kl0u commented on a change in pull request #13357:
URL: https://github.com/apache/flink/pull/13357#discussion_r493267562



##########
File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/SortingThread.java
##########
@@ -0,0 +1,85 @@
+/*
+ * 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.flink.runtime.operators.sort;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.EOF_MARKER;
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.SPILLING_MARKER;
+
+/**
+ * The thread that sorts filled buffers.
+ */
+class SortingThread<E> extends ThreadBase<E> {
+
+       /** Logging. */
+       private static final Logger LOG = 
LoggerFactory.getLogger(SortingThread.class);
+
+       private final IndexedSorter sorter;
+
+       /**
+        * Creates a new sorting thread.
+        *
+        * @param exceptionHandler The exception handler to call for all 
exceptions.
+        * @param dispatcher The queues used to pass buffers between the 
threads.
+        */
+       public SortingThread(
+                       @Nullable ExceptionHandler<IOException> 
exceptionHandler,
+                       StageMessageDispatcher<E> dispatcher) {
+               super(exceptionHandler, "SortMerger sorting thread", 
dispatcher);
+
+               // members
+               this.sorter = new QuickSort();
+       }
+
+       /**
+        * Entry point of the thread.
+        */
+       public void go() throws IOException, InterruptedException {
+               boolean alive = true;
+
+               // loop as long as the thread is marked alive
+               while (isRunning() && alive) {
+                       CircularElement<E> element = 
this.dispatcher.take(SortStage.SORT);

Review comment:
       This could become `final`.

##########
File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/SortingThread.java
##########
@@ -0,0 +1,85 @@
+/*
+ * 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.flink.runtime.operators.sort;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.EOF_MARKER;
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.SPILLING_MARKER;
+
+/**
+ * The thread that sorts filled buffers.
+ */
+class SortingThread<E> extends ThreadBase<E> {
+
+       /** Logging. */
+       private static final Logger LOG = 
LoggerFactory.getLogger(SortingThread.class);
+
+       private final IndexedSorter sorter;
+
+       /**
+        * Creates a new sorting thread.
+        *
+        * @param exceptionHandler The exception handler to call for all 
exceptions.
+        * @param dispatcher The queues used to pass buffers between the 
threads.
+        */
+       public SortingThread(
+                       @Nullable ExceptionHandler<IOException> 
exceptionHandler,
+                       StageMessageDispatcher<E> dispatcher) {
+               super(exceptionHandler, "SortMerger sorting thread", 
dispatcher);
+
+               // members
+               this.sorter = new QuickSort();
+       }
+
+       /**
+        * Entry point of the thread.
+        */
+       public void go() throws IOException, InterruptedException {

Review comment:
       It seems you can remove the `IOException` now.

##########
File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/SpillingThread.java
##########
@@ -0,0 +1,575 @@
+/*
+ * 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.flink.runtime.operators.sort;
+
+import org.apache.flink.api.common.typeutils.TypeComparator;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.core.memory.MemorySegment;
+import org.apache.flink.runtime.io.disk.ChannelReaderInputViewIterator;
+import org.apache.flink.runtime.io.disk.iomanager.BlockChannelReader;
+import org.apache.flink.runtime.io.disk.iomanager.BlockChannelWriter;
+import org.apache.flink.runtime.io.disk.iomanager.ChannelReaderInputView;
+import org.apache.flink.runtime.io.disk.iomanager.ChannelWriterOutputView;
+import org.apache.flink.runtime.io.disk.iomanager.FileIOChannel;
+import org.apache.flink.runtime.io.disk.iomanager.IOManager;
+import org.apache.flink.runtime.memory.MemoryManager;
+import org.apache.flink.runtime.util.EmptyMutableObjectIterator;
+import org.apache.flink.util.MutableObjectIterator;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Queue;
+
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.EOF_MARKER;
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.SPILLING_MARKER;
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * The thread that handles the spilling of intermediate results and sets up 
the merging. It also merges the
+ * channels until sufficiently few channels remain to perform the final 
streamed merge.
+ */
+final class SpillingThread<E> extends ThreadBase<E> {
+       /**
+        * An interface for injecting custom behaviour for spilling and merging 
phases.
+        */
+       interface SpillingBehaviour<E> {
+               default void open() {}
+
+               default void close() {}
+
+               /**
+                * A method that allows adjusting the spilling phase. We can 
inject e.g. combining the elements while spilling.
+                */
+               void spillBuffer(
+                       CircularElement<E> element,
+                       ChannelWriterOutputView output,
+                       LargeRecordHandler<E> largeRecordHandler) throws 
IOException;
+
+               /**
+                * A method that allows adjusting the merging phase. We can 
inject e.g. combining the spilled elements.
+                */
+               void mergeRecords(MergeIterator<E> mergeIterator, 
ChannelWriterOutputView output) throws IOException;
+       }
+
+       /** Logging. */
+       private static final Logger LOG = 
LoggerFactory.getLogger(SpillingThread.class);
+
+       private final MemoryManager memManager;                 // memory 
manager to release memory
+
+       private final IOManager ioManager;                              // I/O 
manager to create channels
+
+       private final TypeSerializer<E> serializer;             // The 
serializer for the data type
+
+       private final TypeComparator<E> comparator;             // The 
comparator that establishes the order relation.
+
+       private final List<MemorySegment> writeMemory;  // memory segments for 
writing
+
+       private final List<MemorySegment> mergeReadMemory;      // memory 
segments for sorting/reading
+
+       private final int maxFanIn;
+
+       private final SpillChannelManager spillChannelManager;
+
+       private final LargeRecordHandler<E> largeRecordHandler;
+
+       private final SpillingBehaviour<E> spillingBehaviour;
+
+       private boolean spillingBehaviourOpened = false;

