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Use Atomic vars in multithreaded env. ++num and num++ operations aren… #16356

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Use Atomic vars in multithreaded env. ++num and num++ operations aren… #16356

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Use Atomic vars in multithreaded env. ++num and num++ operations aren…
dk2k Oct 16, 2024
8b62369
Update server/src/main/java/org/opensearch/action/search/QueryPhaseRe…
dk2k Oct 18, 2024
cdbb13f
Reverted changes in 3 files
dk2k Nov 23, 2024
3fad0c3
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dk2k Nov 23, 2024
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10 changes: 5 additions & 5 deletions server/src/main/java/org/opensearch/action/search/QueryPhaseResultConsumer.java
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Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,7 @@
import java.util.Comparator;
import java.util.List;
import java.util.concurrent.Executor;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Consumer;

Expand Down Expand Up @@ -155,7 +156,7 @@ public SearchPhaseController.ReducedQueryPhase reduce() throws Exception {
aggsList,
topDocsList,
topDocsStats,
pendingMerges.numReducePhases,
pendingMerges.numReducePhases.get(),
false,
aggReduceContextBuilder,
performFinalReduce
Expand Down Expand Up @@ -239,7 +240,7 @@ private MergeResult partialReduce(
}

public int getNumReducePhases() {
return pendingMerges.numReducePhases;
return pendingMerges.numReducePhases.get();
}

/**
Expand All @@ -264,7 +265,7 @@ private class PendingMerges implements Releasable {
private final SearchPhaseController.TopDocsStats topDocsStats;
private volatile MergeResult mergeResult;
private volatile boolean hasPartialReduce;
private volatile int numReducePhases;
private final AtomicInteger numReducePhases = new AtomicInteger();
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Do we still need to change this to AtomicInteger?

If usage of an int hasn't been resulted in any issue so far as we know, then I think we might just add unnecessary overhead of using AtomicInteger over an int.

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There is a definite issue here. But no one digs that deeply and reports it, if faces.
++num and num++ operations aren't atomic, so you can get an out-of-sync value from time to time

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@sandeshkr419 sandeshkr419 Nov 27, 2024 •
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I don't think your analysis is entirely accurate.

The increment operation (++numReducePhases) is performed within a thread-safe block. Specifically, tryExecuteNext() method begins by acquiring a synchronized lock on this.

We’d benefit from AtomicInteger instead of volatile int if numReducePhases were incremented outside of a synchronized block or lock-free code which is not the case.

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@sandeshkr419 You are right. That was the only previously approved change and it's uneecessary. I'm going to close this PR


PendingMerges(int batchReduceSize, int trackTotalHitsUpTo) {
this.batchReduceSize = batchReduceSize;
Expand Down Expand Up @@ -448,8 +449,7 @@ protected void doRun() {
long estimatedMergeSize = estimateRamBytesUsedForReduce(estimatedTotalSize);
addEstimateAndMaybeBreak(estimatedMergeSize);
estimatedTotalSize += estimatedMergeSize;
++numReducePhases;
newMerge = partialReduce(toConsume, task.emptyResults, topDocsStats, thisMergeResult, numReducePhases);
newMerge = partialReduce(toConsume, task.emptyResults, topDocsStats, thisMergeResult, numReducePhases.incrementAndGet());
} catch (Exception t) {
onMergeFailure(t);
return;
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