jelmini commented on code in PR #692:
URL: https://github.com/apache/jackrabbit-oak/pull/692#discussion_r994290034


##########
oak-segment-aws/src/main/java/org/apache/jackrabbit/oak/segment/aws/tool/AwsCompact.java:
##########
@@ -128,6 +133,27 @@ public Builder withGCLogInterval(long gcLogInterval) {
             return this;
         }
 
+        /**
+         * The compactor type to be used by compaction. If not specified it 
defaults to
+         * "parallel" compactor

Review Comment:
   Should default to `diff`.



##########
oak-segment-aws/src/main/java/org/apache/jackrabbit/oak/segment/aws/tool/AwsCompact.java:
##########
@@ -72,6 +73,10 @@ public static class Builder {
 
         private int segmentCacheSize = DEFAULT_SEGMENT_CACHE_MB;
 
+        private CompactorType compactorType = CompactorType.PARALLEL_COMPACTOR;

Review Comment:
   Should default to `CompactorType.CHECKPOINT_COMPACTOR`



##########
oak-segment-tar/pom.xml:
##########
@@ -411,8 +411,6 @@
             <groupId>org.apache.jackrabbit</groupId>
             <artifactId>oak-core</artifactId>
             <version>${project.version}</version>
-            <classifier>tests</classifier>

Review Comment:
   I don't think we should bring `oak-core` as a compile-time dependency. There 
was an effort to depend only on `oak-core-spi` and `oak-api`, related to [this 
Oak issue](https://issues.apache.org/jira/browse/OAK-6073) about allowing for 
independent releases.
   As the objective of this dependency is only to bring a single class, 
`ApproximateCounter`, I would consider 2 options:
   * Copy `ApproximateCounter` to `oak-store-spi`, modify all code to use it 
instead and deprecate the one in `oak-core`. `oak-store-spi` is the module 
where this class has the least dependencies to other modules. Moreover, it's 
depended upon by both `oak-core` and `oak-segment-tar`.
   * If the above is too complex, simply copy `ApproximateCounter` to 
`oak-segment-tar`.



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/RecordCache.java:
##########
@@ -19,29 +19,21 @@
 
 package org.apache.jackrabbit.oak.segment;
 
-import static com.google.common.base.Preconditions.checkNotNull;
-
-import java.util.LinkedHashMap;
-import java.util.Map;
-
 import com.google.common.base.Supplier;
-import com.google.common.cache.CacheStats;
-import com.google.common.cache.Weigher;
-
+import com.google.common.cache.*;

Review Comment:
   [Code Style] We should not use * imports



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/memory/MemoryStore.java:
##########
@@ -72,7 +62,9 @@ public SegmentId newSegmentId(long msb, long lsb) {
         });
         this.revisions = new MemoryStoreRevisions();
         this.segmentReader = new CachingSegmentReader(this::getWriter, null, 
16, 2, NoopStats.INSTANCE);
-        this.segmentWriter = 
defaultSegmentWriterBuilder("sys").withWriterPool().build(this);
+        this.segmentWriter = defaultSegmentWriterBuilder("sys")

Review Comment:
   Given my other comment about keeping `withWriterPool()`, I suggest reverting 
this changes.



##########
oak-segment-azure/src/main/java/org/apache/jackrabbit/oak/segment/azure/tool/AzureCompact.java:
##########
@@ -79,7 +79,9 @@ public static class Builder {
 
         private int segmentCacheSize = 2048;
 
-        private CompactorType compactorType = 
CompactorType.CHECKPOINT_COMPACTOR;
+        private CompactorType compactorType = CompactorType.PARALLEL_COMPACTOR;

Review Comment:
   Should default to CompactorType.CHECKPOINT_COMPACTOR



##########
oak-run/src/main/java/org/apache/jackrabbit/oak/run/CompactCommand.java:
##########
@@ -56,11 +54,16 @@ public void execute(String... args) throws Exception {
                 .withOptionalArg()
                 .ofType(Boolean.class);
         OptionSpec<String> compactor = parser.accepts("compactor",
-                "Allow the user to control compactor type to be used. Valid 
choices are \"classic\" and \"diff\". " +
-                        "While the former is slower, it might be more stable, 
due to lack of optimisations employed " +
-                        "by the \"diff\" compactor which compacts the 
checkpoints on top of each other. If not " +
-                        "specified, \"diff\" compactor is used.")
+                "Allow the user to control compactor type to be used. Valid 
choices are \"classic\", \"diff\", \"parallel\". " +
+                        "While \"classic\" is slower, it might be more stable, 
due to lack of optimisations employed " +
+                        "by the \"diff\" compactor which compacts the 
checkpoints on top of each other and \"parallel\" compactor, which splits " +
+                        "the repository into smaller parts and compacts them 
concurrently. If not specified, \"parallel\" compactor is used.")

