Remove maxMergeAtOnce option from TieredMergePolicy.

lucene/CHANGES.txt

@@ -9,14 +9,16 @@ API Changes
 ---------------------
 * GITHUB#11023: Removing deprecated parameters from CheckIndex. (Jakub Slowinski)
 
+* GITHUB#14165: TieredMergePolicy's maxMergeAtOnce parameter was removed. (Adrien Grand)
+
 New Features
 ---------------------
 * GITHUB#14097: Binary partitioning merge policy over float-valued vector field. (Mike Sokolov)
 
 Improvements
 ---------------------
 
-* GITHUB#266: TieredMergePolicy's maxMergeAtOnce default value was changed from 10 to 30. (Adrien Grand)
+(No changes)
 
 Optimizations
 ---------------------

lucene/MIGRATE.md

@@ -17,6 +17,13 @@
 
 # Apache Lucene Migration Guide
 
+## Migration from Lucene 10.x to Lucene 11.0
+
+### TieredMergePolicy#setMaxMergeAtOnce removed
+
+This parameter has no replacement, TieredMergePolicy no longer bounds the
+number of segments that may be merged together.
+
 ## Migration from Lucene 9.x to Lucene 10.0
 
 ### DataInput#readVLong() may now read negative vlongs

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

@@ -30,9 +30,7 @@
 /**
  * Merges segments of approximately equal size, subject to an allowed number of segments per tier.
  * This is similar to {@link LogByteSizeMergePolicy}, except this merge policy is able to merge
- * non-adjacent segment, and separates how many segments are merged at once ({@link
- * #setMaxMergeAtOnce}) from how many segments are allowed per tier ({@link #setSegmentsPerTier}).
- * This merge policy also does not over-merge (i.e. cascade merges).
+ * non-adjacent segment. This merge policy also does not over-merge (i.e. cascade merges).
  *
  * <p>For normal merging, this policy first computes a "budget" of how many segments are allowed to
  * be in the index. If the index is over-budget, then the policy sorts segments by decreasing size
@@ -84,9 +82,6 @@ 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 for segments above the floor size to avoid suboptimal merging.
-  private int maxMergeAtOnce = 30;
   private long maxMergedSegmentBytes = 5 * 1024 * 1024 * 1024L;
 
   private long floorSegmentBytes = 2 * 1024 * 1024L;
@@ -100,36 +95,12 @@ public TieredMergePolicy() {
     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 30.
-   *
-   * <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 + ")");
-    }
-    maxMergeAtOnce = v;
-    return this;
-  }
-
   private enum MERGE_TYPE {
     NATURAL,
     FORCE_MERGE,
     FORCE_MERGE_DELETES
   }
 
-  /**
-   * Returns the current maxMergeAtOnce setting.
-   *
-   * @see #setMaxMergeAtOnce
-   */
-  public int getMaxMergeAtOnce() {
-    return maxMergeAtOnce;
-  }
-
   // TODO: should addIndexes do explicit merging, too?  And,
   // if user calls IW.maybeMerge "explicitly"
 
@@ -429,7 +400,7 @@ public MergeSpecification findMerges(
     }
     allowedDelCount = Math.max(0, allowedDelCount);
 
-    final int mergeFactor = (int) Math.min(maxMergeAtOnce, segsPerTier);
+    final int mergeFactor = (int) segsPerTier;
     // Compute max allowed segments for the remainder of the index
     long levelSize = Math.max(minSegmentBytes, floorSegmentBytes);
     long bytesLeft = totIndexBytes;
@@ -570,7 +541,6 @@ private MergeSpecification doFindMerges(
         long docCountThisMerge = 0;
         for (int idx = startIdx;
             idx < sortedEligible.size()
-                && 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
@@ -733,7 +703,7 @@ protected MergeScore score(
       // matter in this case because this merge will not
       // "cascade" and so it cannot lead to N^2 merge cost
       // over time:
-      final int mergeFactor = (int) Math.min(maxMergeAtOnce, segsPerTier);
+      int mergeFactor = (int) segsPerTier;
       skew = 1.0 / mergeFactor;
     } else {
       skew =
@@ -1021,7 +991,6 @@ private long floorSize(long bytes) {
   @Override
   public String toString() {
     StringBuilder sb = new StringBuilder("[" + getClass().getSimpleName() + ": ");
-    sb.append("maxMergeAtOnce=").append(maxMergeAtOnce).append(", ");
     sb.append("maxMergedSegmentMB=").append(maxMergedSegmentBytes / 1024. / 1024.).append(", ");
     sb.append("floorSegmentMB=").append(floorSegmentBytes / 1024. / 1024.).append(", ");
     sb.append("forceMergeDeletesPctAllowed=").append(forceMergeDeletesPctAllowed).append(", ");

