Making force merge threadpool 1/8th of total cores (#17255) (#17353)

(cherry picked from commit 38e4b33)

Signed-off-by: Gaurav Bafna <gbbafna@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: opensearch-trigger-bot[bot]

CHANGELOG.md

@@ -16,7 +16,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Bump `reactor_netty` from 1.1.26 to 1.1.27 ([#17322](https://github.com/opensearch-project/OpenSearch/pull/17322))
 
 ### Changed
-- Convert transport-reactor-netty4 to use gradle version catalog [#17233](https://github.com/opensearch-project/OpenSearch/pull/17233))
+- Convert transport-reactor-netty4 to use gradle version catalog [#17233](https://github.com/opensearch-project/OpenSearch/pull/17233)
+- Increase force merge threads to 1/8th of cores [#17255](https://github.com/opensearch-project/OpenSearch/pull/17255)
 
 ### Deprecated
 

server/src/main/java/org/opensearch/threadpool/ThreadPool.java

@@ -289,7 +289,10 @@ public ThreadPool(
             Names.FETCH_SHARD_STARTED,
             new ScalingExecutorBuilder(Names.FETCH_SHARD_STARTED, 1, 2 * allocatedProcessors, TimeValue.timeValueMinutes(5))
         );
-        builders.put(Names.FORCE_MERGE, new FixedExecutorBuilder(settings, Names.FORCE_MERGE, 1, -1));
+        builders.put(
+            Names.FORCE_MERGE,
+            new FixedExecutorBuilder(settings, Names.FORCE_MERGE, oneEighthAllocatedProcessors(allocatedProcessors), -1)
+        );
         builders.put(
             Names.FETCH_SHARD_STORE,
             new ScalingExecutorBuilder(Names.FETCH_SHARD_STORE, 1, 2 * allocatedProcessors, TimeValue.timeValueMinutes(5))
@@ -692,6 +695,10 @@ static int boundedBy(int value, int min, int max) {
         return Math.min(max, Math.max(min, value));
     }
 
+    static int oneEighthAllocatedProcessors(final int allocatedProcessors) {
+        return boundedBy(allocatedProcessors / 8, 1, Integer.MAX_VALUE);
+    }
+
     static int halfAllocatedProcessors(int allocatedProcessors) {
         return (allocatedProcessors + 1) / 2;
     }

server/src/test/java/org/opensearch/threadpool/ThreadPoolTests.java

@@ -196,4 +196,12 @@ public void testThreadPoolResizeFail() {
             terminate(threadPool);
         }
     }
+
+    public void testOneEighthAllocatedProcessors() {
+        assertThat(ThreadPool.oneEighthAllocatedProcessors(1), equalTo(1));
+        assertThat(ThreadPool.oneEighthAllocatedProcessors(4), equalTo(1));
+        assertThat(ThreadPool.oneEighthAllocatedProcessors(8), equalTo(1));
+        assertThat(ThreadPool.oneEighthAllocatedProcessors(32), equalTo(4));
+        assertThat(ThreadPool.oneEighthAllocatedProcessors(128), equalTo(16));
+    }
 }