Review Comment:
   See the other comment about keeping `diff` as default compactor.



##########
oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/AbstractCompactorExternalBlobTest.java:
##########
@@ -0,0 +1,142 @@
+/*
+ * 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.jackrabbit.oak.segment;
+
+import static java.util.concurrent.TimeUnit.DAYS;
+import static 
org.apache.jackrabbit.oak.plugins.memory.EmptyNodeState.EMPTY_NODE;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.addTestContent;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.assertSameRecord;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.assertSameStableId;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.checkGeneration;
+import static org.apache.jackrabbit.oak.segment.CompactorTestUtils.createBlob;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.getCheckpoint;
+import static 
org.apache.jackrabbit.oak.segment.file.FileStoreBuilder.fileStoreBuilder;
+import static 
org.apache.jackrabbit.oak.segment.file.tar.GCGeneration.newGCGeneration;
+import static org.junit.Assert.*;

Review Comment:
   [Code style] We should not use * imports.



##########
oak-segment-aws/src/main/java/org/apache/jackrabbit/oak/segment/aws/tool/AwsToolUtils.java:
##########
@@ -69,14 +70,26 @@ public String description(String pathOrUri) {
     }
 
     public static FileStore newFileStore(SegmentNodeStorePersistence 
persistence, File directory,
-            boolean strictVersionCheck, int segmentCacheSize, long 
gcLogInterval)
-            throws IOException, InvalidFileStoreVersionException, 
URISyntaxException {
+                                         boolean strictVersionCheck, int 
segmentCacheSize, long gcLogInterval)
+            throws IOException, InvalidFileStoreVersionException {
+        return newFileStore(persistence, directory, strictVersionCheck, 
segmentCacheSize,
+                gcLogInterval, CompactorType.PARALLEL_COMPACTOR, 1);

Review Comment:
   Should default to CompactorType.CHECKPOINT_COMPACTOR



##########
oak-segment-azure/src/main/java/org/apache/jackrabbit/oak/segment/azure/tool/AzureCompact.java:
##########
@@ -159,7 +161,7 @@ public Builder withGCLogInterval(long gcLogInterval) {
 
         /**
          * The compactor type to be used by compaction. If not specified it 
defaults to
-         * "diff" compactor
+         * "parallel" compactor

Review Comment:
   Should default to `diff`



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/DefaultSegmentWriterBuilder.java:
##########
@@ -29,10 +30,11 @@
 import org.apache.jackrabbit.oak.segment.file.tar.GCGeneration;
 import org.apache.jackrabbit.oak.segment.memory.MemoryStore;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * Builder for building {@link DefaultSegmentWriter} instances.
- * The returned instances are thread safe if {@link #withWriterPool()}
+ * The returned instances are thread safe if {@link #withWriterPool(PoolType)}
  * was specified and <em>not</em> thread sage if {@link #withoutWriterPool()}

Review Comment:
   Typo: thread sage -> thread safe



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/CheckpointCompactor.java:
##########
@@ -18,13 +18,15 @@
 
 package org.apache.jackrabbit.oak.segment;
 
+import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.collect.Lists.newArrayList;
 import static com.google.common.collect.Maps.newHashMap;
 import static com.google.common.collect.Maps.newLinkedHashMap;
 import static org.apache.jackrabbit.oak.commons.PathUtils.elements;
 import static org.apache.jackrabbit.oak.commons.PathUtils.getName;
 import static org.apache.jackrabbit.oak.commons.PathUtils.getParentPath;
 import static 
org.apache.jackrabbit.oak.plugins.memory.EmptyNodeState.EMPTY_NODE;
+import static 
org.apache.jackrabbit.oak.segment.ClassicCompactor.getStableIdBytes;

Review Comment:
   I appreciate the idea of removing duplication by depending on a single 
implementation of `getStableIdBytes()`. However, this method does not really 
belong to `ClassicCompactor`, nor any other `Compactor` implementation, and now 
there is an arbitrary dependency to `ClassicCompactor` here. I suggest to move 
this method to a new class `CompactorUtils` (under `util` package).



