YARN-6864. FSPreemptionThread cleanup for readability. (Daniel Templeton via Yufei Gu)

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

@@ -66,11 +66,11 @@ class FSPreemptionThread extends Thread {
     schedulerReadLock = scheduler.getSchedulerReadLock();
   }
 
+  @Override
   public void run() {
     while (!Thread.interrupted()) {
-      FSAppAttempt starvedApp;
-      try{
-        starvedApp = context.getStarvedApps().take();
+      try {
+        FSAppAttempt starvedApp = context.getStarvedApps().take();
         // Hold the scheduler readlock so this is not concurrent with the
         // update thread.
         schedulerReadLock.lock();
@@ -82,7 +82,7 @@ class FSPreemptionThread extends Thread {
         starvedApp.preemptionTriggered(delayBeforeNextStarvationCheck);
       } catch (InterruptedException e) {
         LOG.info("Preemption thread interrupted! Exiting.");
-        return;
+        Thread.currentThread().interrupt();
       }
     }
   }
@@ -112,16 +112,19 @@ class FSPreemptionThread extends Thread {
         PreemptableContainers bestContainers = null;
         List<FSSchedulerNode> potentialNodes = scheduler.getNodeTracker()
             .getNodesByResourceName(rr.getResourceName());
+        int maxAMContainers = Integer.MAX_VALUE;
+
         for (FSSchedulerNode node : potentialNodes) {
-          int maxAMContainers = bestContainers == null ?
-              Integer.MAX_VALUE : bestContainers.numAMContainers;
           PreemptableContainers preemptableContainers =
               identifyContainersToPreemptOnNode(
                   rr.getCapability(), node, maxAMContainers);
+
           if (preemptableContainers != null) {
             // This set is better than any previously identified set.
             bestContainers = preemptableContainers;
-            if (preemptableContainers.numAMContainers == 0) {
+            maxAMContainers = bestContainers.numAMContainers;
+
+            if (maxAMContainers == 0) {
               break;
             }
           }
@@ -182,13 +185,10 @@ class FSPreemptionThread extends Thread {
         return preemptableContainers;
       }
     }
-    return null;
-  }
 
-  private boolean isNodeAlreadyReserved(
-      FSSchedulerNode node, FSAppAttempt app) {
-    FSAppAttempt nodeReservedApp = node.getReservedAppSchedulable();
-    return nodeReservedApp != null && !nodeReservedApp.equals(app);
+    // Return null if the sum of all preemptable containers' resources
+    // isn't enough to satisfy the starved request.
+    return null;
   }
 
   private void trackPreemptionsAgainstNode(List<RMContainer> containers,
@@ -214,7 +214,7 @@ class FSPreemptionThread extends Thread {
   }
 
   private class PreemptContainersTask extends TimerTask {
-    private List<RMContainer> containers;
+    private final List<RMContainer> containers;
 
     PreemptContainersTask(List<RMContainer> containers) {
       this.containers = containers;