Add target search concurrency to TieredMergePolicy

lucene/CHANGES.txt

@@ -254,7 +254,11 @@ API Changes
 
 New Features
 ---------------------
-(No changes)
+
+* GITHUB#13430: Allow configuring the search concurrency via
+  TieredMergePolicy#setTargetSearchConcurrency. This in-turn instructs the
+  merge policy to try to have at least this number of segments on the highest
+  tier. (Adrien Grand, Carlos Delgado)
 
 Improvements
 ---------------------

lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java

@@ -93,6 +93,7 @@ public class TieredMergePolicy extends MergePolicy {
   private double segsPerTier = 10.0;
   private double forceMergeDeletesPctAllowed = 10.0;
   private double deletesPctAllowed = 20.0;
+  private int targetSearchConcurrency = 1;
 
   /** Sole constructor, setting all settings to their defaults. */
   public TieredMergePolicy() {
@@ -257,6 +258,26 @@ public double getSegmentsPerTier() {
     return segsPerTier;
   }
 
+  /**
+   * Sets the target search concurrency. This prevents creating segments that are bigger than
+   * maxDoc/targetSearchConcurrency, which in turn makes the work parallelizable into
+   * targetSearchConcurrency slices of similar doc counts. It also makes merging less aggressive, as
+   * higher values result in indices that do less merging and have more segments
+   */
+  public TieredMergePolicy setTargetSearchConcurrency(int targetSearchConcurrency) {
+    if (targetSearchConcurrency < 1) {
+      throw new IllegalArgumentException(
+          "targetSearchConcurrency must be >= 1 (got " + targetSearchConcurrency + ")");
+    }
+    this.targetSearchConcurrency = targetSearchConcurrency;
+    return this;
+  }
+
+  /** Returns the target search concurrency. */
+  public int getTargetSearchConcurrency() {
+    return targetSearchConcurrency;
+  }
+
   private static class SegmentSizeAndDocs {
     private final SegmentCommitInfo segInfo;
     /// Size of the segment in bytes, pro-rated by the number of live documents.
@@ -371,31 +392,40 @@ public MergeSpecification findMerges(
     // If we have too-large segments, grace them out of the maximum segment count
     // If we're above certain thresholds of deleted docs, we can merge very large segments.
     int tooBigCount = 0;
+    // We relax merging for the bigger segments for concurrency reasons, as we want to have several
+    // segments on the highest tier without over-merging on the lower tiers.
+    int concurrencyCount = 0;
     iter = sortedInfos.iterator();
 
+    double allowedSegCount = 0;
+
     // remove large segments from consideration under two conditions.
     // 1> Overall percent deleted docs relatively small and this segment is larger than 50%
     // maxSegSize
     // 2> overall percent deleted docs large and this segment is large and has few deleted docs
-
     while (iter.hasNext()) {
       SegmentSizeAndDocs segSizeDocs = iter.next();
       double segDelPct = 100 * (double) segSizeDocs.delCount / (double) segSizeDocs.maxDoc;
       if (segSizeDocs.sizeInBytes > maxMergedSegmentBytes / 2
           && (totalDelPct <= deletesPctAllowed || segDelPct <= deletesPctAllowed)) {
         iter.remove();
-        tooBigCount++; // Just for reporting purposes.
+        tooBigCount++;
         totIndexBytes -= segSizeDocs.sizeInBytes;
         allowedDelCount -= segSizeDocs.delCount;
+      } else if (concurrencyCount + tooBigCount < targetSearchConcurrency - 1) {
+        // Make sure we count a whole segment for the first targetSearchConcurrency-1 segments to
+        // avoid over merging on the lower levels.
+        concurrencyCount++;
+        allowedSegCount++;
+        totIndexBytes -= segSizeDocs.sizeInBytes;
       }
     }
     allowedDelCount = Math.max(0, allowedDelCount);
 
     final int mergeFactor = (int) Math.min(maxMergeAtOnce, segsPerTier);
-    // Compute max allowed segments in the index
+    // Compute max allowed segments for the remainder of the index
     long levelSize = Math.max(minSegmentBytes, floorSegmentBytes);
     long bytesLeft = totIndexBytes;
-    double allowedSegCount = 0;
     while (true) {
       final double segCountLevel = bytesLeft / (double) levelSize;
       if (segCountLevel < segsPerTier || levelSize == maxMergedSegmentBytes) {
@@ -408,7 +438,8 @@ public MergeSpecification findMerges(
     }
     // allowedSegCount may occasionally be less than segsPerTier
     // if segment sizes are below the floor size
-    allowedSegCount = Math.max(allowedSegCount, segsPerTier);
+    allowedSegCount = Math.max(allowedSegCount, Math.max(segsPerTier, targetSearchConcurrency));
+    int allowedDocCount = getMaxAllowedDocs(totalMaxDoc, totalDelDocs);
 