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/DefaultSegmentWriterBuilder.java:
##########
@@ -106,8 +108,8 @@ public DefaultSegmentWriterBuilder withGeneration(@NotNull 
GCGeneration generati
      * The returned instance is thread safe.
      */
     @NotNull
-    public DefaultSegmentWriterBuilder withWriterPool() {
-        this.pooled = true;
+    public DefaultSegmentWriterBuilder withWriterPool(PoolType writerType) {

Review Comment:
   This is changing the signature of a public method in a public class. If some 
third-party implemented a custom store using this class, then this change could 
break their code.
   I suggest adding an override with the previous signature, setting a default 
value:
   ```java
   public DefaultSegmentWriterBuilder withWriterPool() {
           this.poolType = PoolType.GLOBAL;
           return this;
   }
   ```



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/ParallelCompactor.java:
##########
@@ -0,0 +1,373 @@
+/*
+ * 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.jackrabbit.oak.segment;
+
+import org.apache.jackrabbit.oak.api.PropertyState;
+import org.apache.jackrabbit.oak.plugins.index.counter.ApproximateCounter;
+import org.apache.jackrabbit.oak.plugins.memory.MemoryNodeBuilder;
+import org.apache.jackrabbit.oak.segment.file.GCNodeWriteMonitor;
+import org.apache.jackrabbit.oak.segment.file.cancel.Canceller;
+import org.apache.jackrabbit.oak.spi.blob.BlobStore;
+import org.apache.jackrabbit.oak.spi.gc.GCMonitor;
+import org.apache.jackrabbit.oak.spi.state.NodeState;
+import org.apache.jackrabbit.oak.spi.state.NodeStateDiff;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.*;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+import static com.google.common.base.Preconditions.checkState;
+import static 
org.apache.jackrabbit.oak.plugins.memory.EmptyNodeState.EMPTY_NODE;
+import static 
org.apache.jackrabbit.oak.segment.ClassicCompactor.getStableIdBytes;
+
+/**
+ * This compactor implementation leverages the tree structure of the 
repository for concurrent compaction.
+ * It explores the tree breadth-first until the target node count is reached. 
Every node at this depth will be
+ * an entry point for asynchronous compaction. After the exploration phase, 
the main thread will collect
+ * these compaction results and write their parents' node state to disk.
+ */
+public class ParallelCompactor extends CheckpointCompactor {
+    /**
+     * Expand repository tree until there are this many nodes for each worker 
to compact. Tradeoff
+     * between low efficiency of many small tasks and high risk of at least 
one of the subtrees being
+     * significantly larger than totalSize / numWorkers (unequal work 
distribution).
+     */
+    private static final int MIN_NODES_PER_WORKER = 1000;
+
+    /**
+     * Stop expansion if tree size grows beyond this many nodes per worker at 
the latest.
+     */
+    private static final int MAX_NODES_PER_WORKER = 10_000;
+
+    private final int numWorkers;
+
+    private final long totalSizeEstimate;
+
+    /**
+     * Manages workers for asynchronous compaction.
+     */
+    @Nullable
+    private ExecutorService executorService;
+
+    /**
+     * Create a new instance based on the passed arguments.
+     * @param gcListener listener receiving notifications about the garbage 
collection process
+     * @param reader     segment reader used to read from the segments
+     * @param writer     segment writer used to serialise to segments
+     * @param blobStore  the blob store or {@code null} if none
+     * @param compactionMonitor   notification call back for each compacted 
nodes, properties, and binaries
+     * @param nThreads   number of threads to use for parallel compaction,
+     *                   negative numbers are interpreted relative to the 
number of available processors
+     */
+    public ParallelCompactor(
+            @NotNull GCMonitor gcListener,
+            @NotNull SegmentReader reader,
+            @NotNull SegmentWriter writer,
+            @Nullable BlobStore blobStore,
+            @NotNull GCNodeWriteMonitor compactionMonitor,
+            int nThreads) {
+        super(gcListener, reader, writer, blobStore, compactionMonitor);
+
+        int availableProcessors = Runtime.getRuntime().availableProcessors();
+        if (nThreads < 0) {
+            nThreads += availableProcessors + 1;
+        }
+        numWorkers = Math.max(0, nThreads - 1);

Review Comment:
   Not sure I understand the logic to determine the number of workers. 
   First, I think the `+=` is a mistake and should actually be a simple `=`. 
Otherwise, assuming `availableProcessors=8`, if someone passes for example 
`nThreads=-4` in the constructor, then `nThreads` would be set to `-4 + 8 + 1 = 
5` which would be surprising. 
   Second, we seem to support `numWorkers = 0`. What's the behaviour in this 
case? Does it degrade to the same behaviour than a single-threaded 
`CheckpointCompactor`?