Review comment:
       I think this should become `volatile` because it is called in `close()` 
and in `go()` which run on/ are called by different threads.

##########
File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/ThreadBase.java
##########
@@ -0,0 +1,137 @@
+/*
+ * 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.flink.runtime.operators.sort;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * Base class for all working threads in this sort-merger. The specific 
threads for reading, sorting, spilling,
+ * merging, etc... extend this subclass.
+ * <p>
+ * The threads are designed to terminate themselves when the task they are set 
up to do is completed. Further more,
+ * they terminate immediately when the <code>shutdown()</code> method is 
called.
+ */
+abstract class ThreadBase<E> extends Thread implements 
Thread.UncaughtExceptionHandler, StageRunner {
+
+       /**
+        * The queue of empty buffer that can be used for reading;
+        */
+       protected final StageMessageDispatcher<E> dispatcher;
+
+       /**
+        * The exception handler for any problems.
+        */
+       private final ExceptionHandler<IOException> exceptionHandler;
+
+       /**
+        * The flag marking this thread as alive.
+        */
+       private volatile boolean alive;
+
+       /**
+        * Creates a new thread.
+        *
+        * @param exceptionHandler The exception handler to call for all 
exceptions.
+        * @param name The name of the thread.
+        * @param queues The queues used to pass buffers between the threads.
+        */
+       protected ThreadBase(
+                       @Nullable ExceptionHandler<IOException> 
exceptionHandler,
+                       String name,
+                       StageMessageDispatcher<E> queues) {
+               // thread setup
+               super(checkNotNull(name));
+               this.setDaemon(true);
+
+               // exception handling
+               this.exceptionHandler = exceptionHandler;
+               this.setUncaughtExceptionHandler(this);
+
+               this.dispatcher = checkNotNull(queues);
+               this.alive = true;
+       }
+
+       /**
+        * Implements exception handling and delegates to go().
+        */
+       public void run() {
+               try {
+                       go();
+               }
+               catch (Throwable t) {
+                       internalHandleException(new IOException("Thread '" + 
getName() + "' terminated due to an exception: "
+                               + t.getMessage(), t));
+               }
+       }
+
+       /**
+        * Equivalent to the run() method.
+        *
+        * @throws IOException Exceptions that prohibit correct completion of 
the work may be thrown by the thread.
+        */
+       protected abstract void go() throws IOException, InterruptedException;
+
+       /**
+        * Checks whether this thread is still alive.
+        *
+        * @return true, if the thread is alive, false otherwise.
+        */
+       public boolean isRunning() {

Review comment:
       This can become `protected`.

