Improve cpu utilization with dynamic slice size in doc partitioning

docs/changelog/132774.yaml

@@ -0,0 +1,5 @@
+pr: 132774
+summary: Improve cpu utilization with dynamic slice size in doc partitioning
+area: ES|QL
+type: enhancement
+issues: []

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/lucene/DataPartitioning.java

@@ -9,6 +9,8 @@
 
 import org.elasticsearch.compute.operator.Driver;
 
+import java.util.List;
+
 /**
  * How we partition the data across {@link Driver}s. Each request forks into
  * {@code min(1.5 * cpus, partition_count)} threads on the data node. More partitions
@@ -37,9 +39,20 @@ public enum DataPartitioning {
      */
     SEGMENT,
     /**
-     * Partition each shard into {@code task_concurrency} partitions, splitting
-     * larger segments into slices. This allows bringing the most CPUs to bear on
-     * the problem but adds extra overhead, especially in query preparation.
+     * Partitions into dynamic-sized slices to improve CPU utilization while keeping overhead low.
+     * This approach is more flexible than {@link #SEGMENT} and works as follows:
+     *
+     * <ol>
+     *   <li>The slice size starts from a desired size based on {@code task_concurrency} but is capped
+     *       at around {@link LuceneSliceQueue#MAX_DOCS_PER_SLICE}. This prevents poor CPU usage when
+     *       matching documents are clustered together.</li>
+     *   <li>For small and medium segments (less than five times the desired slice size), it uses a
+     *       slightly different {@link #SEGMENT} strategy, which also splits segments that are larger
+     *       than the desired size. See {@link org.apache.lucene.search.IndexSearcher#slices(List, int, int, boolean)}.</li>
+     *   <li>For very large segments, multiple segments are not combined into a single slice. This allows
+     *       one driver to process an entire large segment until other drivers steal the work after finishing
+     *       their own tasks. See {@link LuceneSliceQueue#nextSlice(LuceneSlice)}.</li>
+     * </ol>
      */
-    DOC,
+    DOC
 }

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/lucene/LuceneOperator.java

@@ -165,7 +165,7 @@ LuceneScorer getCurrentOrLoadNextScorer() {
         while (currentScorer == null || currentScorer.isDone()) {
             if (currentSlice == null || sliceIndex >= currentSlice.numLeaves()) {
                 sliceIndex = 0;
-                currentSlice = sliceQueue.nextSlice();
+                currentSlice = sliceQueue.nextSlice(currentSlice);
                 if (currentSlice == null) {
                     doneCollecting = true;
                     return null;

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/lucene/LuceneSlice.java

@@ -14,7 +14,13 @@
 /**
  * Holds a list of multiple partial Lucene segments
  */
-public record LuceneSlice(ShardContext shardContext, List<PartialLeafReaderContext> leaves, Weight weight, List<Object> tags) {
+public record LuceneSlice(
+    int slicePosition,
+    ShardContext shardContext,
+    List<PartialLeafReaderContext> leaves,
+    Weight weight,
+    List<Object> tags
+) {
     int numLeaves() {
         return leaves.size();
     }

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/lucene/LuceneSliceQueue.java

@@ -16,18 +16,21 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Writeable;
+import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
 import org.elasticsearch.core.Nullable;
 
 import java.io.IOException;
 import java.io.UncheckedIOException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
+import java.util.Comparator;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Queue;
-import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.atomic.AtomicReferenceArray;
 import java.util.function.Function;
 
 /**
@@ -77,18 +80,78 @@ public record QueryAndTags(Query query, List<Object> tags) {}
     public static final int MAX_SEGMENTS_PER_SLICE = 5; // copied from IndexSearcher
 
     private final int totalSlices;
-    private final Queue<LuceneSlice> slices;
     private final Map<String, PartitioningStrategy> partitioningStrategies;
 