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/ParallelCompactor.java:
##########
@@ -0,0 +1,373 @@
+/*
+ * 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.jackrabbit.oak.segment;
+
+import org.apache.jackrabbit.oak.api.PropertyState;
+import org.apache.jackrabbit.oak.plugins.index.counter.ApproximateCounter;
+import org.apache.jackrabbit.oak.plugins.memory.MemoryNodeBuilder;
+import org.apache.jackrabbit.oak.segment.file.GCNodeWriteMonitor;
+import org.apache.jackrabbit.oak.segment.file.cancel.Canceller;
+import org.apache.jackrabbit.oak.spi.blob.BlobStore;
+import org.apache.jackrabbit.oak.spi.gc.GCMonitor;
+import org.apache.jackrabbit.oak.spi.state.NodeState;
+import org.apache.jackrabbit.oak.spi.state.NodeStateDiff;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.*;

Review Comment:
   [Code Style] We should not use * imports



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/FileStore.java:
##########
@@ -40,13 +40,7 @@
 import com.google.common.util.concurrent.UncheckedExecutionException;
 
 import org.apache.jackrabbit.oak.commons.Buffer;
-import org.apache.jackrabbit.oak.segment.RecordId;
-import org.apache.jackrabbit.oak.segment.Segment;
-import org.apache.jackrabbit.oak.segment.SegmentId;
-import org.apache.jackrabbit.oak.segment.SegmentNodeState;
-import org.apache.jackrabbit.oak.segment.SegmentNotFoundException;
-import org.apache.jackrabbit.oak.segment.SegmentNotFoundExceptionListener;
-import org.apache.jackrabbit.oak.segment.SegmentWriter;
+import org.apache.jackrabbit.oak.segment.*;

Review Comment:
   [Code Style] We should not use * imports



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/AbstractCompactionStrategy.java:
##########
@@ -29,12 +29,7 @@
 
 import com.google.common.base.Function;
 
-import org.apache.jackrabbit.oak.segment.CheckpointCompactor;
-import org.apache.jackrabbit.oak.segment.ClassicCompactor;
-import org.apache.jackrabbit.oak.segment.Compactor;
-import org.apache.jackrabbit.oak.segment.RecordId;
-import org.apache.jackrabbit.oak.segment.SegmentNodeState;
-import org.apache.jackrabbit.oak.segment.SegmentWriter;
+import org.apache.jackrabbit.oak.segment.*;

Review Comment:
   [Code Style] We should not use * imports



##########
oak-doc/src/site/markdown/nodestore/segment/overview.md:
##########
@@ -807,24 +823,25 @@ This option is optional and is disabled by default.
 ### <a name="compact"/> Compact
 
 ```
-java -jar oak-run.jar compact [--force] [--mmap] [--compactor] SOURCE 
[--target-path DESTINATION] [--persistent-cache-path PERSISTENT_CACHE_PATH] 
[--persistent-cache-size-gb <PERSISTENT_CACHE_SIZE_GB>]
+java -jar oak-run.jar compact [--force] [--mmap] [--compactor] [--threads] 
SOURCE [--target-path DESTINATION] [--persistent-cache-path 
PERSISTENT_CACHE_PATH] [--persistent-cache-size-gb <PERSISTENT_CACHE_SIZE_GB>]
 ```
 
 The `compact` command performs offline compaction of the local/remote Segment 
Store at `SOURCE`. 
 `SOURCE` must be a valid path/uri to an existing Segment Store. Currently, 
Azure Segment Store and AWS Segment Store the supported remote Segment Stores. 
 Please refer to the [Remote Segment Stores](#remote-segment-stores) section 
for details on how to correctly specify connection URIs.
 
-If the optional `--force [Boolean]` argument is set to `true` the tool ignores 
a non 
-matching Segment Store version. *CAUTION*: this will upgrade the Segment Store 
to the 
+If the optional `--force [Boolean]` argument is set to `true` the tool ignores 
a non-matching Segment Store version. *CAUTION*: this will upgrade the Segment 
Store to the 
 latest version, which is incompatible with older versions. *There is no way to 
downgrade 
 an accidentally upgraded Segment Store*.  
 
 The optional `--mmap [Boolean]` argument can be used to control the file 
access mode. Set
 to `true` for memory mapped access and `false` for file access. If not 
specified, memory 
-mapped access is used on 64 bit systems and file access is used on 32 bit 
systems. On
+mapped access is used on 64-bit systems and file access is used on 32-bit 
systems. On
 Windows, regular file access is always enforced and this option is ignored.
 
-The optional `--compactor [String]` argument can be used to pick the compactor 
type to be used. Valid choices are *classic* and *diff*. While the former is 
slower, it might be more stable, due to lack of optimisations employed by the 
*diff* compactor which compacts the checkpoints on top of each other. If not 
specified, *diff* compactor is used.
+The optional `--compactor [String]` argument can be used to pick the compactor 
type to be used. Valid choices are *classic*, *diff* and *parallel*. While 
*classic* is slower, it might be more stable, due to lack of optimisations 
employed by the *diff* compactor which compacts the checkpoints on top of each 
other and the *parallel* compactor, which additionally divides the repository 
into multiple parts to process in parallel. If not specified, *parallel* 
compactor is used.

