Expose merge events and their memory usage estimate

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -2909,7 +2909,7 @@ private final class EngineThreadPoolMergeScheduler extends ThreadPoolMergeSchedu
             IndexSettings indexSettings,
             ThreadPoolMergeExecutorService threadPoolMergeExecutorService
         ) {
-            super(shardId, indexSettings, threadPoolMergeExecutorService);
+            super(shardId, indexSettings, threadPoolMergeExecutorService, InternalEngine.this::estimateMergeBytes);
         }
 
         @Override

server/src/main/java/org/elasticsearch/index/engine/MergeEventListener.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.index.engine;
+
+import org.elasticsearch.index.merge.OnGoingMerge;
+
+public interface MergeEventListener {
+
+    /**
+     *
+     * @param merge
+     * @param estimateMergeMemoryBytes estimate of the memory needed to perform a merge
+     */
+    void onMergeQueued(OnGoingMerge merge, long estimateMergeMemoryBytes);
+
+    void onMergeCompleted(OnGoingMerge merge);
+
+    void onMergeAborted(OnGoingMerge merge);
+}

server/src/main/java/org/elasticsearch/index/engine/MergeMemoryEstimateProvider.java

@@ -0,0 +1,21 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.index.engine;
+
+import org.apache.lucene.index.MergePolicy;
+
+@FunctionalInterface
+public interface MergeMemoryEstimateProvider {
+
+    /**
+     * Returns an estimate of the memory needed to perform a merge
+     */
+    long estimateMergeMemoryBytes(MergePolicy.OneMerge merge);
+}

server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorService.java

@@ -17,7 +17,9 @@
 import org.elasticsearch.threadpool.ThreadPool;
 
 import java.util.Comparator;
+import java.util.List;
 import java.util.Set;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.PriorityBlockingQueue;
 import java.util.concurrent.RejectedExecutionException;
@@ -73,6 +75,8 @@ public class ThreadPoolMergeExecutorService {
     private final int concurrentMergesFloorLimitForThrottling;
     private final int concurrentMergesCeilLimitForThrottling;
 
+    private final List<MergeEventListener> mergeEventListeners = new CopyOnWriteArrayList<>();
+
     public static @Nullable ThreadPoolMergeExecutorService maybeCreateThreadPoolMergeExecutorService(
         ThreadPool threadPool,
         Settings settings
@@ -127,13 +131,21 @@ boolean submitMergeTask(MergeTask mergeTask) {
                 );
             }
             // then enqueue the merge task proper
-            queuedMergeTasks.add(mergeTask);
+            enqueueMergeTask(mergeTask);
             return true;
         }
     }
 
     void reEnqueueBackloggedMergeTask(MergeTask mergeTask) {
-        queuedMergeTasks.add(mergeTask);
+        enqueueMergeTask(mergeTask);
+    }
+
+    private void enqueueMergeTask(MergeTask mergeTask) {
+        // To ensure that for a given merge onMergeQueued is called before onMergeAborted or onMergeCompleted, we call onMergeQueued
+        // before adding the merge task to the queue. Adding to the queue should not fail.
+        mergeEventListeners.forEach(l -> l.onMergeQueued(mergeTask.getOnGoingMerge(), mergeTask.getMergeMemoryEstimateBytes()));
+        boolean added = queuedMergeTasks.add(mergeTask);
+        assert added;
     }
 
     public boolean allDone() {
@@ -201,6 +213,7 @@ private void runMergeTask(MergeTask mergeTask) {
             if (mergeTask.supportsIOThrottling()) {
                 ioThrottledMergeTasksCount.decrementAndGet();
             }
+            mergeEventListeners.forEach(l -> l.onMergeCompleted(mergeTask.getOnGoingMerge()));
         }
     }
 
@@ -213,6 +226,7 @@ private void abortMergeTask(MergeTask mergeTask) {
             if (mergeTask.supportsIOThrottling()) {
                 ioThrottledMergeTasksCount.decrementAndGet();
             }
+            mergeEventListeners.forEach(l -> l.onMergeAborted(mergeTask.getOnGoingMerge()));
         }
     }
 
@@ -278,6 +292,10 @@ public boolean usingMaxTargetIORateBytesPerSec() {
         return MAX_IO_RATE.getBytes() == targetIORateBytesPerSec.get();
     }
 
