Add mult-leaf optimizations for diversify children collector (#13121)

This adds multi-leaf optimizations for diversified children collector. This means as children vectors are collected within a block join, we can share information between leaves to speed up vector search.

To make this happen, I refactored the multi-leaf collector slightly. Now, instead of inheriting from TopKnnCollector, we inject a inner collector.

lucene/CHANGES.txt

@@ -215,6 +215,9 @@ Optimizations
 
 * GITHUB#13085: Remove unnecessary toString() / substring() calls to save some String allocations (Dmitry Cherniachenko)
 
+* GITHUB#13121: Speedup multi-segment HNSW graph search for diversifying child kNN queries. Builds on GITHUB#12962.
+  (Ben Trent)
+
 Bug Fixes
 ---------------------
 

lucene/core/src/java/org/apache/lucene/search/AbstractKnnCollector.java

@@ -61,6 +61,8 @@ public abstract class AbstractKnnCollector implements KnnCollector {
   @Override
   public abstract boolean collect(int docId, float similarity);
 
+  public abstract int numCollected();
+
   @Override
   public abstract float minCompetitiveSimilarity();
 

lucene/core/src/java/org/apache/lucene/search/TopKnnCollector.java

@@ -63,6 +63,11 @@ public class TopKnnCollector extends AbstractKnnCollector {
     return new TopDocs(new TotalHits(visitedCount(), relation), scoreDocs);
   }
 
+  @Override
+  public int numCollected() {
+    return queue.size();
+  }
+
   @Override
   public String toString() {
     return "TopKnnCollector[k=" + k() + ", size=" + queue.size() + "]";

lucene/core/src/java/org/apache/lucene/search/VectorSimilarityCollector.java

@@ -75,4 +75,9 @@ class VectorSimilarityCollector extends AbstractKnnCollector {
     return new TopDocs(
         new TotalHits(visitedCount(), relation), scoreDocList.toArray(ScoreDoc[]::new));
   }
+
+  @Override
+  public int numCollected() {
+    return scoreDocList.size();
+  }
 }

lucene/core/src/java/org/apache/lucene/search/knn/MultiLeafKnnCollector.java

@@ -17,20 +17,19 @@
 
 package org.apache.lucene.search.knn;
 
-import org.apache.lucene.search.ScoreDoc;
+import org.apache.lucene.search.AbstractKnnCollector;
+import org.apache.lucene.search.KnnCollector;
 import org.apache.lucene.search.TopDocs;
-import org.apache.lucene.search.TopKnnCollector;
-import org.apache.lucene.search.TotalHits;
 import org.apache.lucene.util.hnsw.BlockingFloatHeap;
 import org.apache.lucene.util.hnsw.FloatHeap;
 
 /**
- * MultiLeafTopKnnCollector is a specific KnnCollector that can exchange the top collected results
+ * MultiLeafKnnCollector is a specific KnnCollector that can exchange the top collected results
  * across segments through a shared global queue.
  *
  * @lucene.experimental
  */
-public final class MultiLeafTopKnnCollector extends TopKnnCollector {
+public final class MultiLeafKnnCollector implements KnnCollector {
 
   // greediness of globally non-competitive search: (0,1]
   private static final float DEFAULT_GREEDINESS = 0.9f;
@@ -46,23 +45,55 @@ public final class MultiLeafTopKnnCollector extends TopKnnCollector {
   private final int interval = 0xff; // 255
   private boolean kResultsCollected = false;
   private float cachedGlobalMinSim = Float.NEGATIVE_INFINITY;
+  private final AbstractKnnCollector subCollector;
 
   /**
+   * Create a new MultiLeafKnnCollector.
+   *
    * @param k the number of neighbors to collect
-   * @param visitLimit how many vector nodes the results are allowed to visit
+   * @param globalSimilarityQueue the global queue of the highest similarities collected so far
+   *     across all segments
+   * @param subCollector the local collector
    */
-  public MultiLeafTopKnnCollector(int k, int visitLimit, BlockingFloatHeap globalSimilarityQueue) {
-    super(k, visitLimit);
+  public MultiLeafKnnCollector(
+      int k, BlockingFloatHeap globalSimilarityQueue, AbstractKnnCollector subCollector) {
     this.greediness = DEFAULT_GREEDINESS;
+    this.subCollector = subCollector;
     this.globalSimilarityQueue = globalSimilarityQueue;
     this.nonCompetitiveQueue = new FloatHeap(Math.max(1, Math.round((1 - greediness) * k)));
     this.updatesQueue = new FloatHeap(k);
   }
 