-    private LuceneSliceQueue(List<LuceneSlice> slices, Map<String, PartitioningStrategy> partitioningStrategies) {
-        this.totalSlices = slices.size();
-        this.slices = new ConcurrentLinkedQueue<>(slices);
+    private final AtomicReferenceArray<LuceneSlice> slices;
+    /**
+     * Queue of slice IDs that are the primary entry point for a new group of segments.
+     * A driver should prioritize polling from this queue after failing to get a sequential
+     * slice (the segment affinity). This ensures that threads start work on fresh,
+     * independent segment groups before resorting to work stealing.
+     */
+    private final Queue<Integer> sliceHeads;
+
+    /**
+     * Queue of slice IDs that are not the primary entry point for a segment group.
+     * This queue serves as a fallback pool for work stealing. When a thread has no more independent work,
+     * it will "steal" a slice from this queue to keep itself utilized. A driver should pull tasks from
+     * this queue only when {@code sliceHeads} has been exhausted.
+     */
+    private final Queue<Integer> stealableSlices;
+
+    LuceneSliceQueue(List<LuceneSlice> sliceList, Map<String, PartitioningStrategy> partitioningStrategies) {
+        this.totalSlices = sliceList.size();
+        this.slices = new AtomicReferenceArray<>(sliceList.size());
+        for (int i = 0; i < sliceList.size(); i++) {
+            slices.set(i, sliceList.get(i));
+        }
         this.partitioningStrategies = partitioningStrategies;
+        this.sliceHeads = ConcurrentCollections.newQueue();
+        this.stealableSlices = ConcurrentCollections.newQueue();
+        for (LuceneSlice slice : sliceList) {
+            if (slice.getLeaf(0).minDoc() == 0) {
+                sliceHeads.add(slice.slicePosition());
+            } else {
+                stealableSlices.add(slice.slicePosition());
+            }
+        }
     }
 
+    /**
+     * Retrieves the next available {@link LuceneSlice} for processing.
+     * <p>
+     * This method implements a three-tiered strategy to minimize the overhead of switching between segments:
+     * 1. If a previous slice is provided, it first attempts to return the next sequential slice.
+     * This keeps a thread working on the same segments, minimizing the overhead of segment switching.
+     * 2. If affinity fails, it returns a slice from the {@link #sliceHeads} queue, which is an entry point for
+     * a new, independent group of segments, allowing the calling Driver to work on a fresh set of segments.
+     * 3. If the {@link #sliceHeads} queue is exhausted, it "steals" a slice
+     * from the {@link #stealableSlices} queue. This fallback ensures all threads remain utilized.
+     *
+     * @param prev the previously returned {@link LuceneSlice}, or {@code null} if starting
+     * @return the next available {@link LuceneSlice}, or {@code null} if exhausted
+     */
     @Nullable
-    public LuceneSlice nextSlice() {
-        return slices.poll();
+    public LuceneSlice nextSlice(LuceneSlice prev) {
+        if (prev != null) {
+            final int nextId = prev.slicePosition() + 1;
+            if (nextId < totalSlices) {
+                var slice = slices.getAndSet(nextId, null);
+                if (slice != null) {
+                    return slice;
+                }
+            }
+        }
+        for (var ids : List.of(sliceHeads, stealableSlices)) {
+            Integer nextId;
+            while ((nextId = ids.poll()) != null) {
+                var slice = slices.getAndSet(nextId, null);
+                if (slice != null) {
+                    return slice;
+                }
+            }
+        }
+        return null;
     }
 
     public int totalSlices() {
@@ -103,7 +166,14 @@ public Map<String, PartitioningStrategy> partitioningStrategies() {
     }
 
     public Collection<String> remainingShardsIdentifiers() {
-        return slices.stream().map(slice -> slice.shardContext().shardIdentifier()).toList();
+        List<String> remaining = new ArrayList<>(slices.length());
+        for (int i = 0; i < slices.length(); i++) {
+            LuceneSlice slice = slices.get(i);
+            if (slice != null) {
+                remaining.add(slice.shardContext().shardIdentifier());
+            }
+        }
+        return remaining;
     }
 
     public static LuceneSliceQueue create(
@@ -117,6 +187,7 @@ public static LuceneSliceQueue create(
         List<LuceneSlice> slices = new ArrayList<>();
         Map<String, PartitioningStrategy> partitioningStrategies = new HashMap<>(contexts.size());
 
