedOracle.java
new file mode 100644
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+++ 
b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/ThresholdBasedOracle.java
@@ -0,0 +1,365 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.giraph.ooc.policy;
+
+import com.sun.management.GarbageCollectionNotificationInfo;
+import org.apache.giraph.comm.flow_control.CreditBasedFlowControl;
+import org.apache.giraph.comm.flow_control.FlowControl;
+import org.apache.giraph.comm.netty.NettyClient;
+import org.apache.giraph.conf.FloatConfOption;
+import org.apache.giraph.conf.ImmutableClassesGiraphConfiguration;
+import org.apache.giraph.conf.LongConfOption;
+import org.apache.giraph.ooc.OutOfCoreEngine;
+import org.apache.giraph.ooc.command.IOCommand;
+import org.apache.giraph.utils.CallableFactory;
+import org.apache.giraph.utils.LogStacktraceCallable;
+import org.apache.giraph.utils.MemoryUtils;
+import org.apache.giraph.utils.ThreadUtils;
+import org.apache.log4j.Logger;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+
+import static com.google.common.base.Preconditions.checkState;
+
+/**
+ * Out-of-core oracle to adaptively control data kept in memory, with the goal
+ * of keeping the memory usage at a desired state. Out-of-core policy in this
+ * oracle is based on several user-defined thresholds. Also, this oracle spawns
+ * a thread to periodically check the memory usage. This thread would issue
+ * manual GC calls if JVM fails to call major/full GC for a while and the 
amount
+ * of used memory is about to cause high-memory pressure. This oracle, also,
+ * monitors GC activities. The monitoring mechanism looks for major/full GC
+ * calls, and updates out-of-core decisions based on the amount of available
+ * memory after such GCs. There are three out-of-core decisions:
+ *  - Which IO operations should be done (load/offload of partitions and
+ *    messages)
+ *  - What the incoming messages rate should be (updating credits announced by
+ *    this worker in credit-based flow-control mechanism)
+ *  - How many processing threads should remain active (tethering rate of
+ *    data generation)
+ *
+ * The following table shows the relationship of these decisions and
+ * used-defined thresholds.
+ * --------------------------------------------------------------
+ * Memory Pressure     |  Manual |   IO   | Credit   | Active   |
+ * (memory usage)      |   GC?   | Action |          | Threads  |
+ * --------------------------------------------------------------
+ *                     |  Yes    | hard   |  0       |  0       |
+ *                     |         | store  |          |          |
+ * failPressure -------------------------------------------------
+ *                     |  Yes    | hard   |  0       | fraction |
+ *                     |         | store  |          |          |
+ * emergencyPressure --------------------------------------------
+ *                     |  Yes    | hard   | fraction |  max     |
+ *                     |         | store  |          |          |
+ * highPressure -------------------------------------------------
+ *                     |  No     | soft   | fraction |  max     |
+ *                     |         | store  |          |          |
+ * optimalPressure ----------------------------------------------
+ *                     |  No     | soft   |  max     |  max     |
+ *                     |         | load   |          |          |
+ * lowPressure --------------------------------------------------
+ *                     |  No     | hard   |  max     |  max     |
+ *                     |         | load   |          |          |
+ * --------------------------------------------------------------
+ *
+ */
+public class ThresholdBasedOracle implements OutOfCoreOracle {
+  /** The memory pressure at/above which the job would fail */
+  public static final FloatConfOption FAIL_MEMORY_PRESSURE =
+      new FloatConfOption("giraph.memory.failPressure", 0.975f,
+          "The memory pressure (fraction of used memory) at/above which the " +
+              "job would fail.");
+  /**
+   * The memory pressure at which the job is cloe to fail, even though we were
+   * using maximal disk bandwidth and minimal network rate. We should reduce
+   * job processing rate.
+   */
+  public static final FloatConfOption EMERGENCY_MEMORY_PRESSURE =
+      new FloatConfOption("giraph.memory.emergencyPressure", 0.925f,
+          "The memory pressure (fraction of used memory) at which the job " +
+              "is close to fail, hence we should reduce its processing rate " +
+              "as much as possible.");
+  /** The memory pressure at which the job is suffering from GC overhead. */
+  public static final FloatConfOption HIGH_MEMORY_PRESSURE =
+      new FloatConfOption("giraph.memory.highPressure", 0.875f,
+          "The memory pressure (fraction of used memory) at which the job " +
+              "is suffering from GC overhead.");
+  /**
+   * The memory pressure at which we expect GC to perform optimally for a
+   * memory intensive job.
