Deprecate setting processors to more than available (#44889)

Today the processors setting is permitted to be set to more than the
number of processors available to the JVM. The processors setting
directly sizes the number of threads in the various thread pools, with
most of these sizes being a linear function in the number of
processors. It doesn't make any sense to set processors very high as the
overhead from context switching amongst all the threads will overwhelm,
and changing the setting does not control how many physical CPU
resources there are on which to schedule the additional threads. We have
to draw a line somewhere and this commit deprecates setting processors
to more than the number of available processors. This is the right place
to draw the line given the linear growth as a function of processors in
most of the thread pools, and that some are capped at the number of
available processors already.

server/src/main/java/org/elasticsearch/common/util/concurrent/EsExecutors.java

@@ -19,8 +19,10 @@
 
 package org.elasticsearch.common.util.concurrent;
 
+import org.apache.logging.log4j.LogManager;
 import org.elasticsearch.ExceptionsHelper;
 import org.elasticsearch.common.SuppressForbidden;
+import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Setting.Property;
 import org.elasticsearch.common.settings.Settings;
@@ -46,12 +48,27 @@ import java.util.stream.Collectors;
 
 public class EsExecutors {
 
+    private static final DeprecationLogger deprecationLogger = new DeprecationLogger(LogManager.getLogger(EsExecutors.class));
+
     /**
-     * Settings key to manually set the number of available processors.
-     * This is used to adjust thread pools sizes etc. per node.
+     * Setting to manually set the number of available processors. This setting is used to adjust thread pool sizes per node.
      */
-    public static final Setting<Integer> PROCESSORS_SETTING =
-        Setting.intSetting("processors", Runtime.getRuntime().availableProcessors(), 1, Property.NodeScope);
+    public static final Setting<Integer> PROCESSORS_SETTING = new Setting<>(
+        "processors",
+        s -> Integer.toString(Runtime.getRuntime().availableProcessors()),
+        s -> {
+            final int value = Setting.parseInt(s, 1, "processors");
+            final int availableProcessors = Runtime.getRuntime().availableProcessors();
+            if (value > availableProcessors) {
+                deprecationLogger.deprecatedAndMaybeLog(
+                    "processors",
+                    "setting processors to value [{}] which is more than available processors [{}] is deprecated",
+                    value,
+                    availableProcessors);
+            }
+            return value;
+        },
+        Property.NodeScope);
 
     /**
      * Returns the number of available processors. Defaults to

test/framework/src/main/java/org/elasticsearch/test/InternalTestCluster.java

@@ -450,7 +450,7 @@ public final class InternalTestCluster extends TestCluster {
             builder.put(SearchService.DEFAULT_KEEPALIVE_SETTING.getKey(), timeValueSeconds(100 + random.nextInt(5 * 60)).getStringRep());
         }
 
-        builder.put(EsExecutors.PROCESSORS_SETTING.getKey(), 1 + random.nextInt(3));
+        builder.put(EsExecutors.PROCESSORS_SETTING.getKey(), 1 + random.nextInt(Math.min(4, Runtime.getRuntime().availableProcessors())));
         if (random.nextBoolean()) {
             if (random.nextBoolean()) {
                 builder.put("indices.fielddata.cache.size", 1 + random.nextInt(1000), ByteSizeUnit.MB);

x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/Watcher.java

@@ -515,11 +515,13 @@ public class Watcher extends Plugin implements ActionPlugin, ScriptPlugin, Reloa
      * @param settings The current settings
      * @return A number between 5 and the number of processors
      */
-    static int getWatcherThreadPoolSize(Settings settings) {
-        boolean isDataNode = Node.NODE_DATA_SETTING.get(settings);
+    static int getWatcherThreadPoolSize(final Settings settings) {
+        return getWatcherThreadPoolSize(Node.NODE_DATA_SETTING.get(settings), EsExecutors.numberOfProcessors(settings));
+    }
+
+    static int getWatcherThreadPoolSize(final boolean isDataNode, final int numberOfProcessors) {
         if (isDataNode) {
-            int numberOfProcessors = EsExecutors.numberOfProcessors(settings);
-            long size = Math.max(Math.min(5 * numberOfProcessors, 50), numberOfProcessors);
+            final long size = Math.max(Math.min(5 * numberOfProcessors, 50), numberOfProcessors);
             return Math.toIntExact(size);
         } else {
             return 1;

x-pack/plugin/watcher/src/test/java/org/elasticsearch/xpack/watcher/WatcherPluginTests.java

@@ -86,23 +86,19 @@ public class WatcherPluginTests extends ESTestCase {
 
     public void testThreadPoolSize() {
         // old calculation was 5 * number of processors
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 1).build()), is(5));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 2).build()), is(10));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 4).build()), is(20));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 8).build()), is(40));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 9).build()), is(45));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 10).build()), is(50));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 16).build()), is(50));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 24).build()), is(50));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 50).build()), is(50));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 51).build()), is(51));
-        assertThat(Watcher.getWatcherThreadPoolSize(Settings.builder().put("processors", 96).build()), is(96));
-
-        Settings noDataNodeSettings = Settings.builder()
-                .put("processors", scaledRandomIntBetween(1, 100))
-                .put("node.data", false)
-                .build();
-        assertThat(Watcher.getWatcherThreadPoolSize(noDataNodeSettings), is(1));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 1), is(5));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 2), is(10));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 4), is(20));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 8), is(40));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 9), is(45));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 10), is(50));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 16), is(50));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 24), is(50));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 50), is(50));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 51), is(51));
+        assertThat(Watcher.getWatcherThreadPoolSize(true, 96), is(96));
+
+        assertThat(Watcher.getWatcherThreadPoolSize(false, scaledRandomIntBetween(1, 100)), is(1));
     }
 
     public void testReload() {