+    public void registerMergeEventListener(MergeEventListener consumer) {
+        mergeEventListeners.add(consumer);
+    }
+
     // exposed for tests
     Set<MergeTask> getRunningMergeTasks() {
         return runningMergeTasks;

server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeScheduler.java

@@ -65,11 +65,13 @@ public class ThreadPoolMergeScheduler extends MergeScheduler implements Elastics
     private final AtomicLong doneMergeTaskCount = new AtomicLong();
     private final CountDownLatch closedWithNoRunningMerges = new CountDownLatch(1);
     private volatile boolean closed = false;
+    private final MergeMemoryEstimateProvider mergeMemoryEstimateProvider;
 
     public ThreadPoolMergeScheduler(
         ShardId shardId,
         IndexSettings indexSettings,
-        ThreadPoolMergeExecutorService threadPoolMergeExecutorService
+        ThreadPoolMergeExecutorService threadPoolMergeExecutorService,
+        MergeMemoryEstimateProvider mergeMemoryEstimateProvider
     ) {
         this.shardId = shardId;
         this.config = indexSettings.getMergeSchedulerConfig();
@@ -81,6 +83,7 @@ public ThreadPoolMergeScheduler(
                 : Double.POSITIVE_INFINITY
         );
         this.threadPoolMergeExecutorService = threadPoolMergeExecutorService;
+        this.mergeMemoryEstimateProvider = mergeMemoryEstimateProvider;
     }
 
     @Override
@@ -176,11 +179,13 @@ MergeTask newMergeTask(MergeSource mergeSource, MergePolicy.OneMerge merge, Merg
         // forced merges, as well as merges triggered when closing a shard, always run un-IO-throttled
         boolean isAutoThrottle = mergeTrigger != MergeTrigger.CLOSING && merge.getStoreMergeInfo().mergeMaxNumSegments() == -1;
         // IO throttling cannot be toggled for existing merge tasks, only new merge tasks pick up the updated IO throttling setting
+        long estimateMergeMemoryBytes = mergeMemoryEstimateProvider.estimateMergeMemoryBytes(merge);
         return new MergeTask(
             mergeSource,
             merge,
             isAutoThrottle && config.isAutoThrottle(),
-            "Lucene Merge Task #" + submittedMergeTaskCount.incrementAndGet() + " for shard " + shardId
+            "Lucene Merge Task #" + submittedMergeTaskCount.incrementAndGet() + " for shard " + shardId,
+            estimateMergeMemoryBytes
         );
     }
 
@@ -312,14 +317,22 @@ class MergeTask implements Runnable {
         private final OnGoingMerge onGoingMerge;
         private final MergeRateLimiter rateLimiter;
         private final boolean supportsIOThrottling;
-
-        MergeTask(MergeSource mergeSource, MergePolicy.OneMerge merge, boolean supportsIOThrottling, String name) {
+        private final long mergeMemoryEstimateBytes;
+
+        MergeTask(
+            MergeSource mergeSource,
+            MergePolicy.OneMerge merge,
+            boolean supportsIOThrottling,
+            String name,
+            long mergeMemoryEstimateBytes
+        ) {
             this.name = name;
             this.mergeStartTimeNS = new AtomicLong();
             this.mergeSource = mergeSource;
             this.onGoingMerge = new OnGoingMerge(merge);
             this.rateLimiter = new MergeRateLimiter(merge.getMergeProgress());
             this.supportsIOThrottling = supportsIOThrottling;
+            this.mergeMemoryEstimateBytes = mergeMemoryEstimateBytes;
         }
 
         Schedule schedule() {
@@ -449,6 +462,14 @@ long estimatedMergeSize() {
             return onGoingMerge.getMerge().getStoreMergeInfo().estimatedMergeBytes();
         }
 
+        public long getMergeMemoryEstimateBytes() {
+            return mergeMemoryEstimateBytes;
+        }
+
+        public OnGoingMerge getOnGoingMerge() {
+            return onGoingMerge;
+        }
+
         @Override
         public String toString() {
             return name + (onGoingMerge.getMerge().isAborted() ? " (aborted)" : "");

server/src/test/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorServiceTests.java