##########
File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/SpillingThread.java
##########
@@ -0,0 +1,575 @@
+/*
+ * 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.flink.runtime.operators.sort;
+
+import org.apache.flink.api.common.typeutils.TypeComparator;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.core.memory.MemorySegment;
+import org.apache.flink.runtime.io.disk.ChannelReaderInputViewIterator;
+import org.apache.flink.runtime.io.disk.iomanager.BlockChannelReader;
+import org.apache.flink.runtime.io.disk.iomanager.BlockChannelWriter;
+import org.apache.flink.runtime.io.disk.iomanager.ChannelReaderInputView;
+import org.apache.flink.runtime.io.disk.iomanager.ChannelWriterOutputView;
+import org.apache.flink.runtime.io.disk.iomanager.FileIOChannel;
+import org.apache.flink.runtime.io.disk.iomanager.IOManager;
+import org.apache.flink.runtime.memory.MemoryManager;
+import org.apache.flink.runtime.util.EmptyMutableObjectIterator;
+import org.apache.flink.util.MutableObjectIterator;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Queue;
+
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.EOF_MARKER;
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.SPILLING_MARKER;
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * The thread that handles the spilling of intermediate results and sets up 
the merging. It also merges the
+ * channels until sufficiently few channels remain to perform the final 
streamed merge.
+ */
+final class SpillingThread<E> extends ThreadBase<E> {
+       /**
+        * An interface for injecting custom behaviour for spilling and merging 
phases.
+        */
+       interface SpillingBehaviour<E> {
+               default void open() {}
+
+               default void close() {}
+
+               /**
+                * A method that allows adjusting the spilling phase. We can 
inject e.g. combining the elements while spilling.
+                */
+               void spillBuffer(
+                       CircularElement<E> element,
+                       ChannelWriterOutputView output,
+                       LargeRecordHandler<E> largeRecordHandler) throws 
IOException;
+
+               /**
+                * A method that allows adjusting the merging phase. We can 
inject e.g. combining the spilled elements.
+                */
+               void mergeRecords(MergeIterator<E> mergeIterator, 
ChannelWriterOutputView output) throws IOException;
+       }
+
+       /** Logging. */
+       private static final Logger LOG = 
LoggerFactory.getLogger(SpillingThread.class);
+
+       private final MemoryManager memManager;                 // memory 
manager to release memory
+
+       private final IOManager ioManager;                              // I/O 
manager to create channels
+
+       private final TypeSerializer<E> serializer;             // The 
serializer for the data type
+
+       private final TypeComparator<E> comparator;             // The 
comparator that establishes the order relation.
+
+       private final List<MemorySegment> writeMemory;  // memory segments for 
writing
+
+       private final List<MemorySegment> mergeReadMemory;      // memory 
segments for sorting/reading
+
+       private final int maxFanIn;
+
+       private final SpillChannelManager spillChannelManager;
+
+       private final LargeRecordHandler<E> largeRecordHandler;
+
+       private final SpillingBehaviour<E> spillingBehaviour;
+
+       private boolean spillingBehaviourOpened = false;
+
+       private final int minNumWriteBuffers;
+
+       private final int maxNumWriteBuffers;
+
+       SpillingThread(
+                       @Nullable ExceptionHandler<IOException> 
exceptionHandler,
+                       StageMessageDispatcher<E> dispatcher,
+                       MemoryManager memManager,
+                       IOManager ioManager,
+                       TypeSerializer<E> serializer,
+                       TypeComparator<E> comparator,
+                       List<MemorySegment> sortReadMemory,
+                       List<MemorySegment> writeMemory,
+                       int maxNumFileHandles,
+                       SpillChannelManager spillingChannelManager,
+                       @Nullable LargeRecordHandler<E> largeRecordHandler,
+                       SpillingBehaviour<E> spillingBehaviour,
+                       int minNumWriteBuffers,
+                       int maxNumWriteBuffers) {
+               super(exceptionHandler, "SortMerger spilling thread", 
dispatcher);
+               this.memManager = checkNotNull(memManager);
+               this.ioManager = checkNotNull(ioManager);
+               this.serializer = checkNotNull(serializer);
+               this.comparator = checkNotNull(comparator);
+               this.mergeReadMemory = checkNotNull(sortReadMemory);
+               this.writeMemory = checkNotNull(writeMemory);
+               this.maxFanIn = maxNumFileHandles;
+               this.spillChannelManager = checkNotNull(spillingChannelManager);
+               this.largeRecordHandler = largeRecordHandler;
+               this.spillingBehaviour = checkNotNull(spillingBehaviour);
+               this.minNumWriteBuffers = minNumWriteBuffers;
+               this.maxNumWriteBuffers = maxNumWriteBuffers;
+       }
+
+       /**
+        * Entry point of the thread.
+        */
+       public void go() throws IOException, InterruptedException {
+
+               final Queue<CircularElement<E>> cache = new ArrayDeque<>();
+               boolean cacheOnly = readCache(cache);
+
+               // check whether the thread was canceled
+               if (!isRunning()) {
+                       return;
+               }
+
+               MutableObjectIterator<E> largeRecords = null;
+
+               // check if we can stay in memory with the large record handler
+               if (cacheOnly && largeRecordHandler != null && 
largeRecordHandler.hasData()) {
+                       List<MemorySegment> memoryForLargeRecordSorting = new 
ArrayList<>();
+
+                       CircularElement<E> circElement;
+                       while ((circElement = 
this.dispatcher.poll(SortStage.READ)) != null) {
+                               circElement.getBuffer().dispose();
+                               
memoryForLargeRecordSorting.addAll(circElement.getMemory());
+                       }
+
+                       if (memoryForLargeRecordSorting.isEmpty()) {
+                               cacheOnly = false;
+                               LOG.debug("Going to disk-based merge because of 
large records.");
+                       } else {
+                               LOG.debug("Sorting large records, to add them 
to in-memory merge.");
+                               largeRecords = 
largeRecordHandler.finishWriteAndSortKeys(memoryForLargeRecordSorting);
+                       }
+               }
+
+               // ------------------- In-Memory Merge ------------------------
+               if (cacheOnly) {
+                       mergeInMemory(cache, largeRecords);
+                       return;
+               }
+
+               // ------------------- Spilling Phase ------------------------
+               List<ChannelWithBlockCount> channelIDs = startSpilling(cache);
+
+               // ------------------- Merging Phase ------------------------
+
+               mergeOnDisk(channelIDs);
+       }
+
+       @Override
+       public void close() throws InterruptedException {
+               super.close();
+               if (spillingBehaviourOpened) {
+                       this.spillingBehaviour.close();
+                       this.spillingBehaviourOpened = false;
+               }
+       }
+
+       private boolean readCache(Queue<CircularElement<E>> cache) throws 
InterruptedException {
+               CircularElement<E> element;

