Process more expensive allocation deciders last (#20724)

Today, the individual allocation deciders appear in random
order when initialized in AllocationDeciders, which means
potentially more performance intensive allocation deciders
could run before less expensive deciders. This adds to the
execution time when a less expensive decider could terminate
the decision making process early with a NO decision. This
commit orders the initialization of allocation deciders,
based on a general assessment of the big O runtime of each
decider, moving the likely more expensive deciders last.

Closes #12815

core/src/main/java/org/elasticsearch/cluster/ClusterModule.java

@@ -19,12 +19,10 @@
 
 package org.elasticsearch.cluster;
 
-import org.apache.logging.log4j.Logger;
 import org.elasticsearch.cluster.action.index.MappingUpdatedAction;
 import org.elasticsearch.cluster.action.index.NodeMappingRefreshAction;
 import org.elasticsearch.cluster.action.shard.ShardStateAction;
 import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
-import org.elasticsearch.cluster.metadata.IndexTemplateFilter;
 import org.elasticsearch.cluster.metadata.MetaDataCreateIndexService;
 import org.elasticsearch.cluster.metadata.MetaDataDeleteIndexService;
 import org.elasticsearch.cluster.metadata.MetaDataIndexAliasesService;
@@ -55,18 +53,17 @@ import org.elasticsearch.cluster.routing.allocation.decider.SnapshotInProgressAl
 import org.elasticsearch.cluster.routing.allocation.decider.ThrottlingAllocationDecider;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.inject.AbstractModule;
-import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Setting.Property;
 import org.elasticsearch.common.settings.Settings;
-import org.elasticsearch.common.util.ExtensionPoint;
 import org.elasticsearch.gateway.GatewayAllocator;
 import org.elasticsearch.plugins.ClusterPlugin;
 import org.elasticsearch.tasks.TaskResultsService;
 
 import java.util.Collection;
 import java.util.HashMap;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -109,21 +106,21 @@ public class ClusterModule extends AbstractModule {
     public static Collection<AllocationDecider> createAllocationDeciders(Settings settings, ClusterSettings clusterSettings,
                                                                          List<ClusterPlugin> clusterPlugins) {
         // collect deciders by class so that we can detect duplicates
-        Map<Class, AllocationDecider> deciders = new HashMap<>();
+        Map<Class, AllocationDecider> deciders = new LinkedHashMap<>();
         addAllocationDecider(deciders, new MaxRetryAllocationDecider(settings));
-        addAllocationDecider(deciders, new SameShardAllocationDecider(settings));
-        addAllocationDecider(deciders, new FilterAllocationDecider(settings, clusterSettings));
         addAllocationDecider(deciders, new ReplicaAfterPrimaryActiveAllocationDecider(settings));
-        addAllocationDecider(deciders, new ThrottlingAllocationDecider(settings, clusterSettings));
         addAllocationDecider(deciders, new RebalanceOnlyWhenActiveAllocationDecider(settings));
         addAllocationDecider(deciders, new ClusterRebalanceAllocationDecider(settings, clusterSettings));
         addAllocationDecider(deciders, new ConcurrentRebalanceAllocationDecider(settings, clusterSettings));
         addAllocationDecider(deciders, new EnableAllocationDecider(settings, clusterSettings));
-        addAllocationDecider(deciders, new AwarenessAllocationDecider(settings, clusterSettings));
-        addAllocationDecider(deciders, new ShardsLimitAllocationDecider(settings, clusterSettings));
         addAllocationDecider(deciders, new NodeVersionAllocationDecider(settings));
-        addAllocationDecider(deciders, new DiskThresholdDecider(settings, clusterSettings));
         addAllocationDecider(deciders, new SnapshotInProgressAllocationDecider(settings, clusterSettings));
+        addAllocationDecider(deciders, new FilterAllocationDecider(settings, clusterSettings));
+        addAllocationDecider(deciders, new SameShardAllocationDecider(settings));
+        addAllocationDecider(deciders, new DiskThresholdDecider(settings, clusterSettings));
+        addAllocationDecider(deciders, new ThrottlingAllocationDecider(settings, clusterSettings));
+        addAllocationDecider(deciders, new ShardsLimitAllocationDecider(settings, clusterSettings));
+        addAllocationDecider(deciders, new AwarenessAllocationDecider(settings, clusterSettings));
 
