Added logs to understand the time taken by different merge related items

Signed-off-by: Navneet Verma <[email protected]>

src/main/java/org/opensearch/knn/index/codec/KNN990Codec/NativeEngines990KnnVectorsWriter.java

@@ -108,31 +108,35 @@ public void flush(int maxDoc, final Sorter.DocMap sortMap) throws IOException {
     @Override
     public void mergeOneField(final FieldInfo fieldInfo, final MergeState mergeState) throws IOException {
         // This will ensure that we are merging the FlatIndex during force merge.
+        StopWatch stopwatch = new StopWatch().start();
         flatVectorsWriter.mergeOneField(fieldInfo, mergeState);
-
+        log.info("Merge : Reading and writing merged FlatVector Took : {} ms", stopwatch.stop().totalTime().millis());
         final VectorDataType vectorDataType = extractVectorDataType(fieldInfo);
         final Supplier<KNNVectorValues<?>> knnVectorValuesSupplier = () -> getKNNVectorValuesForMerge(
             vectorDataType,
             fieldInfo,
             mergeState
         );
+        stopwatch = new StopWatch().start();
         int totalLiveDocs = getLiveDocs(knnVectorValuesSupplier.get());
+        log.info("Merge : Reading live docs Took : {} ms", stopwatch.stop().totalTime().millis());
         if (totalLiveDocs == 0) {
             log.debug("[Merge] No live docs for field {}", fieldInfo.getName());
             return;
         }
 
         final QuantizationState quantizationState = train(fieldInfo, knnVectorValuesSupplier, totalLiveDocs);
-        final NativeIndexWriter writer = NativeIndexWriter.getWriter(fieldInfo, segmentWriteState, quantizationState);
-        final KNNVectorValues<?> knnVectorValues = knnVectorValuesSupplier.get();
 
+        StopWatch stopWatch2 = new StopWatch().start();
+        final KNNVectorValues<?> knnVectorValues = knnVectorValuesSupplier.get();
+        log.info("Merge : Creating merged VectorValues Took : {} ms", stopWatch2.stop().totalTime().millis());
         StopWatch stopWatch = new StopWatch().start();
-
+        final NativeIndexWriter writer = NativeIndexWriter.getWriter(fieldInfo, segmentWriteState, quantizationState);
         writer.mergeIndex(knnVectorValues, totalLiveDocs);
 
         long time_in_millis = stopWatch.stop().totalTime().millis();
         KNNGraphValue.MERGE_TOTAL_TIME_IN_MILLIS.incrementBy(time_in_millis);
-        log.debug("Merge took {} ms for vector field [{}]", time_in_millis, fieldInfo.getName());
+        log.info("Merge took {} ms for vector field [{}]", time_in_millis, fieldInfo.getName());
     }
 
     /**