reuse slot objects for lookups

Signed-off-by: Sandesh Kumar <sandeshkr419@gmail.com>

Author: sandeshkr419

CHANGELOG.md

@@ -17,6 +17,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add BooleanQuery rewrite moving constant-scoring must clauses to filter clauses ([#18510](https://github.com/opensearch-project/OpenSearch/issues/18510))
 - Add functionality for plugins to inject QueryCollectorContext during QueryPhase ([#18637](https://github.com/opensearch-project/OpenSearch/pull/18637))
 - Add support for non-timing info in profiler ([#18460](https://github.com/opensearch-project/OpenSearch/issues/18460))
+- Optimize Composite Aggregations by removing unnecessary object allocations ([#18531](https://github.com/opensearch-project/OpenSearch/pull/18531))
 
 ### Changed
 - Update Subject interface to use CheckedRunnable ([#18570](https://github.com/opensearch-project/OpenSearch/issues/18570))

server/src/main/java/org/opensearch/search/aggregations/bucket/composite/CompositeValuesCollectorQueue.java

@@ -60,6 +60,11 @@ private class Slot {
             this.value = initial;
         }
 
+        // This is to be only for reusable slot
+        public void set(int newValue) {
+            this.value = newValue;
+        }
+
         @Override
         public boolean equals(Object o) {
             if (this == o) return true;
@@ -82,6 +87,14 @@ public int hashCode() {
     private final Map<Slot, Integer> map; // to quickly find the slot for a value
     private final SingleDimensionValuesSource<?>[] arrays;
 
+    /**
+     * A reusable, flyweight Slot instance to avoid object allocation and reduce GC pressure
+     * during map lookups in the high-frequency collect() path. This object is NOT
+     * thread-safe, but is safe here because each collector instance is confined to a
+     * single thread.
+     */
+    private final Slot reusableSlot = new Slot(0);
+
     private LongArray docCounts;
     private boolean afterKeyIsSet = false;
 
@@ -125,7 +138,8 @@ boolean isFull() {
      * the slot if the candidate is already in the queue or null if the candidate is not present.
      */
     Integer getCurrentSlot() {
-        return map.get(new Slot(CANDIDATE_SLOT));
+        reusableSlot.set(CANDIDATE_SLOT); // Update the state of the reusable slot
+        return map.get(reusableSlot);     // Use the single reusable slot instance for the lookup
     }
 
     /**
@@ -322,7 +336,8 @@ boolean addIfCompetitive(int indexSortSourcePrefix, long inc) {
         if (size() >= maxSize) {
             // the queue is full, we replace the last key with this candidate
             int slot = pop();
-            map.remove(new Slot(slot));
+            reusableSlot.set(slot);          // Use reusable for remove
+            map.remove(reusableSlot);
             // and we recycle the deleted slot
             newSlot = slot;
         } else {