Review comment:
       This can go in the `while()` loop now and become `final`.

##########
File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/SpillingThread.java
##########
@@ -0,0 +1,575 @@
+/*
+ * 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.flink.runtime.operators.sort;
+
+import org.apache.flink.api.common.typeutils.TypeComparator;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.core.memory.MemorySegment;
+import org.apache.flink.runtime.io.disk.ChannelReaderInputViewIterator;
+import org.apache.flink.runtime.io.disk.iomanager.BlockChannelReader;
+import org.apache.flink.runtime.io.disk.iomanager.BlockChannelWriter;
+import org.apache.flink.runtime.io.disk.iomanager.ChannelReaderInputView;
+import org.apache.flink.runtime.io.disk.iomanager.ChannelWriterOutputView;
+import org.apache.flink.runtime.io.disk.iomanager.FileIOChannel;
+import org.apache.flink.runtime.io.disk.iomanager.IOManager;
+import org.apache.flink.runtime.memory.MemoryManager;
+import org.apache.flink.runtime.util.EmptyMutableObjectIterator;
+import org.apache.flink.util.MutableObjectIterator;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Queue;
+
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.EOF_MARKER;
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.SPILLING_MARKER;
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * The thread that handles the spilling of intermediate results and sets up 
the merging. It also merges the
+ * channels until sufficiently few channels remain to perform the final 
streamed merge.
+ */
+final class SpillingThread<E> extends ThreadBase<E> {
+       /**
+        * An interface for injecting custom behaviour for spilling and merging 
phases.
+        */
+       interface SpillingBehaviour<E> {
+               default void open() {}
+
+               default void close() {}
+
+               /**
+                * A method that allows adjusting the spilling phase. We can 
inject e.g. combining the elements while spilling.
+                */
+               void spillBuffer(
+                       CircularElement<E> element,
+                       ChannelWriterOutputView output,
+                       LargeRecordHandler<E> largeRecordHandler) throws 
IOException;
+
+               /**
+                * A method that allows adjusting the merging phase. We can 
inject e.g. combining the spilled elements.
+                */
+               void mergeRecords(MergeIterator<E> mergeIterator, 
ChannelWriterOutputView output) throws IOException;
+       }
+
+       /** Logging. */
+       private static final Logger LOG = 
LoggerFactory.getLogger(SpillingThread.class);
+
+       private final MemoryManager memManager;                 // memory 
manager to release memory
+
+       private final IOManager ioManager;                              // I/O 
manager to create channels
+
+       private final TypeSerializer<E> serializer;             // The 
serializer for the data type
+
+       private final TypeComparator<E> comparator;             // The 
comparator that establishes the order relation.
+
+       private final List<MemorySegment> writeMemory;  // memory segments for 
writing
+
+       private final List<MemorySegment> mergeReadMemory;      // memory 
segments for sorting/reading
+
+       private final int maxFanIn;
+
+       private final SpillChannelManager spillChannelManager;
+
+       private final LargeRecordHandler<E> largeRecordHandler;
+
+       private final SpillingBehaviour<E> spillingBehaviour;
+
+       private boolean spillingBehaviourOpened = false;
+
+       private final int minNumWriteBuffers;
+
+       private final int maxNumWriteBuffers;
+
+       SpillingThread(
+                       @Nullable ExceptionHandler<IOException> 
exceptionHandler,
+                       StageMessageDispatcher<E> dispatcher,
+                       MemoryManager memManager,
+                       IOManager ioManager,
+                       TypeSerializer<E> serializer,
+                       TypeComparator<E> comparator,
+                       List<MemorySegment> sortReadMemory,
+                       List<MemorySegment> writeMemory,
+                       int maxNumFileHandles,
+                       SpillChannelManager spillingChannelManager,
+                       @Nullable LargeRecordHandler<E> largeRecordHandler,
+                       SpillingBehaviour<E> spillingBehaviour,
+                       int minNumWriteBuffers,
+                       int maxNumWriteBuffers) {
+               super(exceptionHandler, "SortMerger spilling thread", 
dispatcher);
+               this.memManager = checkNotNull(memManager);
+               this.ioManager = checkNotNull(ioManager);
+               this.serializer = checkNotNull(serializer);