         clusterPlugins.stream()
             .flatMap(p -> p.createAllocationDeciders(settings, clusterSettings).stream())

core/src/test/java/org/elasticsearch/cluster/ClusterModuleTests.java

@@ -19,17 +19,25 @@
 
 package org.elasticsearch.cluster;
 
-import org.elasticsearch.action.admin.indices.create.CreateIndexClusterStateUpdateRequest;
-import org.elasticsearch.cluster.metadata.IndexTemplateFilter;
-import org.elasticsearch.cluster.metadata.IndexTemplateMetaData;
 import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
-import org.elasticsearch.cluster.routing.allocation.allocator.BalancedShardsAllocator;
 import org.elasticsearch.cluster.routing.allocation.allocator.ShardsAllocator;
 import org.elasticsearch.cluster.routing.allocation.decider.AllocationDecider;
-import org.elasticsearch.cluster.routing.allocation.decider.AllocationDeciders;
+import org.elasticsearch.cluster.routing.allocation.decider.AwarenessAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.ClusterRebalanceAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.ConcurrentRebalanceAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.DiskThresholdDecider;
 import org.elasticsearch.cluster.routing.allocation.decider.EnableAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.FilterAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.MaxRetryAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.NodeVersionAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.RebalanceOnlyWhenActiveAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.ReplicaAfterPrimaryActiveAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.SameShardAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.ShardsLimitAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.SnapshotInProgressAllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.ThrottlingAllocationDecider;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.inject.ModuleTestCase;
 import org.elasticsearch.common.settings.ClusterSettings;
@@ -40,9 +48,12 @@ import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.settings.SettingsModule;
 import org.elasticsearch.plugins.ClusterPlugin;
 
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
 import java.util.Map;
 import java.util.function.Supplier;
 
@@ -156,4 +167,34 @@ public class ClusterModuleTests extends ModuleTestCase {
         NullPointerException e = expectThrows(NullPointerException.class, () ->
             newClusterModuleWithShardsAllocator(settings, "bad", () -> null));
     }
+
+    // makes sure that the allocation deciders are setup in the correct order, such that the
+    // slower allocation deciders come last and we can exit early if there is a NO decision without
+    // running them. If the order of the deciders is changed for a valid reason, the order should be
+    // changed in the test too.
+    public void testAllocationDeciderOrder() {
+        List<Class<? extends AllocationDecider>> expectedDeciders = Arrays.asList(
+            MaxRetryAllocationDecider.class,
+            ReplicaAfterPrimaryActiveAllocationDecider.class,
+            RebalanceOnlyWhenActiveAllocationDecider.class,
+            ClusterRebalanceAllocationDecider.class,
+            ConcurrentRebalanceAllocationDecider.class,
+            EnableAllocationDecider.class,
+            NodeVersionAllocationDecider.class,
+            SnapshotInProgressAllocationDecider.class,
+            FilterAllocationDecider.class,
+            SameShardAllocationDecider.class,
+            DiskThresholdDecider.class,
+            ThrottlingAllocationDecider.class,
+            ShardsLimitAllocationDecider.class,
+            AwarenessAllocationDecider.class);
+        Collection<AllocationDecider> deciders = ClusterModule.createAllocationDeciders(Settings.EMPTY,
+            new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS), Collections.emptyList());
+        Iterator<AllocationDecider> iter = deciders.iterator();
+        int idx = 0;
+        while (iter.hasNext()) {
+            AllocationDecider decider = iter.next();
+            assertSame(decider.getClass(), expectedDeciders.get(idx++));
+        }
+    }
 }