+   */
+  public static final FloatConfOption OPTIMAL_MEMORY_PRESSURE =
+      new FloatConfOption("giraph.memory.optimalPressure", 0.8f,
+          "The memory pressure (fraction of used memory) at which a " +
+              "memory-intensive job shows the optimal GC behavior.");
+  /**
+   * The memory pressure at/below which the job can use more memory without
+   * suffering from GC overhead.
+   */
+  public static final FloatConfOption LOW_MEMORY_PRESSURE =
+      new FloatConfOption("giraph.memory.lowPressure", 0.7f,
+          "The memory pressure (fraction of used memory) at/below which the " +
+              "job can use more memory without suffering the performance.");
+  /** The interval at which memory observer thread wakes up. */
+  public static final LongConfOption CHECK_MEMORY_INTERVAL =
+      new LongConfOption("giraph.checkMemoryInterval", 2500,
+          "The interval/period where memory observer thread wakes up and " +
+              "monitors memory footprint (in milliseconds)");
+  /**
+   * Memory observer thread would manually call GC if major/full GC has not
+   * been called for a while. The period where we expect GC to be happened in
+   * past is specified in this parameter
+   */
+  public static final LongConfOption LAST_GC_CALL_INTERVAL =
+      new LongConfOption("giraph.lastGcCallInterval", 10 * 1000,
+          "How long after last major/full GC should we call manual GC?");
+
+  /** Class logger */
+  private static final Logger LOG =
+      Logger.getLogger(ThresholdBasedOracle.class);
+  /** Cached value for FAIL_MEMORY_PRESSURE */
+  private final float failMemoryPressure;
+  /** Cached value for EMERGENCY_MEMORY_PRESSURE */
+  private final float emergencyMemoryPressure;
+  /** Cached value for HIGH_MEMORY_PRESSURE */
+  private final float highMemoryPressure;
+  /** Cached value for OPTIMAL_MEMORY_PRESSURE */
+  private final float optimalMemoryPressure;
+  /** Cached value for LOW_MEMORY_PRESSURE */
+  private final float lowMemoryPressure;
+  /** Cached value for CHECK_MEMORY_INTERVAL */
+  private final long checkMemoryInterval;
+  /** Cached value for LAST_GC_CALL_INTERVAL */
+  private final long lastGCCallInterval;
+  /**
+   * Cached value for NettyClient.MAX_NUM_OF_OPEN_REQUESTS_PER_WORKER (max
+   * credit used for credit-based flow-control mechanism)
+   */
+  private final short maxRequestsCredit;
+  /**
+   * Whether the job is shutting down. Used for terminating the memory
+   * observer thread.
+   */
+  private final CountDownLatch shouldTerminate;
+  /** Result of memory observer thread */
+  private final Future<Void> checkMemoryThreadResult;
+  /** Out-of-core engine */
+  private final OutOfCoreEngine oocEngine;
+  /** Last time a major/full GC has been called (in milliseconds) */
+  private volatile long lastMajorGCTime;
+  /** Last time a non major/full GC has been called (in milliseconds) */
+  private volatile long lastMinorGCTime;
+
+  /**
+   * Constructor
+   *
+   * @param conf configuration
+   * @param oocEngine out-of-core engine
+   */
+  public ThresholdBasedOracle(ImmutableClassesGiraphConfiguration conf,
+                              OutOfCoreEngine oocEngine) {
+    this.failMemoryPressure = FAIL_MEMORY_PRESSURE.get(conf);
+    this.emergencyMemoryPressure = EMERGENCY_MEMORY_PRESSURE.get(conf);
+    this.highMemoryPressure = HIGH_MEMORY_PRESSURE.get(conf);
+    this.optimalMemoryPressure = OPTIMAL_MEMORY_PRESSURE.get(conf);
+    this.lowMemoryPressure = LOW_MEMORY_PRESSURE.get(conf);
+    this.checkMemoryInterval = CHECK_MEMORY_INTERVAL.get(conf);
+    this.lastGCCallInterval = LAST_GC_CALL_INTERVAL.get(conf);
+    this.maxRequestsCredit = (short)
+        CreditBasedFlowControl.MAX_NUM_OF_OPEN_REQUESTS_PER_WORKER.get(conf);
+    NettyClient.LIMIT_OPEN_REQUESTS_PER_WORKER.setIfUnset(conf, true);
+    boolean useCredit = NettyClient.LIMIT_OPEN_REQUESTS_PER_WORKER.get(conf);
+    checkState(useCredit, "ThresholdBasedOracle: credit-based flow control " +