Review Comment:
   It says here that `if not specified, *parallel* compactor is used`. I think 
it's safer to keep the previous behaviour, which is to use the `diff` 
compactor. 
   In my opinion the new parallel compactor should be considered in preview 
mode at first. Once we have more experience with it in production (validating 
it on a limited number of customers) and we become certain of its robustness, 
we may make it the default compactor.



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/PriorityCache.java:
##########
@@ -156,13 +154,40 @@ public static long nextPowerOfTwo(int size) {
      * @param weigher   Needed to provide an estimation of the cache weight in 
memory
      */
     public PriorityCache(int size, int rehash, @NotNull Weigher<K, V> weigher) 
{
+        this(size, rehash, weigher, 1024);
+    }
+
+    /**
+     * Create a new instance of the given {@code size}. {@code rehash} 
specifies the number
+     * of rehashes to resolve a clash.
+     * @param size        Size of the cache. Must be a power of {@code 2}.
+     * @param rehash      Number of rehashes. Must be greater or equal to 
{@code 0} and
+     *                    smaller than {@code 32 - 
numberOfTrailingZeros(size)}.
+     * @param weigher     Needed to provide an estimation of the cache weight 
in memory
+     * @param numSegments Number of separately locked segments
+     */
+    public PriorityCache(int size, int rehash, @NotNull Weigher<K, V> weigher, 
int numSegments) {
         checkArgument(bitCount(size) == 1);
         checkArgument(rehash >= 0);
         checkArgument(rehash < 32 - numberOfTrailingZeros(size));
         this.rehash = rehash;
         entries = new Entry<?,?>[size];
         fill(entries, Entry.NULL);
         this.weigher = checkNotNull(weigher);
+
+        numSegments = Math.min(numSegments, size);
+        checkArgument((size % numSegments) == 0);

Review Comment:
   I suggest to add an error message explaining why this check failed and how 
to fix it, ie what are acceptables values of `numSegments` in relation to 
`size`. 
   The Javadoc for `numSegments` should also explain this.



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/FileStore.java:
##########
@@ -198,7 +192,7 @@ private static GarbageCollectionStrategy 
newGarbageCollectionStrategy() {
                 defaultSegmentWriterBuilder("c")
                     
.with(builder.getCacheManager().withAccessTracking("COMPACT", statsProvider))
                     .withGeneration(generation)
-                    .withoutWriterPool()
+                    
.withWriterPool(SegmentBufferWriterPool.PoolType.THREAD_SPECIFIC)

Review Comment:
   Why not keep using `withoutWriterPool()` here? 



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/PriorityCache.java:
##########
@@ -218,42 +259,45 @@ public synchronized boolean put(@NotNull K key, @NotNull 
V value, int generation
                     initialCost++;
                 }
                 eviction = false;
-                break;
             } else if (entry.generation < generation) {
                 // Old generation -> use this index
                 index = i;
                 eviction = false;
-                break;
             } else if (entry.cost < cheapest) {
                 // Candidate slot, keep on searching for even cheaper slots
                 cheapest = entry.cost;
                 index = i;
                 eviction = true;
+                if (k < rehash) {
+                    continue;
+                }
+            } else {

Review Comment:
   Is there a reason other than stylistic for the use of `continue` instead of 
`break` in this loop?
   I find the previous version using `break` more readable than this one.



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/PriorityCache.java:
##########
@@ -289,35 +338,42 @@ public synchronized V get(@NotNull K key, int generation) 
{
      * passed {@code purge} predicate.
      * @param purge
      */
-    public synchronized void purgeGenerations(@NotNull Predicate<Integer> 
purge) {
-        for (int i = 0; i < entries.length; i++) {
-            Entry<?, ?> entry = entries[i];
-            if (entry != Entry.NULL && purge.apply(entry.generation)) {
-                entries[i] = Entry.NULL;
-                size--;
-                weight -= weighEntry(entry);
+    public void purgeGenerations(@NotNull Predicate<Integer> purge) {
+        int numSegments = segments.length;
+        int entriesPerSegment = entries.length / numSegments;
+        for (int s = 0; s < numSegments; s++) {
+            segments[s].lock();
+            for (int i = 0; i < entriesPerSegment; i++) {
+                int j = i + s * entriesPerSegment;
+                Entry<?, ?> entry = entries[j];
+                if (entry != Entry.NULL && purge.apply(entry.generation)) {
+                    entries[j] = Entry.NULL;
+                    size.decrementAndGet();
+                    weight.addAndGet(-weighEntry(entry));
+                }
             }
+            segments[s].unlock();

Review Comment:
   In case of exception, `unlock()` will not be called. It's safer to call it 
in a `finally` clause. The `try` block should start just after `lock()` is 
called.