+               this.comparator = checkNotNull(comparator);
+               this.mergeReadMemory = checkNotNull(sortReadMemory);
+               this.writeMemory = checkNotNull(writeMemory);
+               this.maxFanIn = maxNumFileHandles;
+               this.spillChannelManager = checkNotNull(spillingChannelManager);
+               this.largeRecordHandler = largeRecordHandler;
+               this.spillingBehaviour = checkNotNull(spillingBehaviour);
+               this.minNumWriteBuffers = minNumWriteBuffers;
+               this.maxNumWriteBuffers = maxNumWriteBuffers;
+       }
+
+       /**
+        * Entry point of the thread.
+        */
+       public void go() throws IOException, InterruptedException {
+
+               final Queue<CircularElement<E>> cache = new ArrayDeque<>();
+               boolean cacheOnly = readCache(cache);
+
+               // check whether the thread was canceled
+               if (!isRunning()) {
+                       return;
+               }
+
+               MutableObjectIterator<E> largeRecords = null;
+
+               // check if we can stay in memory with the large record handler
+               if (cacheOnly && largeRecordHandler != null && 
largeRecordHandler.hasData()) {
+                       List<MemorySegment> memoryForLargeRecordSorting = new 
ArrayList<>();
+
+                       CircularElement<E> circElement;
+                       while ((circElement = 
this.dispatcher.poll(SortStage.READ)) != null) {
+                               circElement.getBuffer().dispose();
+                               
memoryForLargeRecordSorting.addAll(circElement.getMemory());
+                       }
+
+                       if (memoryForLargeRecordSorting.isEmpty()) {
+                               cacheOnly = false;
+                               LOG.debug("Going to disk-based merge because of 
large records.");
+                       } else {
+                               LOG.debug("Sorting large records, to add them 
to in-memory merge.");
+                               largeRecords = 
largeRecordHandler.finishWriteAndSortKeys(memoryForLargeRecordSorting);
+                       }
+               }
+
+               // ------------------- In-Memory Merge ------------------------
+               if (cacheOnly) {
+                       mergeInMemory(cache, largeRecords);
+                       return;
+               }
+
+               // ------------------- Spilling Phase ------------------------
+               List<ChannelWithBlockCount> channelIDs = startSpilling(cache);
+
+               // ------------------- Merging Phase ------------------------
+
+               mergeOnDisk(channelIDs);
+       }
+
+       @Override
+       public void close() throws InterruptedException {
+               super.close();
+               if (spillingBehaviourOpened) {
+                       this.spillingBehaviour.close();
+                       this.spillingBehaviourOpened = false;
+               }
+       }
+
+       private boolean readCache(Queue<CircularElement<E>> cache) throws 
InterruptedException {
+               CircularElement<E> element;
+
+               // ------------------- In-Memory Cache ------------------------
+               // fill cache
+               while (isRunning()) {
+                       // take next element from queue
+                       element = this.dispatcher.take(SortStage.SPILL);
+
+                       if (element == SPILLING_MARKER) {
+                               return false;
+                       } else if (element == EOF_MARKER) {
+                               return true;
+                       }
+                       cache.add(element);
+               }
+               return false;
+       }
+
+       private void mergeOnDisk(List<ChannelWithBlockCount> channelIDs) throws 
IOException {
+               // make sure we have enough memory to merge and for large 
record handling
+               List<MemorySegment> mergeReadMemory;
+               MutableObjectIterator<E> largeRecords = null;
+
+               if (largeRecordHandler != null && largeRecordHandler.hasData()) 
{
+
+                       List<MemorySegment> longRecMem;
+                       if (channelIDs.isEmpty()) {
+                               // only long records
+                               longRecMem = this.mergeReadMemory;
+                               mergeReadMemory = Collections.emptyList();
+                       }
+                       else {
+                               int maxMergedStreams = Math.min(this.maxFanIn, 
channelIDs.size());
+
+                               int pagesPerStream = Math.max(