+        int nextSliceId = 0;
         for (ShardContext ctx : contexts) {
             for (QueryAndTags queryAndExtra : queryFunction.apply(ctx)) {
                 var scoreMode = scoreModeFunction.apply(ctx);
@@ -140,7 +211,7 @@ public static LuceneSliceQueue create(
                 Weight weight = weight(ctx, query, scoreMode);
                 for (List<PartialLeafReaderContext> group : groups) {
                     if (group.isEmpty() == false) {
-                        slices.add(new LuceneSlice(ctx, group, weight, queryAndExtra.tags));
+                        slices.add(new LuceneSlice(nextSliceId++, ctx, group, weight, queryAndExtra.tags));
                     }
                 }
             }
@@ -158,7 +229,7 @@ public enum PartitioningStrategy implements Writeable {
          */
         SHARD(0) {
             @Override
-            List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int requestedNumSlices) {
+            List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int taskConcurrency) {
                 return List.of(searcher.getLeafContexts().stream().map(PartialLeafReaderContext::new).toList());
             }
         },
@@ -167,7 +238,7 @@ List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int requeste
          */
         SEGMENT(1) {
             @Override
-            List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int requestedNumSlices) {
+            List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int taskConcurrency) {
                 IndexSearcher.LeafSlice[] gs = IndexSearcher.slices(
                     searcher.getLeafContexts(),
                     MAX_DOCS_PER_SLICE,
@@ -182,52 +253,11 @@ List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int requeste
          */
         DOC(2) {
             @Override
-            List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int requestedNumSlices) {
+            List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int taskConcurrency) {
                 final int totalDocCount = searcher.getIndexReader().maxDoc();
-                final int normalMaxDocsPerSlice = totalDocCount / requestedNumSlices;
-                final int extraDocsInFirstSlice = totalDocCount % requestedNumSlices;
-                final List<List<PartialLeafReaderContext>> slices = new ArrayList<>();
-                int docsAllocatedInCurrentSlice = 0;
-                List<PartialLeafReaderContext> currentSlice = null;
-                int maxDocsPerSlice = normalMaxDocsPerSlice + extraDocsInFirstSlice;
-                for (LeafReaderContext ctx : searcher.getLeafContexts()) {
-                    final int numDocsInLeaf = ctx.reader().maxDoc();
-                    int minDoc = 0;
-                    while (minDoc < numDocsInLeaf) {
-                        int numDocsToUse = Math.min(maxDocsPerSlice - docsAllocatedInCurrentSlice, numDocsInLeaf - minDoc);
-                        if (numDocsToUse <= 0) {
-                            break;
-                        }
-                        if (currentSlice == null) {
-                            currentSlice = new ArrayList<>();
-                        }
-                        currentSlice.add(new PartialLeafReaderContext(ctx, minDoc, minDoc + numDocsToUse));
-                        minDoc += numDocsToUse;
-                        docsAllocatedInCurrentSlice += numDocsToUse;
-                        if (docsAllocatedInCurrentSlice == maxDocsPerSlice) {
-                            slices.add(currentSlice);
-                            // once the first slice with the extra docs is added, no need for extra docs
-                            maxDocsPerSlice = normalMaxDocsPerSlice;
-                            currentSlice = null;
-                            docsAllocatedInCurrentSlice = 0;
-                        }
-                    }
-                }
-                if (currentSlice != null) {
-                    slices.add(currentSlice);
-                }
-                if (requestedNumSlices < totalDocCount && slices.size() != requestedNumSlices) {
-                    throw new IllegalStateException("wrong number of slices, expected " + requestedNumSlices + " but got " + slices.size());
-                }
-                if (slices.stream()
-                    .flatMapToInt(
-                        l -> l.stream()
-                            .mapToInt(partialLeafReaderContext -> partialLeafReaderContext.maxDoc() - partialLeafReaderContext.minDoc())
-                    )
-                    .sum() != totalDocCount) {
-                    throw new IllegalStateException("wrong doc count");
-                }
-                return slices;
+                // Cap the desired slice to prevent CPU underutilization when matching documents are concentrated in one segment region.
+                int desiredSliceSize = Math.clamp(Math.ceilDiv(totalDocCount, taskConcurrency), 1, MAX_DOCS_PER_SLICE);
+                return new AdaptivePartitioner(Math.max(1, desiredSliceSize), MAX_SEGMENTS_PER_SLICE).partition(searcher.getLeafContexts());
             }
         };
 