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/PriorityCache.java:
##########
@@ -218,42 +259,45 @@ public synchronized boolean put(@NotNull K key, @NotNull 
V value, int generation
                     initialCost++;
                 }
                 eviction = false;
-                break;
             } else if (entry.generation < generation) {
                 // Old generation -> use this index
                 index = i;
                 eviction = false;
-                break;
             } else if (entry.cost < cheapest) {
                 // Candidate slot, keep on searching for even cheaper slots
                 cheapest = entry.cost;
                 index = i;
                 eviction = true;
+                if (k < rehash) {
+                    continue;
+                }
+            } else {
+                continue;
             }
-        }
 
-        if (index >= 0) {
             Entry<?, ?> old = entries[index];
             Entry<?, ?> newE = new Entry<>(key, value, generation, 
initialCost);
             entries[index] = newE;
-            loadCount++;
-            costs[initialCost - Byte.MIN_VALUE]++;
+            loadCount.incrementAndGet();
+            costs[initialCost - Byte.MIN_VALUE].incrementAndGet();
             if (old != Entry.NULL) {
-                costs[old.cost - Byte.MIN_VALUE]--;
+                costs[old.cost - Byte.MIN_VALUE].decrementAndGet();
                 if (eviction) {
-                    evictions[old.cost - Byte.MIN_VALUE]++;
-                    evictionCount++;
+                    evictions[old.cost - Byte.MIN_VALUE].incrementAndGet();
+                    evictionCount.incrementAndGet();
                 }
-                weight -= weighEntry(old);
+                weight.addAndGet(-weighEntry(old)) ;
             } else {
-                size++;
+                size.incrementAndGet();
             }
-            weight += weighEntry(newE);
+            weight.addAndGet(weighEntry(newE));
+            lockedSegment.unlock();
             return true;
-        } else {
-            loadExceptionCount++;
-            return false;
         }
+
+        checkNotNull(lockedSegment).unlock();

Review Comment:
   In case of exception, `unlock()` will not be called. It's safer to call it 
in a `finally` clause. The `try` block should start just after `lockedSegment` 
is defined. The `unlock()` call at line 294 can then be removed.



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/tool/Compact.java:
##########
@@ -164,7 +166,7 @@ public Builder withGCLogInterval(long gcLogInterval) {
 
         /**
          * The compactor type to be used by compaction. If not specified it 
defaults to
-         * "diff" compactor
+         * "parallel" compactor

Review Comment:
   Should default to `diff`.



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/memory/MemoryStore.java:
##########
@@ -27,17 +27,7 @@
 import com.google.common.collect.Maps;
 
 import org.apache.jackrabbit.oak.commons.Buffer;
-import org.apache.jackrabbit.oak.segment.CachingSegmentReader;
-import org.apache.jackrabbit.oak.segment.Revisions;
-import org.apache.jackrabbit.oak.segment.Segment;
-import org.apache.jackrabbit.oak.segment.SegmentId;
-import org.apache.jackrabbit.oak.segment.SegmentIdFactory;
-import org.apache.jackrabbit.oak.segment.SegmentIdProvider;
-import org.apache.jackrabbit.oak.segment.SegmentNotFoundException;
-import org.apache.jackrabbit.oak.segment.SegmentReader;
-import org.apache.jackrabbit.oak.segment.SegmentStore;
-import org.apache.jackrabbit.oak.segment.SegmentTracker;
-import org.apache.jackrabbit.oak.segment.SegmentWriter;
+import org.apache.jackrabbit.oak.segment.*;

Review Comment:
   [Code Style] We should not use * imports



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/PriorityCache.java:
##########
@@ -265,22 +309,27 @@ public synchronized boolean put(@NotNull K key, @NotNull 
V value, int generation
      */
     @SuppressWarnings("unchecked")
     @Nullable
-    public synchronized V get(@NotNull K key, int generation) {
+    public V get(@NotNull K key, int generation) {
         int hashCode = key.hashCode();
         for (int k = 0; k <= rehash; k++) {
             int i = project(hashCode, k);
+            Segment segment = getSegment(i);
+            segment.lock();
             Entry<?, ?> entry = entries[i];
             if (generation == entry.generation && key.equals(entry.key)) {
                 if (entry.cost < Byte.MAX_VALUE) {
-                    costs[entry.cost - Byte.MIN_VALUE]--;
+                    costs[entry.cost - Byte.MIN_VALUE].decrementAndGet();
                     entry.cost++;
-                    costs[entry.cost - Byte.MIN_VALUE]++;
+                    costs[entry.cost - Byte.MIN_VALUE].incrementAndGet();
                 }
-                hitCount++;
-                return (V) entry.value;
+                hitCount.incrementAndGet();
+                V value = (V) entry.value;
+                segment.unlock();
+                return value;
             }
+            segment.unlock();