lucene/core/src/test/org/apache/lucene/document/TestManyKnnDocs.java

@@ -43,7 +43,6 @@ public void testLargeSegment() throws Exception {
             128)); // Make sure to use the ConfigurableMCodec instead of a random one
     iwc.setRAMBufferSizeMB(64); // Use a 64MB buffer to create larger initial segments
     TieredMergePolicy mp = new TieredMergePolicy();
-    mp.setMaxMergeAtOnce(256); // avoid intermediate merges (waste of time with HNSW?)
     mp.setSegmentsPerTier(256); // only merge once at the end when we ask
     iwc.setMergePolicy(mp);
     String fieldName = "field";

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

@@ -109,7 +109,7 @@ public void runTest(Directory directory) throws Exception {
 
     IndexWriterConfig conf =
         new IndexWriterConfig(new MockAnalyzer(random())).setMaxBufferedDocs(7);
-    ((TieredMergePolicy) conf.getMergePolicy()).setMaxMergeAtOnce(3);
+    ((TieredMergePolicy) conf.getMergePolicy()).setSegmentsPerTier(3);
     IndexWriter writer = RandomIndexWriter.mockIndexWriter(directory, conf, random());
 
     // Establish a base index of 100 docs:

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

@@ -375,7 +375,6 @@ protected void doMerge(MergeSource mergeSource, MergePolicy.OneMerge merge)
 
     TieredMergePolicy tmp = new TieredMergePolicy();
     iwc.setMergePolicy(tmp);
-    tmp.setMaxMergeAtOnce(2);
     tmp.setSegmentsPerTier(2);
 
     IndexWriter w = new IndexWriter(dir, iwc);

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

@@ -36,7 +36,7 @@ public class TestForTooMuchCloning extends LuceneTestCase {
   public void test() throws Exception {
     final MockDirectoryWrapper dir = newMockDirectory();
     final TieredMergePolicy tmp = new TieredMergePolicy();
-    tmp.setMaxMergeAtOnce(2);
+    tmp.setSegmentsPerTier(2);
     final RandomIndexWriter w =
         new RandomIndexWriter(
             random(),

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

@@ -70,7 +70,7 @@ public void test() throws Exception {
     MergePolicy mp = w.getConfig().getMergePolicy();
     final int mergeAtOnce = 1 + w.cloneSegmentInfos().size();
     if (mp instanceof TieredMergePolicy) {
-      ((TieredMergePolicy) mp).setMaxMergeAtOnce(mergeAtOnce);
+      ((TieredMergePolicy) mp).setSegmentsPerTier(mergeAtOnce);
     } else if (mp instanceof LogMergePolicy) {
       ((LogMergePolicy) mp).setMergeFactor(mergeAtOnce);
     } else {

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

@@ -104,7 +104,7 @@ protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws
 
     // 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());
+    int mergeFactor = (int) tmp.getSegmentsPerTier();
     while (true) {
       final double segCountLevel = bytesLeft / (double) levelSizeBytes;
       if (segCountLevel <= tmp.getSegmentsPerTier()
@@ -145,12 +145,11 @@ protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws
     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 targetNumSegments=%d",
+            "mergeFactor=%d minSegmentBytes=%,d maxMergedSegmentBytes=%,d segmentsPerTier=%g numSegments=%d allowed=%g totalBytes=%,d delPercentage=%g deletesPctAllowed=%g targetNumSegments=%d",
             mergeFactor,
             minSegmentBytes,
             maxMergedSegmentBytes,
             tmp.getSegmentsPerTier(),
-            tmp.getMaxMergeAtOnce(),
             numSegments,
             allowedSegCount,
             totalBytes,
@@ -162,10 +161,7 @@ protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws
 