@@ -252,7 +282,7 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeByte(id);
         }
 
-        abstract List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int requestedNumSlices);
+        abstract List<List<PartialLeafReaderContext>> groups(IndexSearcher searcher, int taskConcurrency);
 
         private static PartitioningStrategy pick(
             DataPartitioning dataPartitioning,
@@ -291,4 +321,67 @@ static Weight weight(ShardContext ctx, Query query, ScoreMode scoreMode) {
             throw new UncheckedIOException(e);
         }
     }
+
+    static final class AdaptivePartitioner {
+        final int desiredDocsPerSlice;
+        final int maxDocsPerSlice;
+        final int maxSegmentsPerSlice;
+
+        AdaptivePartitioner(int desiredDocsPerSlice, int maxSegmentsPerSlice) {
+            this.desiredDocsPerSlice = desiredDocsPerSlice;
+            this.maxDocsPerSlice = desiredDocsPerSlice * 5 / 4;
+            this.maxSegmentsPerSlice = maxSegmentsPerSlice;
+        }
+
+        List<List<PartialLeafReaderContext>> partition(List<LeafReaderContext> leaves) {
+            List<LeafReaderContext> smallSegments = new ArrayList<>();
+            List<LeafReaderContext> largeSegments = new ArrayList<>();
+            List<List<PartialLeafReaderContext>> results = new ArrayList<>();
+            for (LeafReaderContext leaf : leaves) {
+                if (leaf.reader().maxDoc() >= 5 * desiredDocsPerSlice) {
+                    largeSegments.add(leaf);
+                } else {
+                    smallSegments.add(leaf);
+                }
+            }
+            largeSegments.sort(Collections.reverseOrder(Comparator.comparingInt(l -> l.reader().maxDoc())));
+            for (LeafReaderContext segment : largeSegments) {
+                results.addAll(partitionOneLargeSegment(segment));
+            }
+            results.addAll(partitionSmallSegments(smallSegments));
+            return results;
+        }
+
+        List<List<PartialLeafReaderContext>> partitionOneLargeSegment(LeafReaderContext leaf) {
+            int numDocsInLeaf = leaf.reader().maxDoc();
+            int numSlices = Math.max(1, numDocsInLeaf / desiredDocsPerSlice);
+            while (Math.ceilDiv(numDocsInLeaf, numSlices) > maxDocsPerSlice) {
+                numSlices++;
+            }
+            int docPerSlice = numDocsInLeaf / numSlices;
+            int leftoverDocs = numDocsInLeaf % numSlices;
+            int minDoc = 0;
+            List<List<PartialLeafReaderContext>> results = new ArrayList<>();
+            while (minDoc < numDocsInLeaf) {
+                int docsToUse = docPerSlice;
+                if (leftoverDocs > 0) {
+                    --leftoverDocs;
+                    docsToUse++;
+                }
+                int maxDoc = Math.min(minDoc + docsToUse, numDocsInLeaf);
+                results.add(List.of(new PartialLeafReaderContext(leaf, minDoc, maxDoc)));
+                minDoc = maxDoc;
+            }
+            assert leftoverDocs == 0 : leftoverDocs;
+            assert results.stream().allMatch(s -> s.size() == 1) : "must have one partial leaf per slice";
+            assert results.stream().flatMapToInt(ss -> ss.stream().mapToInt(s -> s.maxDoc() - s.minDoc())).sum() == numDocsInLeaf;
+            return results;
+        }
+
+        List<List<PartialLeafReaderContext>> partitionSmallSegments(List<LeafReaderContext> leaves) {
+            var slices = IndexSearcher.slices(leaves, maxDocsPerSlice, maxSegmentsPerSlice, true);
+            return Arrays.stream(slices).map(g -> Arrays.stream(g.partitions).map(PartialLeafReaderContext::new).toList()).toList();
+        }
+    }
+
 }

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/lucene/TimeSeriesSourceOperator.java

@@ -97,7 +97,7 @@ public Page getCheckedOutput() throws IOException {
         long startInNanos = System.nanoTime();
         try {
             if (iterator == null) {
-                var slice = sliceQueue.nextSlice();
+                var slice = sliceQueue.nextSlice(null);
                 if (slice == null) {
                     doneCollecting = true;
                     return null;