@@ -15,6 +15,7 @@
 import org.elasticsearch.common.util.concurrent.EsExecutors;
 import org.elasticsearch.index.engine.ThreadPoolMergeScheduler.MergeTask;
 import org.elasticsearch.index.engine.ThreadPoolMergeScheduler.Schedule;
+import org.elasticsearch.index.merge.OnGoingMerge;
 import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.threadpool.TestThreadPool;
 import org.elasticsearch.threadpool.ThreadPool;
@@ -80,10 +81,14 @@ public void testEnqueuedAndBackloggedMergesAreStillExecutedWhenThreadPoolIsShutd
             .build();
         TestThreadPool testThreadPool = new TestThreadPool("test", settings);
         ThreadPoolMergeExecutorService threadPoolMergeExecutorService = getThreadPoolMergeExecutorService(testThreadPool);
+        var countingListener = new CountingMergeEventListener();
+        threadPoolMergeExecutorService.registerMergeEventListener(countingListener);
         assertThat(threadPoolMergeExecutorService.getMaxConcurrentMerges(), equalTo(mergeExecutorThreadCount));
         Semaphore runMergeSemaphore = new Semaphore(0);
         ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) testThreadPool.executor(ThreadPool.Names.MERGE);
         AtomicInteger doneMergesCount = new AtomicInteger(0);
+        AtomicInteger reEnqueuedBackloggedMergesCount = new AtomicInteger();
+        AtomicInteger abortedMergesCount = new AtomicInteger();
         // submit more merge tasks than there are threads so that some are enqueued
         for (int i = 0; i < mergesToSubmit; i++) {
             MergeTask mergeTask = mock(MergeTask.class);
@@ -95,6 +100,7 @@ public void testEnqueuedAndBackloggedMergesAreStillExecutedWhenThreadPoolIsShutd
                 if (schedule == BACKLOG) {
                     // reenqueue backlogged merge task
                     new Thread(() -> threadPoolMergeExecutorService.reEnqueueBackloggedMergeTask(mergeTask)).start();
+                    reEnqueuedBackloggedMergesCount.incrementAndGet();
                 }
                 return schedule;
             }).when(mergeTask).schedule();
@@ -114,6 +120,7 @@ public void testEnqueuedAndBackloggedMergesAreStillExecutedWhenThreadPoolIsShutd
                 }
                 runMergeSemaphore.acquireUninterruptibly();
                 doneMergesCount.incrementAndGet();
+                abortedMergesCount.incrementAndGet();
                 return null;
             }).when(mergeTask).abort();
             threadPoolMergeExecutorService.submitMergeTask(mergeTask);
@@ -125,6 +132,12 @@ public void testEnqueuedAndBackloggedMergesAreStillExecutedWhenThreadPoolIsShutd
             // with the other merge tasks enqueued
             assertThat(threadPoolExecutor.getQueue().size(), is(mergesToSubmit - mergeExecutorThreadCount));
         });
+        assertBusy(
+            () -> assertThat(
+                countingListener.queued.get(),
+                equalTo(threadPoolExecutor.getActiveCount() + threadPoolExecutor.getQueue().size() + reEnqueuedBackloggedMergesCount.get())
+            )
+        );
         // shutdown prevents new merge tasks to be enqueued but existing ones should be allowed to continue
         testThreadPool.shutdown();
         // assert all executors, except the merge one, are terminated
@@ -165,6 +178,8 @@ public void testEnqueuedAndBackloggedMergesAreStillExecutedWhenThreadPoolIsShutd
             assertTrue(threadPoolExecutor.isTerminated());
             assertTrue(threadPoolMergeExecutorService.allDone());
         });
+        assertThat(countingListener.aborted.get() + countingListener.completed.get(), equalTo(doneMergesCount.get()));
+        assertThat(countingListener.aborted.get(), equalTo(abortedMergesCount.get()));
     }
 
     public void testTargetIORateChangesWhenSubmittingMergeTasks() throws Exception {
@@ -660,6 +675,27 @@ public void testMergeTasksExecuteInSizeOrder() {
         }
     }
 
+    private static class CountingMergeEventListener implements MergeEventListener {
+        AtomicInteger queued = new AtomicInteger();
+        AtomicInteger aborted = new AtomicInteger();
+        AtomicInteger completed = new AtomicInteger();
+
+        @Override
+        public void onMergeQueued(OnGoingMerge merge, long estimateMergeMemoryBytes) {
+            queued.incrementAndGet();
+        }
+
+        @Override
+        public void onMergeCompleted(OnGoingMerge merge) {
+            completed.incrementAndGet();
+        }
+
+        @Override
+        public void onMergeAborted(OnGoingMerge merge) {
+            aborted.incrementAndGet();
+        }
+    }
+
     static ThreadPoolMergeExecutorService getThreadPoolMergeExecutorService(ThreadPool threadPool) {
         ThreadPoolMergeExecutorService threadPoolMergeExecutorService = ThreadPoolMergeExecutorService
             .maybeCreateThreadPoolMergeExecutorService(