LUCENE-10073: Reduce merging overhead of NRT by using a greater mergeFactor on tiny segments. (#266)

Closes #11111

lucene/CHANGES.txt

@@ -30,6 +30,10 @@ Improvements
   system property to increase the set of Java versions that Panama Vectorization will
   provide optimized implementations for. (Chris Hegarty)
 
+* GITHUB#266: TieredMergePolicy now allows merging up to maxMergeAtOnce
+  segments for merges below the floor segment size, even if maxMergeAtOnce is
+  bigger than segsPerTier. (Adrien Grand)
+
 Optimizations
 ---------------------
 

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

@@ -85,7 +85,7 @@ public class TieredMergePolicy extends MergePolicy {
   public static final double DEFAULT_NO_CFS_RATIO = 0.1;
 
   // User-specified maxMergeAtOnce. In practice we always take the min of its
-  // value and segsPerTier to avoid suboptimal merging.
+  // value and segsPerTier for segments above the floor size to avoid suboptimal merging.
   private int maxMergeAtOnce = 10;
   private long maxMergedSegmentBytes = 5 * 1024 * 1024 * 1024L;
 
@@ -100,7 +100,13 @@ public class TieredMergePolicy extends MergePolicy {
     super(DEFAULT_NO_CFS_RATIO, MergePolicy.DEFAULT_MAX_CFS_SEGMENT_SIZE);
   }
 
-  /** Maximum number of segments to be merged at a time during "normal" merging. Default is 10. */
+  /**
+   * Maximum number of segments to be merged at a time during "normal" merging. Default is 10.
+   *
+   * <p><b>NOTE</b>: Merges above the {@link #setFloorSegmentMB(double) floor segment size} also
+   * bound the number of merged segments by {@link #setSegmentsPerTier(double) the number of
+   * segments per tier}.
+   */
   public TieredMergePolicy setMaxMergeAtOnce(int v) {
     if (v < 2) {
       throw new IllegalArgumentException("maxMergeAtOnce must be > 1 (got " + v + ")");
@@ -557,34 +563,34 @@ public class TieredMergePolicy extends MergePolicy {
 
       for (int startIdx = 0; startIdx < sortedEligible.size(); startIdx++) {
 
-        long totAfterMergeBytes = 0;
-
         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
+                && candidate.size() < maxMergeAtOnce
+                // We allow merging more than mergeFactor segments together if the merged segment
+                // would be less than the floor segment size. This is important because segments
+                // below the floor segment size are more aggressively merged by this policy, so we
+                // need to grow them as quickly as possible.
+                && (candidate.size() < mergeFactor || bytesThisMerge < floorSegmentBytes)
                 && bytesThisMerge < maxMergedSegmentBytes
                 && (bytesThisMerge < floorSegmentBytes || docCountThisMerge <= allowedDocCount);
             idx++) {
           final SegmentSizeAndDocs segSizeDocs = sortedEligible.get(idx);
           final long segBytes = segSizeDocs.sizeInBytes;
           int segDocCount = segSizeDocs.maxDoc - segSizeDocs.delCount;
-          if (totAfterMergeBytes + segBytes > maxMergedSegmentBytes
-              || (totAfterMergeBytes > floorSegmentBytes
+          if (bytesThisMerge + segBytes > maxMergedSegmentBytes
+              || (bytesThisMerge > 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) {
+            hitTooLarge |= bytesThisMerge + segBytes > maxMergedSegmentBytes;
             // We should never have something coming in that _cannot_ be merged, so handle
             // singleton merges
-              candidate.add(segSizeDocs.segInfo);
-              bytesThisMerge += segBytes;
-            }
+            if (candidate.size() > 0) {
               // NOTE: we continue, so that we can try
               // "packing" smaller segments into this merge
               // to see if we can get closer to the max
@@ -593,10 +599,10 @@ public class TieredMergePolicy extends MergePolicy {
               // to try different permutations.
               continue;
             }
+          }
           candidate.add(segSizeDocs.segInfo);
           bytesThisMerge += segBytes;
           docCountThisMerge += segDocCount;
-          totAfterMergeBytes += segBytes;
         }
 