Review Comment:
   In case of exception, `unlock()` will not be called. It's safer to call it 
in a `finally` clause. The `try` block should start just after `lock()` is 
called. The call to `unlock()` at line 327 can be removed.



##########
oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/RecordCacheStatsTest.java:
##########
@@ -37,18 +37,9 @@ public class RecordCacheStatsTest {
     private final Random rnd = new Random();
     private final MemoryStore store = new MemoryStore();
 
-    private final RecordCache<Integer> cache = newRecordCache(KEYS);
+    private final RecordCache<Integer> cache = newRecordCache(KEYS * 4 / 3);

Review Comment:
   I've noticed that if I remove your change and initialise the `RecordCache` 
with only KEYS (=100) values like before, then some tests fail. This means the 
behaviour of `RecordCache` is not preserved. This could break other code 
relying on the previous behaviour. 
   Can you explain why the stats are not the same? Can you restore the previous 
behaviour?
   



##########
oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/AbstractCompactorTest.java:
##########
@@ -0,0 +1,118 @@
+/*
+ * 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.jackrabbit.oak.segment;
+
+import static java.util.concurrent.TimeUnit.DAYS;
+import static 
org.apache.jackrabbit.oak.plugins.memory.EmptyNodeState.EMPTY_NODE;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.addTestContent;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.assertSameRecord;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.assertSameStableId;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.checkGeneration;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.getCheckpoint;
+import static 
org.apache.jackrabbit.oak.segment.file.FileStoreBuilder.fileStoreBuilder;
+import static 
org.apache.jackrabbit.oak.segment.file.tar.GCGeneration.newGCGeneration;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+import java.io.File;
+import java.io.IOException;
+
+import org.apache.jackrabbit.oak.segment.file.FileStore;
+import org.apache.jackrabbit.oak.segment.file.InvalidFileStoreVersionException;
+import org.apache.jackrabbit.oak.segment.file.cancel.Canceller;
+import org.apache.jackrabbit.oak.segment.file.tar.GCGeneration;
+import org.jetbrains.annotations.NotNull;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+public abstract class AbstractCompactorTest {
+    @Rule
+    public TemporaryFolder folder = new TemporaryFolder(new File("target"));
+
+    private FileStore fileStore;
+
+    private SegmentNodeStore nodeStore;
+
+    private Compactor compactor;
+
+    private GCGeneration compactedGeneration;
+
+    @Before
+    public void setup() throws IOException, InvalidFileStoreVersionException {
+        fileStore = fileStoreBuilder(folder.getRoot()).build();
+        nodeStore = SegmentNodeStoreBuilders.builder(fileStore).build();
+        compactedGeneration = newGCGeneration(1,1, true);
+        compactor = createCompactor(fileStore, compactedGeneration);
+    }
+
+    protected abstract Compactor createCompactor(@NotNull FileStore fileStore, 
@NotNull GCGeneration generation);
+
+    @After
+    public void tearDown() {
+        fileStore.close();
+    }
+
+    @Test
+    public void testCompact() throws Exception {
+        addTestContent("cp1", nodeStore, 42);
+        String cp1 = nodeStore.checkpoint(DAYS.toMillis(1));
+        addTestContent("cp2", nodeStore, 42);
+        String cp2 = nodeStore.checkpoint(DAYS.toMillis(1));
+
+        SegmentNodeState uncompacted1 = fileStore.getHead();
+        SegmentNodeState compacted1 = compactor.compact(EMPTY_NODE, 
uncompacted1, EMPTY_NODE, Canceller.newCanceller());
+        assertNotNull(compacted1);
+        assertFalse(uncompacted1 == compacted1);

Review Comment:
   Can be simplified to:
   ```java
   assertNotSame(uncompacted1, compacted1);
   ```



##########
oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/NodeRecordTest.java:
##########
@@ -105,7 +105,7 @@ public void baseNodeStateShouldBeReusedAcrossGenerations() 
throws Exception {
 
             SegmentWriter writer = defaultSegmentWriterBuilder("test")
                     .withGeneration(generation)
-                    .withWriterPool()
+                    .withWriterPool(SegmentBufferWriterPool.PoolType.GLOBAL)

Review Comment:
   Given my other comment about keeping `withWriterPool()`, I suggest reverting 
this changes.