+  @Override
+  public boolean earlyTerminated() {
+    return subCollector.earlyTerminated();
+  }
+
+  @Override
+  public void incVisitedCount(int count) {
+    subCollector.incVisitedCount(count);
+  }
+
+  @Override
+  public long visitedCount() {
+    return subCollector.visitedCount();
+  }
+
+  @Override
+  public long visitLimit() {
+    return subCollector.visitLimit();
+  }
+
+  @Override
+  public int k() {
+    return subCollector.k();
+  }
+
   @Override
   public boolean collect(int docId, float similarity) {
-    boolean localSimUpdated = queue.insertWithOverflow(docId, similarity);
-    boolean firstKResultsCollected = (kResultsCollected == false && queue.size() == k());
+    boolean localSimUpdated = subCollector.collect(docId, similarity);
+    boolean firstKResultsCollected =
+        (kResultsCollected == false && subCollector.numCollected() == k());
     if (firstKResultsCollected) {
       kResultsCollected = true;
     }
@@ -71,7 +102,7 @@ public final class MultiLeafTopKnnCollector extends TopKnnCollector {
 
     if (kResultsCollected) {
       // as we've collected k results, we can start do periodic updates with the global queue
-      if (firstKResultsCollected || (visitedCount & interval) == 0) {
+      if (firstKResultsCollected || (subCollector.visitedCount() & interval) == 0) {
         cachedGlobalMinSim = globalSimilarityQueue.offer(updatesQueue.getHeap());
         updatesQueue.clear();
         globalSimUpdated = true;
@@ -85,26 +116,18 @@ public final class MultiLeafTopKnnCollector extends TopKnnCollector {
     if (kResultsCollected == false) {
       return Float.NEGATIVE_INFINITY;
     }
-    return Math.max(queue.topScore(), Math.min(nonCompetitiveQueue.peek(), cachedGlobalMinSim));
+    return Math.max(
+        subCollector.minCompetitiveSimilarity(),
+        Math.min(nonCompetitiveQueue.peek(), cachedGlobalMinSim));
   }
 
   @Override
   public TopDocs topDocs() {
-    assert queue.size() <= k() : "Tried to collect more results than the maximum number allowed";
-    ScoreDoc[] scoreDocs = new ScoreDoc[queue.size()];
-    for (int i = 1; i <= scoreDocs.length; i++) {
-      scoreDocs[scoreDocs.length - i] = new ScoreDoc(queue.topNode(), queue.topScore());
-      queue.pop();
-    }
-    TotalHits.Relation relation =
-        earlyTerminated()
-            ? TotalHits.Relation.GREATER_THAN_OR_EQUAL_TO
-            : TotalHits.Relation.EQUAL_TO;
-    return new TopDocs(new TotalHits(visitedCount(), relation), scoreDocs);
+    return subCollector.topDocs();
   }
 
   @Override
   public String toString() {
-    return "MultiLeafTopKnnCollector[k=" + k() + ", size=" + queue.size() + "]";
+    return "MultiLeafKnnCollector[subCollector=" + subCollector + "]";
   }
 }

lucene/core/src/java/org/apache/lucene/search/knn/TopKnnCollectorManager.java

@@ -20,6 +20,7 @@ package org.apache.lucene.search.knn;
 import java.io.IOException;
 import org.apache.lucene.index.LeafReaderContext;
 import org.apache.lucene.search.IndexSearcher;
+import org.apache.lucene.search.KnnCollector;
 import org.apache.lucene.search.TopKnnCollector;
 import org.apache.lucene.util.hnsw.BlockingFloatHeap;
 
@@ -48,12 +49,11 @@ public class TopKnnCollectorManager implements KnnCollectorManager {
    * @param context the leaf reader context
    */
   @Override
-  public TopKnnCollector newCollector(int visitedLimit, LeafReaderContext context)
-      throws IOException {
+  public KnnCollector newCollector(int visitedLimit, LeafReaderContext context) throws IOException {
     if (globalScoreQueue == null) {
       return new TopKnnCollector(k, visitedLimit);
     } else {
-      return new MultiLeafTopKnnCollector(k, visitedLimit, globalScoreQueue);
+      return new MultiLeafKnnCollector(k, globalScoreQueue, new TopKnnCollector(k, visitedLimit));
     }
   }
 }

lucene/join/src/java/org/apache/lucene/search/join/DiversifyingChildrenByteKnnVectorQuery.java

@@ -126,7 +126,7 @@ public class DiversifyingChildrenByteKnnVectorQuery extends KnnByteVectorQuery {
 
   @Override
   protected KnnCollectorManager getKnnCollectorManager(int k, IndexSearcher searcher) {
-    return new DiversifyingNearestChildrenKnnCollectorManager(k, parentsFilter);
+    return new DiversifyingNearestChildrenKnnCollectorManager(k, parentsFilter, searcher);
   }
 