     if (verbose(mergeContext) && tooBigCount > 0) {
       message(
@@ -419,7 +450,11 @@ public MergeSpecification findMerges(
               + " (eligible count="
               + sortedInfos.size()
               + ") tooBigCount= "
-              + tooBigCount,
+              + tooBigCount
+              + "  allowedDocCount="
+              + allowedDocCount
+              + " vs doc count="
+              + infos.totalMaxDoc(),
           mergeContext);
     }
     return doFindMerges(
@@ -428,6 +463,7 @@ public MergeSpecification findMerges(
         mergeFactor,
         (int) allowedSegCount,
         allowedDelCount,
+        allowedDocCount,
         MERGE_TYPE.NATURAL,
         mergeContext,
         mergingBytes >= maxMergedSegmentBytes);
@@ -439,6 +475,7 @@ private MergeSpecification doFindMerges(
       final int mergeFactor,
       final int allowedSegCount,
       final int allowedDelCount,
+      final int allowedDocCount,
       final MERGE_TYPE mergeType,
       MergeContext mergeContext,
       boolean maxMergeIsRunning)
@@ -522,16 +559,23 @@ private MergeSpecification doFindMerges(
         final List<SegmentCommitInfo> candidate = new ArrayList<>();
         boolean hitTooLarge = false;
         long bytesThisMerge = 0;
+        long docCountThisMerge = 0;
         for (int idx = startIdx;
             idx < sortedEligible.size()
                 && candidate.size() < mergeFactor
-                && bytesThisMerge < maxMergedSegmentBytes;
+                && bytesThisMerge < maxMergedSegmentBytes
+                && (bytesThisMerge < floorSegmentBytes || docCountThisMerge <= allowedDocCount);
             idx++) {
           final SegmentSizeAndDocs segSizeDocs = sortedEligible.get(idx);
           final long segBytes = segSizeDocs.sizeInBytes;
-
-          if (totAfterMergeBytes + segBytes > maxMergedSegmentBytes) {
-            hitTooLarge = true;
+          int segDocCount = segSizeDocs.maxDoc - segSizeDocs.delCount;
+          if (totAfterMergeBytes + segBytes > maxMergedSegmentBytes
+              || (totAfterMergeBytes > floorSegmentBytes
+                  && docCountThisMerge + segDocCount > allowedDocCount)) {
+            // Only set hitTooLarge when reaching the maximum byte size, as this will create
+            // segments of the maximum size which will no longer be eligible for merging for a long
+            // time (until they accumulate enough deletes).
+            hitTooLarge |= totAfterMergeBytes + segBytes > maxMergedSegmentBytes;
             if (candidate.size() == 0) {
               // We should never have something coming in that _cannot_ be merged, so handle
               // singleton merges
@@ -548,6 +592,7 @@ private MergeSpecification doFindMerges(
           }
           candidate.add(segSizeDocs.segInfo);
           bytesThisMerge += segBytes;
+          docCountThisMerge += segDocCount;
           totAfterMergeBytes += segBytes;
         }
 
@@ -916,14 +961,13 @@ public MergeSpecification findForcedDeletesMerges(SegmentInfos infos, MergeConte
     final Set<SegmentCommitInfo> merging = mergeContext.getMergingSegments();
 
     boolean haveWork = false;
+    int totalDelCount = 0;
     for (SegmentCommitInfo info : infos) {
       int delCount = mergeContext.numDeletesToMerge(info);
       assert assertDelCount(delCount, info);
+      totalDelCount += delCount;
       double pctDeletes = 100. * ((double) delCount) / info.info.maxDoc();
-      if (pctDeletes > forceMergeDeletesPctAllowed && !merging.contains(info)) {
-        haveWork = true;
-        break;
-      }
+      haveWork = haveWork || (pctDeletes > forceMergeDeletesPctAllowed && !merging.contains(info));
     }
 
     if (haveWork == false) {
@@ -950,11 +994,16 @@ public MergeSpecification findForcedDeletesMerges(SegmentInfos infos, MergeConte
         Integer.MAX_VALUE,
         Integer.MAX_VALUE,
         0,
+        getMaxAllowedDocs(infos.totalMaxDoc(), totalDelCount),
         MERGE_TYPE.FORCE_MERGE_DELETES,
         mergeContext,
         false);
   }
 
+  int getMaxAllowedDocs(int totalMaxDoc, int totalDelDocs) {
+    return Math.ceilDiv(totalMaxDoc - totalDelDocs, targetSearchConcurrency);
+  }
+
   private long floorSize(long bytes) {
     return Math.max(floorSegmentBytes, bytes);
   }
@@ -969,7 +1018,8 @@ public String toString() {
     sb.append("segmentsPerTier=").append(segsPerTier).append(", ");
     sb.append("maxCFSSegmentSizeMB=").append(getMaxCFSSegmentSizeMB()).append(", ");
     sb.append("noCFSRatio=").append(noCFSRatio).append(", ");
-    sb.append("deletesPctAllowed=").append(deletesPctAllowed);
+    sb.append("deletesPctAllowed=").append(deletesPctAllowed).append(", ");
+    sb.append("targetSearchConcurrency=").append(targetSearchConcurrency);
     return sb.toString();
   }
 }

lucene/core/src/test/org/apache/lucene/index/TestTieredMergePolicy.java

@@ -65,6 +65,7 @@ protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws
       segmentSizes.add(weightedByteSize);
       minSegmentBytes = Math.min(minSegmentBytes, weightedByteSize);
     }
+    Collections.sort(segmentSizes);
 
