Decrease minimum deletes percentage in TMP (#14893)

Author: stefanvodita

lucene/CHANGES.txt

@@ -120,6 +120,9 @@ API Changes
 * GITHUB#14426: Support determining desired off-heap memory requirements through
   KnnVectorsReader::getOffHeapByteSize (Chris Hegarty)
 
+* GITHUB#14893: TieredMergePolicy minimum deletes percentage decreased from 5% inclusive to 0% exclusive.
+  (Stefan Vodita)
+
 * GITHUB#14899: Deprecate MergeSpecification#segString(Directory) (kitoha)
 
 * GITHUB#14978: Add a bulk scoring interface to RandomVectorScorer

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

@@ -130,18 +130,25 @@ public double getMaxMergedSegmentMB() {
   /**
    * Sets the maximum percentage of doc id space taken by deleted docs. The denominator includes
    * both active and deleted documents. Lower values make the index more space efficient at the
-   * expense of increased CPU and I/O activity. Values must be between 5 and 50. Default value is
+   * expense of increased CPU and I/O activity. Values must be between 0 and 50. Default value is
    * 20.
    *
    * <p>When the maximum delete percentage is lowered, the indexing thread will call for merges more
    * often, meaning that write amplification factor will be increased. Write amplification factor
    * measures the number of times each document in the index is written. A higher write
    * amplification factor will lead to higher CPU and I/O activity as indicated above.
+   *
+   * <p>Values below 5% can lead to exceptionally high merge cost where indexing will continuously
+   * merge nearly all segments, and select newly merged segments immediately for merging again,
+   * often forcing degenerate merge selection like singleton merges. If you venture into this dark
+   * forest, consider limiting the maximum number of concurrent merges and threads (see {@link
+   * ConcurrentMergeScheduler#setMaxMergesAndThreads}) as a coarse attempt to bound the otherwise
+   * pathological indexing behavior.
    */
   public TieredMergePolicy setDeletesPctAllowed(double v) {
-    if (v < 5 || v > 50) {
+    if (v <= 0 || v > 50) {
       throw new IllegalArgumentException(
-          "indexPctDeletedTarget must be >= 5.0 and <= 50 (got " + v + ")");
+          "indexPctDeletedTarget must be > 0 and <= 50 (got " + v + ")");
     }
     deletesPctAllowed = v;
     return this;