Configurable limit on concurrent shard closing (#121267)

Today we limit the number of shards concurrently closed by the
`IndicesClusterStateService`, but this limit is currently a function of
the CPU count of the node. On nodes with plentiful CPU but poor IO
performance we may want to restrict this limit further. This commit
exposes the throttling limit as a setting.

Author: DaveCTurner

server/src/main/java/org/elasticsearch/common/settings/ClusterSettings.java

@@ -614,6 +614,7 @@ public void apply(Settings value, Settings current, Settings previous) {
         DataStreamLifecycle.CLUSTER_LIFECYCLE_DEFAULT_ROLLOVER_SETTING,
         IndicesClusterStateService.SHARD_LOCK_RETRY_INTERVAL_SETTING,
         IndicesClusterStateService.SHARD_LOCK_RETRY_TIMEOUT_SETTING,
+        IndicesClusterStateService.CONCURRENT_SHARD_CLOSE_LIMIT,
         IngestSettings.GROK_WATCHDOG_INTERVAL,
         IngestSettings.GROK_WATCHDOG_MAX_EXECUTION_TIME,
         TDigestExecutionHint.SETTING,

server/src/main/java/org/elasticsearch/indices/cluster/IndicesClusterStateService.java

@@ -116,6 +116,18 @@ public class IndicesClusterStateService extends AbstractLifecycleComponent imple
         Setting.Property.NodeScope
     );
 
+    /**
+     * Maximum number of shards to try and close concurrently. Defaults to the smaller of {@code node.processors} and {@code 10}, but can be
+     * set to any positive integer.
+     */
+    public static final Setting<Integer> CONCURRENT_SHARD_CLOSE_LIMIT = Setting.intSetting(
+        "indices.store.max_concurrent_closing_shards",
+        settings -> Integer.toString(Math.min(10, EsExecutors.NODE_PROCESSORS_SETTING.get(settings).roundUp())),
+        1,
+        Integer.MAX_VALUE,
+        Setting.Property.NodeScope
+    );
+
     final AllocatedIndices<? extends Shard, ? extends AllocatedIndex<? extends Shard>> indicesService;
     private final ClusterService clusterService;
     private final ThreadPool threadPool;
@@ -1347,7 +1359,7 @@ enum IndexRemovalReason {
         }
     }
 
-    private static class ShardCloseExecutor implements Executor {
+    static class ShardCloseExecutor implements Executor {
 
         private final ThrottledTaskRunner throttledTaskRunner;
 
@@ -1360,8 +1372,11 @@ private static class ShardCloseExecutor implements Executor {
             // can't close the old ones down fast enough. Maybe we could block or throttle new shards starting while old shards are still
             // shutting down, given that starting new shards is already async. Since this seems unlikely in practice, we opt for the simple
             // approach here.
-            final var maxThreads = Math.max(EsExecutors.NODE_PROCESSORS_SETTING.get(settings).roundUp(), 10);
-            throttledTaskRunner = new ThrottledTaskRunner(IndicesClusterStateService.class.getCanonicalName(), maxThreads, delegate);
+            throttledTaskRunner = new ThrottledTaskRunner(
+                IndicesClusterStateService.class.getCanonicalName(),
+                CONCURRENT_SHARD_CLOSE_LIMIT.get(settings),
+                delegate
+            );
         }
 
         @Override

server/src/test/java/org/elasticsearch/indices/cluster/ShardCloseExecutorTests.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.indices.cluster;
+
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.util.concurrent.EsExecutors;
+import org.elasticsearch.test.ESTestCase;
+
+import java.util.ArrayList;
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class ShardCloseExecutorTests extends ESTestCase {
+
+    public void testThrottling() {
+        // This defaults to the number of CPUs of the machine running the tests which could be either side of 10.
+        final var defaultProcessors = EsExecutors.NODE_PROCESSORS_SETTING.get(Settings.EMPTY).roundUp();
+        ensureThrottling(Math.min(10, defaultProcessors), Settings.EMPTY);
+
+        if (10 < defaultProcessors) {
+            ensureThrottling(
+                10,
+                Settings.builder().put(EsExecutors.NODE_PROCESSORS_SETTING.getKey(), between(10, defaultProcessors - 1)).build()
+            );
+        } // else we cannot run this check, the machine running the tests doesn't have enough CPUs
+
+        if (1 < defaultProcessors) {
+            final var fewProcessors = between(1, Math.min(10, defaultProcessors - 1));
+            ensureThrottling(fewProcessors, Settings.builder().put(EsExecutors.NODE_PROCESSORS_SETTING.getKey(), fewProcessors).build());
+        } // else we cannot run this check, the machine running the tests has less than 2 whole CPUs (and we already tested the 1 case)
+
+        // but in any case we can override the throttle regardless of its default value
+        final var override = between(1, defaultProcessors * 2);
+        ensureThrottling(
+            override,
+            Settings.builder().put(IndicesClusterStateService.CONCURRENT_SHARD_CLOSE_LIMIT.getKey(), override).build()
+        );
+    }
+
+    private static void ensureThrottling(int expectedLimit, Settings settings) {
+        final var tasksToRun = new ArrayList<Runnable>(expectedLimit + 1);
+        final var executor = new IndicesClusterStateService.ShardCloseExecutor(settings, tasksToRun::add);
+        final var runCount = new AtomicInteger();
+
+        // enqueue one more task than the throttling limit
+        for (int i = 0; i < expectedLimit + 1; i++) {
+            executor.execute(runCount::incrementAndGet);
+        }
+
+        // check that we submitted tasks up to the expected limit, holding back the final task behind the throttle for now
+        assertEquals(expectedLimit, tasksToRun.size());
+
+        // now execute all the tasks one by one
+        for (int i = 0; i < expectedLimit + 1; i++) {
+            assertEquals(i, runCount.get());
+            tasksToRun.get(i).run();
+            assertEquals(i + 1, runCount.get());
+
+            // executing the first task enqueues the final task
+            assertEquals(expectedLimit + 1, tasksToRun.size());
+        }
+    }
+}