+        "must be enabled. Use giraph.waitForPerWorkerRequests=true");
+    this.shouldTerminate = new CountDownLatch(1);
+    this.oocEngine = oocEngine;
+    this.lastMajorGCTime = 0;
+
+    CallableFactory<Void> callableFactory = new CallableFactory<Void>() {
+      @Override
+      public Callable<Void> newCallable(int callableId) {
+        return new Callable<Void>() {
+          @Override
+          public Void call() throws Exception {
+            while (true) {
+              boolean done = shouldTerminate.await(checkMemoryInterval,
+                  TimeUnit.MILLISECONDS);
+              if (done) {
+                break;
+              }
+              double usedMemoryFraction = 1 - MemoryUtils.freeMemoryFraction();
+              long time = System.currentTimeMillis();
+              if ((usedMemoryFraction > highMemoryPressure &&
+                  time - lastMajorGCTime >= lastGCCallInterval) ||
+                  (usedMemoryFraction > optimalMemoryPressure &&
+                  time - lastMajorGCTime >= lastGCCallInterval &&
+                  time - lastMinorGCTime >= lastGCCallInterval)) {
+                if (LOG.isInfoEnabled()) {
+                  LOG.info("call: last GC happened a while ago and the " +
+                      "amount of used memory is high (used memory " +
+                      "fraction is " +
+                      String.format("%.2f", usedMemoryFraction) + "). " +
+                      "Calling GC manually");
+                }
+                System.gc();
+                time = System.currentTimeMillis() - time;
+                usedMemoryFraction = 1 - MemoryUtils.freeMemoryFraction();
+                if (LOG.isInfoEnabled()) {
+                  LOG.info("call: manual GC is done. It took " +
+                      String.format("%.2f", (double) time / 1000) +
+                      " seconds. Used memory fraction is " +
+                      String.format("%.2f", usedMemoryFraction));
+                }
+              }
+              updateRates(usedMemoryFraction);
+            }
+            return null;
+          }
+        };
+      }
+    };
+    ExecutorService executor = Executors.newSingleThreadExecutor(
+        ThreadUtils.createThreadFactory("check-memory"));
+    this.checkMemoryThreadResult = executor.submit(new LogStacktraceCallable<>(
+        callableFactory.newCallable(0)));
+    executor.shutdown();
+  }
+
+  /**
+   * upon major/full GC calls.
+   */
+  /**
+   * Update statistics and rate regarding communication credits and number of
+   * active threads.
+   *
+   * @param usedMemoryFraction the fraction of used memory over max memory
+   */
+  public void updateRates(double usedMemoryFraction) {
+    // Update the fraction of processing threads that should remain active
+    if (usedMemoryFraction >= failMemoryPressure) {
+      oocEngine.updateActiveThreadsFraction(0);
+    } else if (usedMemoryFraction < emergencyMemoryPressure) {
+      oocEngine.updateActiveThreadsFraction(1);
+    } else {
+      oocEngine.updateActiveThreadsFraction(1 -
+          (usedMemoryFraction - emergencyMemoryPressure) /
+              (failMemoryPressure - emergencyMemoryPressure));
+    }
+
+    // Update the fraction of credit that should be used in credit-based flow-
+    // control
+    if (usedMemoryFraction >= emergencyMemoryPressure) {
+      updateRequestsCredit((short) 0);
+    } else if (usedMemoryFraction < optimalMemoryPressure) {
+      updateRequestsCredit(maxRequestsCredit);
+    } else {
+      updateRequestsCredit((short) (maxRequestsCredit *
+          (1 - (usedMemoryFraction - optimalMemoryPressure) /
+              (emergencyMemoryPressure - optimalMemoryPressure))));
+    }
+  }
+
+  @Override
+  public IOAction[] getNextIOActions() {
+    double usedMemoryFraction = 1 - MemoryUtils.freeMemoryFraction();
+    if (LOG.isInfoEnabled()) {
+      LOG.info(String.format("getNextIOActions: usedMemoryFraction = %.2f",