   @Override
   protected void assertMerge(MergePolicy policy, MergeSpecification merges) {
-    TieredMergePolicy tmp = (TieredMergePolicy) policy;
-    for (OneMerge merge : merges.merges) {
-      assertTrue(merge.segments.size() <= tmp.getMaxMergeAtOnce());
-    }
+    // anything to assert?
   }
 
   public void testForceMergeDeletes() throws Exception {
@@ -174,7 +170,6 @@ public void testForceMergeDeletes() throws Exception {
     TieredMergePolicy tmp = newTieredMergePolicy();
     conf.setMergePolicy(tmp);
     conf.setMaxBufferedDocs(4);
-    tmp.setMaxMergeAtOnce(100);
     tmp.setSegmentsPerTier(100);
     tmp.setDeletesPctAllowed(50.0);
     tmp.setForceMergeDeletesPctAllowed(30.0);
@@ -219,8 +214,8 @@ public void testPartialMerge() throws Exception {
       TieredMergePolicy tmp = newTieredMergePolicy();
       conf.setMergePolicy(tmp);
       conf.setMaxBufferedDocs(2);
-      tmp.setMaxMergeAtOnce(3);
       tmp.setSegmentsPerTier(6);
+      tmp.setFloorSegmentMB(Double.MIN_VALUE);
 
       IndexWriter w = new IndexWriter(dir, conf);
       int maxCount = 0;
@@ -231,7 +226,7 @@ public void testPartialMerge() throws Exception {
         w.addDocument(doc);
         int count = w.getSegmentCount();
         maxCount = Math.max(count, maxCount);
-        assertTrue("count=" + count + " maxCount=" + maxCount, count >= maxCount - 3);
+        assertTrue("count=" + count + " maxCount=" + maxCount, count >= maxCount - 6);
       }
 
       w.flush(true, true);
@@ -973,15 +968,13 @@ public void testMergeSizeIsLessThanFloorSize() throws IOException {
       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.
+    // Segments are below the floor segment size. We get one merge that merges the 50 segments
+    // together.
     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());
+    assertEquals(1, mergeSpec.merges.size());
+    assertEquals(50, mergeSpec.merges.get(0).segments.size());
   }
 
   public void testFullFlushMerges() throws IOException {
@@ -1008,6 +1001,6 @@ public void testFullFlushMerges() throws IOException {
       segmentInfos =
           applyMerge(segmentInfos, merge, "_" + segNameGenerator.getAndIncrement(), stats);
     }
-    assertEquals(2, segmentInfos.size());
+    assertEquals(1, segmentInfos.size());
   }
 }

lucene/replicator/src/test/org/apache/lucene/replicator/nrt/SimplePrimaryNode.java

@@ -164,7 +164,6 @@ private static IndexWriter initWriter(
     if (mp instanceof TieredMergePolicy) {
       TieredMergePolicy tmp = (TieredMergePolicy) mp;
       tmp.setSegmentsPerTier(3);
-      tmp.setMaxMergeAtOnce(3);
     } else if (mp instanceof LogMergePolicy) {
       LogMergePolicy lmp = (LogMergePolicy) mp;
       lmp.setMergeFactor(3);

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

@@ -1096,11 +1096,6 @@ private static void configureRandom(Random r, MergePolicy mergePolicy) {
 
   public static TieredMergePolicy newTieredMergePolicy(Random r) {
     TieredMergePolicy tmp = new TieredMergePolicy();
-    if (rarely(r)) {
-      tmp.setMaxMergeAtOnce(TestUtil.nextInt(r, 2, 9));
-    } else {
-      tmp.setMaxMergeAtOnce(TestUtil.nextInt(r, 10, 50));
-    }
     if (rarely(r)) {
       tmp.setMaxMergedSegmentMB(0.2 + r.nextDouble() * 2.0);
     } else {
@@ -1235,11 +1230,6 @@ public static void maybeChangeLiveIndexWriterConfig(Random r, LiveIndexWriterCon
         }
       } else if (mp instanceof TieredMergePolicy) {
         TieredMergePolicy tmp = (TieredMergePolicy) mp;
-        if (rarely(r)) {
-          tmp.setMaxMergeAtOnce(TestUtil.nextInt(r, 2, 9));
-        } else {
-          tmp.setMaxMergeAtOnce(TestUtil.nextInt(r, 10, 50));
-        }
         if (rarely(r)) {
           tmp.setMaxMergedSegmentMB(0.2 + r.nextDouble() * 2.0);
         } else {

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

@@ -1453,7 +1453,6 @@ public static void reduceOpenFiles(IndexWriter w) {
       lmp.setMergeFactor(Math.min(5, lmp.getMergeFactor()));
     } else if (mp instanceof TieredMergePolicy) {
       TieredMergePolicy tmp = (TieredMergePolicy) mp;
-      tmp.setMaxMergeAtOnce(Math.min(5, tmp.getMaxMergeAtOnce()));
       tmp.setSegmentsPerTier(Math.min(5, tmp.getSegmentsPerTier()));
     }
     MergeScheduler ms = w.getConfig().getMergeScheduler();