Adjusting the merge policy setting to make merges less aggressive

CHANGELOG.md

@@ -24,4 +24,6 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 * Use pre-quantized vectors from native engines for exact search [#3095](https://github.com/opensearch-project/k-NN/pull/3095)
 * Use right Vector Scorer when segments are initialized using SPI and also corrected the maxConn for MOS [#3117](https://github.com/opensearch-project/k-NN/pull/3117)
 * Use pre-quantized vectors for ADC [#3113](https://github.com/opensearch-project/k-NN/pull/3113)
+* Adjusting the merge policy setting to make merges less aggressive [#3128](https://github.com/opensearch-project/k-NN/pull/3128)
 * Upgrade Lucene to 10.4.0 [#3135](https://github.com/opensearch-project/k-NN/pull/3135)
+

src/main/java/org/opensearch/knn/plugin/KNNPlugin.java

@@ -22,11 +22,14 @@
 import org.opensearch.core.action.ActionResponse;
 import org.opensearch.core.common.io.stream.NamedWriteableRegistry;
 import org.opensearch.core.common.settings.SecureString;
+import org.opensearch.core.common.unit.ByteSizeUnit;
+import org.opensearch.core.common.unit.ByteSizeValue;
 import org.opensearch.core.xcontent.NamedXContentRegistry;
 import org.opensearch.env.Environment;
 import org.opensearch.env.NodeEnvironment;
 import org.opensearch.index.IndexModule;
 import org.opensearch.index.IndexSettings;
+import org.opensearch.index.TieredMergePolicyProvider;
 import org.opensearch.index.codec.CodecServiceFactory;
 import org.opensearch.index.engine.EngineFactory;
 import org.opensearch.index.mapper.Mapper;
@@ -284,7 +287,18 @@ public Collection<IndexSettingProvider> getAdditionalIndexSettingProviders() {
             @Override
             public Settings getAdditionalIndexSettings(String indexName, boolean isDataStreamIndex, Settings templateAndRequestSettings) {
                 if (templateAndRequestSettings.getAsBoolean(KNNSettings.KNN_INDEX, false)) {
-                    return Settings.builder().put(KNN_DERIVED_SOURCE_ENABLED, true).build();
+                    return Settings.builder()
+                        .put(KNN_DERIVED_SOURCE_ENABLED, true)
+                        // Aggressive merges for k-NN can hogg CPU which can degrade search performance
+                        // These settings are being overridden to make it less aggressive
+                        // Core has max_merge_at_once default as 30, which can make merges more aggressive
+                        .put(TieredMergePolicyProvider.INDEX_MERGE_POLICY_MAX_MERGE_AT_ONCE_SETTING.getKey(), 10)
+                        // Core has max_merge_at_once default as 16MB, which can make merges more aggressive
+                        .put(
+                            TieredMergePolicyProvider.INDEX_MERGE_POLICY_FLOOR_SEGMENT_SETTING.getKey(),
+                            new ByteSizeValue(2, ByteSizeUnit.MB)
+                        )
+                        .build();
                 }
                 return Settings.EMPTY;
             }

src/test/java/org/opensearch/knn/index/DefaultKNNIndexSettingsIT.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.knn.index;
+
+import org.opensearch.index.TieredMergePolicyProvider;
+import org.opensearch.knn.KNNRestTestCase;
+import org.opensearch.common.settings.Settings;
+
+import static org.opensearch.knn.index.KNNSettings.KNN_DERIVED_SOURCE_ENABLED;
+
+public class DefaultKNNIndexSettingsIT extends KNNRestTestCase {
+
+    public void testKnnIndexSettings_whenisKNN_thenOverride() throws Exception {
+        String indexName = randomLowerCaseString();
+        String fieldName = randomLowerCaseString();
+        boolean isKnnIndex = randomBoolean();
+
+        Settings indexSettings = Settings.builder().put("index.knn", isKnnIndex).build();
+
+        if (isKnnIndex) {
+            String mapping = createKnnIndexMapping(fieldName, 128);
+            createKnnIndex(indexName, indexSettings, mapping);
+        } else {
+            createIndex(indexName, indexSettings);
+        }
+
+        // Verify settings based on knn flag
+        String derivedSourceEnabled = getIndexSettingByName(indexName, KNN_DERIVED_SOURCE_ENABLED);
+        assertEquals(Boolean.toString(isKnnIndex), derivedSourceEnabled);
+
+        String maxMergeAtOnce = getIndexSettingByName(
+            indexName,
+            TieredMergePolicyProvider.INDEX_MERGE_POLICY_MAX_MERGE_AT_ONCE_SETTING.getKey(),
+            !isKnnIndex
+        );
+        String floorSegment = getIndexSettingByName(
+            indexName,
+            TieredMergePolicyProvider.INDEX_MERGE_POLICY_FLOOR_SEGMENT_SETTING.getKey(),
+            !isKnnIndex
+        );
+
+        if (isKnnIndex) {
+            assertEquals("10", maxMergeAtOnce);
+            assertEquals("2mb", floorSegment);
+        } else {
+            assertNotEquals("10", maxMergeAtOnce);
+            assertNotEquals("2mb", floorSegment);
+        }
+
+        deleteKNNIndex(indexName);
+    }
+}