+                                               minNumWriteBuffers,
+                                               Math.min(maxNumWriteBuffers, 
this.mergeReadMemory.size() / 2 / maxMergedStreams));
+
+                               int totalMergeReadMemory = maxMergedStreams * 
pagesPerStream;
+
+                               // grab the merge memory
+                               mergeReadMemory = new 
ArrayList<>(totalMergeReadMemory);
+                               for (int i = 0; i < totalMergeReadMemory; i++) {
+                                       
mergeReadMemory.add(this.mergeReadMemory.get(i));
+                               }
+
+                               // the remainder of the memory goes to the long 
record sorter
+                               longRecMem = new ArrayList<>();
+                               for (int i = totalMergeReadMemory; i < 
this.mergeReadMemory.size(); i++) {
+                                       
longRecMem.add(this.mergeReadMemory.get(i));
+                               }
+                       }
+
+                       LOG.debug("Sorting keys for large records.");
+                       largeRecords = 
largeRecordHandler.finishWriteAndSortKeys(longRecMem);
+               }
+               else {
+                       mergeReadMemory = this.mergeReadMemory;
+               }
+
+               // merge channels until sufficient file handles are available
+               while (isRunning() && channelIDs.size() > this.maxFanIn) {
+                       channelIDs = mergeChannelList(channelIDs, 
mergeReadMemory, this.writeMemory);
+               }
+
+               // from here on, we won't write again
+               this.memManager.release(this.writeMemory);
+               this.writeMemory.clear();
+
+               // check if we have spilled some data at all
+               if (channelIDs.isEmpty()) {
+                       if (largeRecords == null) {
+                               
this.dispatcher.sendResult(EmptyMutableObjectIterator.get());
+                       } else {
+                               this.dispatcher.sendResult(largeRecords);
+                       }
+               }
+               else {
+                       LOG.debug("Beginning final merge.");
+
+                       // allocate the memory for the final merging step
+                       List<List<MemorySegment>> readBuffers = new 
ArrayList<>(channelIDs.size());
+
+                       // allocate the read memory and register it to be 
released
+                       getSegmentsForReaders(readBuffers, mergeReadMemory, 
channelIDs.size());
+
+                       // get the readers and register them to be released
+                       
this.dispatcher.sendResult(getMergingIterator(channelIDs, readBuffers,
+                               new ArrayList<>(channelIDs.size()), 
largeRecords));
+               }
+
+               // done
+               LOG.debug("Spilling and merging thread done.");
+       }
+
+       private void mergeInMemory(
+                       Queue<CircularElement<E>> cache,
+                       MutableObjectIterator<E> largeRecords) throws 
IOException {
+               // operates on in-memory buffers only
+               LOG.debug("Initiating in memory merge.");
+
+               List<MutableObjectIterator<E>> iterators = new 
ArrayList<>(cache.size() + 1);
+
+               // iterate buffers and collect a set of iterators
+               for (CircularElement<E> cached : cache) {
+                       // note: the yielded iterator only operates on the 
buffer heap (and disregards the stack)
+                       iterators.add(cached.getBuffer().getIterator());
+               }
+
+               if (largeRecords != null) {
+                       iterators.add(largeRecords);
+               }
+
+               // release the remaining sort-buffers
+               LOG.debug("Releasing unused sort-buffer memory.");
+               disposeSortBuffers(true);
+
+               // set lazy iterator
+               this.dispatcher.sendResult(iterators.isEmpty() ? 
EmptyMutableObjectIterator.get() :
+                       iterators.size() == 1 ? iterators.get(0) :
+                               new MergeIterator<>(iterators, 
this.comparator));
+       }
+
+       private List<ChannelWithBlockCount> 
startSpilling(Queue<CircularElement<E>> cache) throws IOException, 
InterruptedException {
+               final FileIOChannel.Enumerator enumerator = 
this.ioManager.createChannelEnumerator();
+               List<ChannelWithBlockCount> channelIDs = new ArrayList<>();
+
+               // loop as long as the thread is marked alive and we do not see 
the final element
+               CircularElement<E> element;