+          usedMemoryFraction));
+    }
+    if (usedMemoryFraction > highMemoryPressure) {
+      return new IOAction[]{
+        IOAction.STORE_MESSAGES_AND_BUFFERS,
+        IOAction.STORE_PARTITION};
+    } else if (usedMemoryFraction > optimalMemoryPressure) {
+      return new IOAction[]{
+        IOAction.LOAD_UNPROCESSED_PARTITION,
+        IOAction.STORE_MESSAGES_AND_BUFFERS,
+        IOAction.STORE_PROCESSED_PARTITION};
+    } else if (usedMemoryFraction > lowMemoryPressure) {
+      return new IOAction[]{
+        IOAction.LOAD_UNPROCESSED_PARTITION,
+        IOAction.STORE_MESSAGES_AND_BUFFERS,
+        IOAction.LOAD_PARTITION};
+    } else {
+      return new IOAction[]{IOAction.LOAD_PARTITION};
+    }
+  }
+
+  @Override
+  public boolean approve(IOCommand command) {
+    return true;
+  }
+
+  @Override
+  public void commandCompleted(IOCommand command) {
+    // Do nothing
+  }
+
+  @Override
+  public void gcCompleted(GarbageCollectionNotificationInfo gcInfo) {
+    String gcAction = gcInfo.getGcAction().toLowerCase();
+    if (gcAction.contains("full") || gcAction.contains("major")) {
+      if (!gcInfo.getGcCause().contains("No GC")) {
+        lastMajorGCTime = System.currentTimeMillis();
+      }
+    } else {
+      lastMinorGCTime = System.currentTimeMillis();
+    }
+  }
+
+  @Override
+  public void shutdown() {
+    shouldTerminate.countDown();
+    try {
+      checkMemoryThreadResult.get();
+    } catch (InterruptedException | ExecutionException e) {
+      LOG.error("shutdown: caught exception while waiting on check-memory " +
+          "thread to terminate!");
+      throw new IllegalStateException(e);
+    }
+    if (LOG.isInfoEnabled()) {
+      LOG.info("shutdown: ThresholdBasedOracle shutdown complete!");
+    }
+  }
+
+  /**
+   * Update the credit announced for this worker in Netty. The lower the credit
+   * is, the lower rate incoming messages arrive at this worker. Thus, credit
+   * is an indirect way of controlling amount of memory incoming messages would
+   * take.
+   *
+   * @param newCredit the new credit to announce to other workers
+   */
+  private void updateRequestsCredit(short newCredit) {
+    if (LOG.isInfoEnabled()) {
+      LOG.info("updateRequestsCredit: updating the credit to " + newCredit);
+    }
+    FlowControl flowControl = oocEngine.getFlowControl();
+    if (flowControl != null) {
+      ((CreditBasedFlowControl) flowControl).updateCredit(newCredit);
+    }
+  }
+}

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/ooc/policy/package-info.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/ooc/policy/package-info.java 
b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/package-info.java
new file mode 100644
index 0000000..c58289f
--- /dev/null
+++ b/giraph-core/src/main/java/org/apache/giraph/ooc/policy/package-info.java
@@ -0,0 +1,21 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+/**
+ * Package of classes related to out-of-core policy
+ */
+package org.apache.giraph.ooc.policy;

http://git-wip-us.apache.org/repos/asf/giraph/blob/3793c9ef/giraph-core/src/main/java/org/apache/giraph/zk/ZooKeeperManager.java
----------------------------------------------------------------------
diff --git 
a/giraph-core/src/main/java/org/apache/giraph/zk/ZooKeeperManager.java 
b/giraph-core/src/main/java/org/apache/giraph/zk/ZooKeeperManager.java
index d3ace99..c54e7b2 100644
--- a/giraph-core/src/main/java/org/apache/giraph/zk/ZooKeeperManager.java
+++ b/giraph-core/src/main/java/org/apache/giraph/zk/ZooKeeperManager.java
@@ -123,7 +123,7 @@ public class ZooKeeperManager {
     this.context = context;
     this.conf = configuration;
     taskPartition = conf.getTaskPartition();
-    jobId = conf.get("mapred.job.id", "Unknown Job");
+    jobId = conf.getJobId();
     baseDirectory =
         new Path(ZOOKEEPER_MANAGER_DIRECTORY.getWithDefault(conf,
             getFinalZooKeeperPath()));

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