##########
oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/FileStore.java:
##########
@@ -146,7 +140,7 @@ private static GarbageCollectionStrategy 
newGarbageCollectionStrategy() {
 
         this.segmentWriter = defaultSegmentWriterBuilder("sys")
                 .withGeneration(() -> getGcGeneration().nonGC())
-                .withWriterPool()
+                .withWriterPool(SegmentBufferWriterPool.PoolType.GLOBAL)

Review Comment:
   Given my other comment about keeping withWriterPool(), I suggest reverting 
this change.



##########
oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/AbstractCompactorTest.java:
##########
@@ -0,0 +1,118 @@
+/*
+ * 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.jackrabbit.oak.segment;
+
+import static java.util.concurrent.TimeUnit.DAYS;
+import static 
org.apache.jackrabbit.oak.plugins.memory.EmptyNodeState.EMPTY_NODE;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.addTestContent;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.assertSameRecord;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.assertSameStableId;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.checkGeneration;
+import static 
org.apache.jackrabbit.oak.segment.CompactorTestUtils.getCheckpoint;
+import static 
org.apache.jackrabbit.oak.segment.file.FileStoreBuilder.fileStoreBuilder;
+import static 
org.apache.jackrabbit.oak.segment.file.tar.GCGeneration.newGCGeneration;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+import java.io.File;
+import java.io.IOException;
+
+import org.apache.jackrabbit.oak.segment.file.FileStore;
+import org.apache.jackrabbit.oak.segment.file.InvalidFileStoreVersionException;
+import org.apache.jackrabbit.oak.segment.file.cancel.Canceller;
+import org.apache.jackrabbit.oak.segment.file.tar.GCGeneration;
+import org.jetbrains.annotations.NotNull;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+public abstract class AbstractCompactorTest {
+    @Rule
+    public TemporaryFolder folder = new TemporaryFolder(new File("target"));
+
+    private FileStore fileStore;
+
+    private SegmentNodeStore nodeStore;
+
+    private Compactor compactor;
+
+    private GCGeneration compactedGeneration;
+
+    @Before
+    public void setup() throws IOException, InvalidFileStoreVersionException {
+        fileStore = fileStoreBuilder(folder.getRoot()).build();
+        nodeStore = SegmentNodeStoreBuilders.builder(fileStore).build();
+        compactedGeneration = newGCGeneration(1,1, true);
+        compactor = createCompactor(fileStore, compactedGeneration);
+    }
+
+    protected abstract Compactor createCompactor(@NotNull FileStore fileStore, 
@NotNull GCGeneration generation);
+
+    @After
+    public void tearDown() {
+        fileStore.close();
+    }
+
+    @Test
+    public void testCompact() throws Exception {
+        addTestContent("cp1", nodeStore, 42);
+        String cp1 = nodeStore.checkpoint(DAYS.toMillis(1));
+        addTestContent("cp2", nodeStore, 42);
+        String cp2 = nodeStore.checkpoint(DAYS.toMillis(1));
+
+        SegmentNodeState uncompacted1 = fileStore.getHead();
+        SegmentNodeState compacted1 = compactor.compact(EMPTY_NODE, 
uncompacted1, EMPTY_NODE, Canceller.newCanceller());
+        assertNotNull(compacted1);
+        assertFalse(uncompacted1 == compacted1);
+        checkGeneration(compacted1, compactedGeneration);
+
+        assertSameStableId(uncompacted1, compacted1);
+        assertSameStableId(getCheckpoint(uncompacted1, cp1), 
getCheckpoint(compacted1, cp1));
+        assertSameStableId(getCheckpoint(uncompacted1, cp2), 
getCheckpoint(compacted1, cp2));
+        assertSameRecord(getCheckpoint(compacted1, cp2), 
compacted1.getChildNode("root"));
+
+        // Simulate a 2nd compaction cycle
+        addTestContent("cp3", nodeStore, 42);
+        String cp3 = nodeStore.checkpoint(DAYS.toMillis(1));
+        addTestContent("cp4", nodeStore, 42);
+        String cp4 = nodeStore.checkpoint(DAYS.toMillis(1));
+
+        SegmentNodeState uncompacted2 = fileStore.getHead();
+        SegmentNodeState compacted2 = compactor.compact(uncompacted1, 
uncompacted2, compacted1, Canceller.newCanceller());
+        assertNotNull(compacted2);
+        assertFalse(uncompacted2 == compacted2);

Review Comment:
   Can be simplified to:
   ```java
   assertNotSame(uncompacted2, compacted2);
   ```



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