   @Override
@@ -136,12 +136,10 @@ public class DiversifyingChildrenByteKnnVectorQuery extends KnnByteVectorQuery {
       int visitedLimit,
       KnnCollectorManager knnCollectorManager)
       throws IOException {
-    BitSet parentBitSet = parentsFilter.getBitSet(context);
-    if (parentBitSet == null) {
+    KnnCollector collector = knnCollectorManager.newCollector(visitedLimit, context);
+    if (collector == null) {
       return NO_RESULTS;
     }
-    KnnCollector collector =
-        new DiversifyingNearestChildrenKnnCollector(k, visitedLimit, parentBitSet);
     context.reader().searchNearestVectors(field, query, collector, acceptDocs);
     return collector.topDocs();
   }

lucene/join/src/java/org/apache/lucene/search/join/DiversifyingChildrenFloatKnnVectorQuery.java

@@ -126,7 +126,7 @@ public class DiversifyingChildrenFloatKnnVectorQuery extends KnnFloatVectorQuery
 
   @Override
   protected KnnCollectorManager getKnnCollectorManager(int k, IndexSearcher searcher) {
-    return new DiversifyingNearestChildrenKnnCollectorManager(k, parentsFilter);
+    return new DiversifyingNearestChildrenKnnCollectorManager(k, parentsFilter, searcher);
   }
 
   @Override

lucene/join/src/java/org/apache/lucene/search/join/DiversifyingNearestChildrenKnnCollector.java

@@ -96,6 +96,11 @@ class DiversifyingNearestChildrenKnnCollector extends AbstractKnnCollector {
     return new TopDocs(new TotalHits(visitedCount(), relation), scoreDocs);
   }
 
+  @Override
+  public int numCollected() {
+    return heap.size();
+  }
+
   /**
    * This is a minimum binary heap, inspired by {@link org.apache.lucene.util.LongHeap}. But instead
    * of encoding and using `long` values. Node ids and scores are kept separate. Additionally, this

lucene/join/src/java/org/apache/lucene/search/join/DiversifyingNearestChildrenKnnCollectorManager.java

@@ -19,8 +19,12 @@ package org.apache.lucene.search.join;
 
 import java.io.IOException;
 import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.search.IndexSearcher;
+import org.apache.lucene.search.KnnCollector;
 import org.apache.lucene.search.knn.KnnCollectorManager;
+import org.apache.lucene.search.knn.MultiLeafKnnCollector;
 import org.apache.lucene.util.BitSet;
+import org.apache.lucene.util.hnsw.BlockingFloatHeap;
 
 /**
  * DiversifyingNearestChildrenKnnCollectorManager responsible for creating {@link
@@ -32,6 +36,7 @@ public class DiversifyingNearestChildrenKnnCollectorManager implements KnnCollec
   private final int k;
   // filter identifying the parent documents.
   private final BitSetProducer parentsFilter;
+  private final BlockingFloatHeap globalScoreQueue;
 
   /**
    * Constructor
@@ -39,9 +44,12 @@ public class DiversifyingNearestChildrenKnnCollectorManager implements KnnCollec
    * @param k - the number of top k vectors to collect
    * @param parentsFilter Filter identifying the parent documents.
    */
-  public DiversifyingNearestChildrenKnnCollectorManager(int k, BitSetProducer parentsFilter) {
+  public DiversifyingNearestChildrenKnnCollectorManager(
+      int k, BitSetProducer parentsFilter, IndexSearcher indexSearcher) {
     this.k = k;
     this.parentsFilter = parentsFilter;
+    this.globalScoreQueue =
+        indexSearcher.getIndexReader().leaves().size() > 1 ? new BlockingFloatHeap(k) : null;
   }
 
   /**
@@ -51,12 +59,18 @@ public class DiversifyingNearestChildrenKnnCollectorManager implements KnnCollec
    * @param context the leaf reader context
    */
   @Override
-  public DiversifyingNearestChildrenKnnCollector newCollector(
-      int visitedLimit, LeafReaderContext context) throws IOException {
+  public KnnCollector newCollector(int visitedLimit, LeafReaderContext context) throws IOException {
     BitSet parentBitSet = parentsFilter.getBitSet(context);
     if (parentBitSet == null) {
       return null;
     }
+    if (globalScoreQueue == null) {
       return new DiversifyingNearestChildrenKnnCollector(k, visitedLimit, parentBitSet);
+    } else {
+      return new MultiLeafKnnCollector(
+          k,
+          globalScoreQueue,
+          new DiversifyingNearestChildrenKnnCollector(k, visitedLimit, parentBitSet));
+    }
   }
 }