YARN-7401. Reduce lock contention in ClusterNodeTracker#getClusterCapacity()

(cherry picked from commit 8db9d61ac2e04888cb228b29fe54b41c730cf0e6)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/ClusterNodeTracker.java

@@ -55,8 +55,9 @@ public class ClusterNodeTracker<N extends SchedulerNode> {
   private Map<String, N> nodeNameToNodeMap = new HashMap<>();
   private Map<String, List<N>> nodesPerRack = new HashMap<>();
 
-  private Resource clusterCapacity = Resources.clone(Resources.none());
-  private Resource staleClusterCapacity = null;
+  private final Resource clusterCapacity = Resources.clone(Resources.none());
+  private volatile Resource staleClusterCapacity =
+      Resources.clone(Resources.none());
 
   // Max allocation
   private long maxNodeMemory = -1;
@@ -82,6 +83,7 @@ public class ClusterNodeTracker<N extends SchedulerNode> {
 
       // Update cluster capacity
       Resources.addTo(clusterCapacity, node.getTotalResource());
+      staleClusterCapacity = Resources.clone(clusterCapacity);
 
       // Update maximumAllocation
       updateMaxResources(node, true);
@@ -139,16 +141,7 @@ public class ClusterNodeTracker<N extends SchedulerNode> {
   }
 
   public Resource getClusterCapacity() {
-    readLock.lock();
-    try {
-      if (staleClusterCapacity == null ||
-          !Resources.equals(staleClusterCapacity, clusterCapacity)) {
-        staleClusterCapacity = Resources.clone(clusterCapacity);
-      }
-      return staleClusterCapacity;
-    } finally {
-      readLock.unlock();
-    }
+    return staleClusterCapacity;
   }
 
   public N removeNode(NodeId nodeId) {
@@ -175,6 +168,7 @@ public class ClusterNodeTracker<N extends SchedulerNode> {
 
       // Update cluster capacity
       Resources.subtractFrom(clusterCapacity, node.getTotalResource());
+      staleClusterCapacity = Resources.clone(clusterCapacity);
 
       // Update maximumAllocation
       updateMaxResources(node, false);