Review comment:
       This can go in the `while()` loop now and become `final`.

##########
File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/SpillingThread.java
##########
@@ -0,0 +1,575 @@
+/*
+ * 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.flink.runtime.operators.sort;
+
+import org.apache.flink.api.common.typeutils.TypeComparator;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.core.memory.MemorySegment;
+import org.apache.flink.runtime.io.disk.ChannelReaderInputViewIterator;
+import org.apache.flink.runtime.io.disk.iomanager.BlockChannelReader;
+import org.apache.flink.runtime.io.disk.iomanager.BlockChannelWriter;
+import org.apache.flink.runtime.io.disk.iomanager.ChannelReaderInputView;
+import org.apache.flink.runtime.io.disk.iomanager.ChannelWriterOutputView;
+import org.apache.flink.runtime.io.disk.iomanager.FileIOChannel;
+import org.apache.flink.runtime.io.disk.iomanager.IOManager;
+import org.apache.flink.runtime.memory.MemoryManager;
+import org.apache.flink.runtime.util.EmptyMutableObjectIterator;
+import org.apache.flink.util.MutableObjectIterator;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Queue;
+
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.EOF_MARKER;
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.SPILLING_MARKER;
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * The thread that handles the spilling of intermediate results and sets up 
the merging. It also merges the
+ * channels until sufficiently few channels remain to perform the final 
streamed merge.
+ */
+final class SpillingThread<E> extends ThreadBase<E> {
+       /**
+        * An interface for injecting custom behaviour for spilling and merging 
phases.
+        */
+       interface SpillingBehaviour<E> {
+               default void open() {}
+
+               default void close() {}
+
+               /**
+                * A method that allows adjusting the spilling phase. We can 
inject e.g. combining the elements while spilling.
+                */
+               void spillBuffer(
+                       CircularElement<E> element,
+                       ChannelWriterOutputView output,
+                       LargeRecordHandler<E> largeRecordHandler) throws 
IOException;
+
+               /**
+                * A method that allows adjusting the merging phase. We can 
inject e.g. combining the spilled elements.
+                */
+               void mergeRecords(MergeIterator<E> mergeIterator, 
ChannelWriterOutputView output) throws IOException;
+       }
+
+       /** Logging. */
+       private static final Logger LOG = 
LoggerFactory.getLogger(SpillingThread.class);
+
+       private final MemoryManager memManager;                 // memory 
manager to release memory
+
+       private final IOManager ioManager;                              // I/O 
manager to create channels
+
+       private final TypeSerializer<E> serializer;             // The 
serializer for the data type
+
+       private final TypeComparator<E> comparator;             // The 
comparator that establishes the order relation.
+
+       private final List<MemorySegment> writeMemory;  // memory segments for 
writing
+
+       private final List<MemorySegment> mergeReadMemory;      // memory 
segments for sorting/reading
+
+       private final int maxFanIn;
+
+       private final SpillChannelManager spillChannelManager;
+
+       private final LargeRecordHandler<E> largeRecordHandler;
+
+       private final SpillingBehaviour<E> spillingBehaviour;
+
+       private boolean spillingBehaviourOpened = false;
+
+       private final int minNumWriteBuffers;
+
+       private final int maxNumWriteBuffers;
+
+       SpillingThread(
+                       @Nullable ExceptionHandler<IOException> 
exceptionHandler,
+                       StageMessageDispatcher<E> dispatcher,
+                       MemoryManager memManager,
+                       IOManager ioManager,
+                       TypeSerializer<E> serializer,
+                       TypeComparator<E> comparator,
+                       List<MemorySegment> sortReadMemory,
+                       List<MemorySegment> writeMemory,
+                       int maxNumFileHandles,
+                       SpillChannelManager spillingChannelManager,
+                       @Nullable LargeRecordHandler<E> largeRecordHandler,
+                       SpillingBehaviour<E> spillingBehaviour,
+                       int minNumWriteBuffers,
+                       int maxNumWriteBuffers) {
+               super(exceptionHandler, "SortMerger spilling thread", 
dispatcher);
+               this.memManager = checkNotNull(memManager);
+               this.ioManager = checkNotNull(ioManager);
+               this.serializer = checkNotNull(serializer);
+               this.comparator = checkNotNull(comparator);
+               this.mergeReadMemory = checkNotNull(sortReadMemory);
+               this.writeMemory = checkNotNull(writeMemory);
+               this.maxFanIn = maxNumFileHandles;
+               this.spillChannelManager = checkNotNull(spillingChannelManager);
+               this.largeRecordHandler = largeRecordHandler;
+               this.spillingBehaviour = checkNotNull(spillingBehaviour);
+               this.minNumWriteBuffers = minNumWriteBuffers;
+               this.maxNumWriteBuffers = maxNumWriteBuffers;
+       }
+
+       /**
+        * Entry point of the thread.
+        */