         // We should never see an empty candidate: we iterated over maxMergeAtOnce
@@ -645,7 +651,7 @@ public class TieredMergePolicy extends MergePolicy {
                   + " tooLarge="
                   + hitTooLarge
                   + " size="
-                  + String.format(Locale.ROOT, "%.3f MB", totAfterMergeBytes / 1024. / 1024.),
+                  + String.format(Locale.ROOT, "%.3f MB", bytesThisMerge / 1024. / 1024.),
               mergeContext);
         }
 
@@ -654,7 +660,7 @@ public class TieredMergePolicy extends MergePolicy {
           best = candidate;
           bestScore = score;
           bestTooLarge = hitTooLarge;
-          bestMergeBytes = totAfterMergeBytes;
+          bestMergeBytes = bytesThisMerge;
         }
       }
 

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

@@ -163,9 +163,8 @@ public class TestTieredMergePolicy extends BaseMergePolicyTestCase {
   @Override
   protected void assertMerge(MergePolicy policy, MergeSpecification merges) {
     TieredMergePolicy tmp = (TieredMergePolicy) policy;
-    final int mergeFactor = (int) Math.min(tmp.getMaxMergeAtOnce(), tmp.getSegmentsPerTier());
     for (OneMerge merge : merges.merges) {
-      assertTrue(merge.segments.size() <= mergeFactor);
+      assertTrue(merge.segments.size() <= tmp.getMaxMergeAtOnce());
     }
   }
 
@@ -943,6 +942,49 @@ public class TestTieredMergePolicy extends BaseMergePolicyTestCase {
     doTestSimulateUpdates(mergePolicy, numDocs, 2500);
   }
 
+  public void testMergeSizeIsLessThanFloorSize() throws IOException {
+    MergeContext mergeContext = new MockMergeContext(SegmentCommitInfo::getDelCount);
+
+    SegmentInfos infos = new SegmentInfos(Version.LATEST.major);
+    // 50 1MB segments
+    for (int i = 0; i < 50; ++i) {
+      infos.add(makeSegmentCommitInfo("_0", 1_000_000, 0, 1, IndexWriter.SOURCE_FLUSH));
+    }
+
+    TieredMergePolicy mergePolicy = new TieredMergePolicy();
+    mergePolicy.setMaxMergeAtOnce(30);
+    mergePolicy.setFloorSegmentMB(0.1);
+
+    // Segments are above the floor segment size, we get 4 merges of mergeFactor=10 segments each
+    MergeSpecification mergeSpec =
+        mergePolicy.findMerges(MergeTrigger.FULL_FLUSH, infos, mergeContext);
+    assertNotNull(mergeSpec);
+    assertEquals(4, mergeSpec.merges.size());
+    for (OneMerge oneMerge : mergeSpec.merges) {
+      assertEquals(mergePolicy.getSegmentsPerTier(), oneMerge.segments.size(), 0d);
+    }
+
+    // Segments are below the floor segment size and it takes 15 segments to go above the floor
+    // segment size. We get 3 merges of 15 segments each
+    mergePolicy.setFloorSegmentMB(15);
+    mergeSpec = mergePolicy.findMerges(MergeTrigger.FULL_FLUSH, infos, mergeContext);
+    assertNotNull(mergeSpec);
+    assertEquals(3, mergeSpec.merges.size());
+    for (OneMerge oneMerge : mergeSpec.merges) {
+      assertEquals(15, oneMerge.segments.size());
+    }
+
+    // Segments are below the floor segment size and we'd need to merge more than maxMergeAtOnce
+    // segments to go above the minimum segment size. We get 1 merge of maxMergeAtOnce=30 segments
+    // and 1 merge of 50-30=20 segments.
+    mergePolicy.setFloorSegmentMB(60);
+    mergeSpec = mergePolicy.findMerges(MergeTrigger.FULL_FLUSH, infos, mergeContext);
+    assertNotNull(mergeSpec);
+    assertEquals(2, mergeSpec.merges.size());
+    assertEquals(30, mergeSpec.merges.get(0).segments.size());
+    assertEquals(20, mergeSpec.merges.get(1).segments.size());
+  }
+
   public void testFullFlushMerges() throws IOException {
     AtomicLong segNameGenerator = new AtomicLong();
     IOStats stats = new IOStats();