Add CHANGES entry for GH-13581

lucene/CHANGES.txt

@@ -326,6 +326,8 @@ Optimizations
   performance for queries that need skipping such as conjunctions.
   (Adrien Grand)
 
+* GITHUB#13581: OnHeapHnswGraph no longer allocates a lock for every graph node (Mike Sokolov)
+
 Changes in runtime behavior
 ---------------------
 

lucene/core/src/java/org/apache/lucene/util/hnsw/HnswLock.java

@@ -26,7 +26,7 @@ import java.util.concurrent.locks.ReentrantReadWriteLock;
  * Provide (read-and-write) locked access to rows of an OnHeapHnswGraph. For use by
  * HnswConcurrentMerger and its HnswGraphBuilders.
  */
-public class HnswLock {
+class HnswLock {
   private static final int NUM_LOCKS = 512;
   private final ReentrantReadWriteLock[] locks;
   private final OnHeapHnswGraph graph;