Review comment:
       Probably add the `@Override`.

##########
File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/SortingThread.java
##########
@@ -0,0 +1,85 @@
+/*
+ * 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.flink.runtime.operators.sort;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.EOF_MARKER;
+import static 
org.apache.flink.runtime.operators.sort.CircularElement.SPILLING_MARKER;
+
+/**
+ * The thread that sorts filled buffers.
+ */
+class SortingThread<E> extends ThreadBase<E> {
+
+       /** Logging. */
+       private static final Logger LOG = 
LoggerFactory.getLogger(SortingThread.class);
+
+       private final IndexedSorter sorter;
+
+       /**
+        * Creates a new sorting thread.
+        *
+        * @param exceptionHandler The exception handler to call for all 
exceptions.
+        * @param dispatcher The queues used to pass buffers between the 
threads.
+        */
+       public SortingThread(
+                       @Nullable ExceptionHandler<IOException> 
exceptionHandler,
+                       StageMessageDispatcher<E> dispatcher) {
+               super(exceptionHandler, "SortMerger sorting thread", 
dispatcher);
+
+               // members
+               this.sorter = new QuickSort();
+       }
+
+       /**
+        * Entry point of the thread.
+        */

Review comment:
       I would also add the `@Override` annotation.

##########
File path: 
flink-runtime/src/main/java/org/apache/flink/runtime/operators/sort/ReadingThread.java
##########
@@ -0,0 +1,75 @@
+/*
+ * 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.flink.runtime.operators.sort;
+
+import org.apache.flink.util.MutableObjectIterator;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * The thread that consumes the input data and puts it into a buffer that will 
be sorted.
+ */
+final class ReadingThread<E> extends ThreadBase<E> {
+
+       /** The input channels to read from. */
+       private final MutableObjectIterator<E> reader;
+
+       /** The object into which the thread reads the data from the input. */
+       private final E readTarget;
+
+       private final SorterInputGateway<E> sorterGateway;
+
+       /**
+        * Creates a new reading thread.
+        *
+        * @param exceptionHandler The exception handler to call for all 
exceptions.
+        * @param reader The reader to pull the data from.
+        * @param dispatcher The queues used to pass buffers between the 
threads.
+        */
+       ReadingThread(
+                       @Nullable ExceptionHandler<IOException> 
exceptionHandler,
+                       MutableObjectIterator<E> reader,
+                       StageMessageDispatcher<E> dispatcher,
+                       @Nullable LargeRecordHandler<E> largeRecordsHandler,
+                       @Nullable E readTarget,
+                       long startSpillingBytes) {
+               super(exceptionHandler, "SortMerger Reading Thread", 
dispatcher);
+
+               // members
+               this.sorterGateway = new SorterInputGateway<>(dispatcher, 
largeRecordsHandler, startSpillingBytes);
+               this.reader = checkNotNull(reader);
+               this.readTarget = readTarget;
+       }
+

Review comment:
       Probably add `@Override`.




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