     final double delPercentage = 100.0 * totalDelCount / totalMaxDoc;
     assertTrue(
@@ -77,12 +78,26 @@ protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws
     long levelSizeBytes = Math.max(minSegmentBytes, (long) (tmp.getFloorSegmentMB() * 1024 * 1024));
     long bytesLeft = totalBytes;
     double allowedSegCount = 0;
+    List<Long> biggestSegments = segmentSizes;
+    if (biggestSegments.size() > tmp.getTargetSearchConcurrency() - 1) {
+      biggestSegments =
+          biggestSegments.subList(
+              biggestSegments.size() - tmp.getTargetSearchConcurrency() + 1,
+              biggestSegments.size());
+    }
+    // Allow whole segments for the targetSearchConcurrency-1 biggest segments
+    for (long size : biggestSegments) {
+      bytesLeft -= size;
+      allowedSegCount++;
+    }
+
     // below we make the assumption that segments that reached the max segment
     // size divided by 2 don't need merging anymore
     int mergeFactor = (int) Math.min(tmp.getSegmentsPerTier(), tmp.getMaxMergeAtOnce());
     while (true) {
       final double segCountLevel = bytesLeft / (double) levelSizeBytes;
-      if (segCountLevel < tmp.getSegmentsPerTier() || levelSizeBytes >= maxMergedSegmentBytes / 2) {
+      if (segCountLevel <= tmp.getSegmentsPerTier()
+          || levelSizeBytes >= maxMergedSegmentBytes / 2) {
         allowedSegCount += Math.ceil(segCountLevel);
         break;
       }
@@ -94,7 +109,6 @@ protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws
 
     // It's ok to be over the allowed segment count if none of the most balanced merges are balanced
     // enough
-    Collections.sort(segmentSizes);
     boolean hasBalancedMerges = false;
     for (int i = 0; i < segmentSizes.size() - mergeFactor; ++i) {
       long maxMergeSegmentSize = segmentSizes.get(i + mergeFactor - 1);
@@ -111,11 +125,24 @@ protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws
       }
     }
 
+    // There can be more segments if we can't merge docs because they are balanced between segments.
+    // At least the
+    //  2 smallest segments should be mergeable.
+    // should be 2 segments to merge
+    int maxDocsPerSegment = tmp.getMaxAllowedDocs(infos.totalMaxDoc(), totalDelCount);
+    List<Integer> segmentDocs =
+        infos.asList().stream()
+            .map(info -> info.info.maxDoc() - info.getDelCount())
+            .sorted()
+            .toList();
+    boolean eligibleDocsMerge =
+        segmentDocs.size() >= 2 && segmentDocs.get(0) + segmentDocs.get(1) < maxDocsPerSegment;
+
     int numSegments = infos.asList().size();
     assertTrue(
         String.format(
             Locale.ROOT,
-            "mergeFactor=%d minSegmentBytes=%,d maxMergedSegmentBytes=%,d segmentsPerTier=%g maxMergeAtOnce=%d numSegments=%d allowed=%g totalBytes=%,d delPercentage=%g deletesPctAllowed=%g",
+            "mergeFactor=%d minSegmentBytes=%,d maxMergedSegmentBytes=%,d segmentsPerTier=%g maxMergeAtOnce=%d numSegments=%d allowed=%g totalBytes=%,d delPercentage=%g deletesPctAllowed=%g targetNumSegments=%d",
             mergeFactor,
             minSegmentBytes,
             maxMergedSegmentBytes,
@@ -125,8 +152,9 @@ protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws
             allowedSegCount,
             totalBytes,
             delPercentage,
-            tmp.getDeletesPctAllowed()),
-        numSegments <= allowedSegCount || hasBalancedMerges == false);
+            tmp.getDeletesPctAllowed(),
+            tmp.getTargetSearchConcurrency()),
+        numSegments <= allowedSegCount || hasBalancedMerges == false || eligibleDocsMerge == false);
   }
 
   @Override
@@ -208,6 +236,7 @@ public void testPartialMerge() throws Exception {
 
       int segmentCount = w.getSegmentCount();
       int targetCount = TestUtil.nextInt(random(), 1, segmentCount);
+
       if (VERBOSE) {
         System.out.println(
             "TEST: merge to " + targetCount + " segs (current count=" + segmentCount + ")");

lucene/test-framework/src/java/org/apache/lucene/tests/util/LuceneTestCase.java

@@ -1095,6 +1095,12 @@ public static TieredMergePolicy newTieredMergePolicy(Random r) {
     } else {
       tmp.setSegmentsPerTier(TestUtil.nextInt(r, 10, 50));
     }
+    if (rarely(r)) {
+      tmp.setTargetSearchConcurrency(TestUtil.nextInt(r, 10, 50));
+    } else {
+      tmp.setTargetSearchConcurrency(TestUtil.nextInt(r, 2, 20));
+    }
+
     configureRandom(r, tmp);
     tmp.setDeletesPctAllowed(20 + random().nextDouble() * 30);
     return tmp;