HDFS-11742. Improve balancer usability after HDFS-8818. Contributed by Kihwal Lee

Updated CHANGES.txt
(cherry picked from commit e229ffee64e8abd1df4d819b81151582ed3a16e2)

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -394,6 +394,8 @@ Release 2.7.4 - UNRELEASED
     HDFS-12177. NameNode exits due to setting BlockPlacementPolicy loglevel to Debug.
     (Jiandan Yang via Brahma Reddy Battula)
 
+    HDFS-11742. Improve balancer usability after HDFS-8818. (kihwal)
+
 Release 2.7.3 - 2016-08-25
 
   INCOMPATIBLE CHANGES

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/Dispatcher.java

@@ -118,6 +118,7 @@ public class Dispatcher {
 
   /** The maximum number of concurrent blocks moves at a datanode */
   private final int maxConcurrentMovesPerNode;
+  private final int maxMoverThreads;
 
   private final long getBlocksSize;
   private final long getBlocksMinBlockSize;
@@ -131,11 +132,13 @@ public class Dispatcher {
   static class Allocator {
     private final int max;
     private int count = 0;
+    private int lotSize = 1;
 
     Allocator(int max) {
       this.max = max;
     }
 
+    /** Allocate specified number of items */
     synchronized int allocate(int n) {
       final int remaining = max - count;
       if (remaining <= 0) {
@@ -147,9 +150,19 @@ public class Dispatcher {
       }
     }
 
+    /** Aloocate a single lot of items */
+    int allocate() {
+      return allocate(lotSize);
+    }
+
     synchronized void reset() {
       count = 0;
     }
+
+    /** Set the lot size */
+    synchronized void setLotSize(int lotSize) {
+      this.lotSize = lotSize;
+    }
   }
 
   private static class GlobalBlockMap {
@@ -905,6 +918,7 @@ public class Dispatcher {
     this.dispatchExecutor = dispatcherThreads == 0? null
         : Executors.newFixedThreadPool(dispatcherThreads);
     this.moverThreadAllocator = new Allocator(moverThreads);
+    this.maxMoverThreads = moverThreads;
     this.maxConcurrentMovesPerNode = maxConcurrentMovesPerNode;
 
     this.getBlocksSize = getBlocksSize;
@@ -999,7 +1013,7 @@ public class Dispatcher {
     final DDatanode targetDn = p.target.getDDatanode();
     ExecutorService moveExecutor = targetDn.getMoveExecutor();
     if (moveExecutor == null) {
-      final int nThreads = moverThreadAllocator.allocate(maxConcurrentMovesPerNode);
+      final int nThreads = moverThreadAllocator.allocate();
       if (nThreads > 0) {
         moveExecutor = targetDn.initMoveExecutor(nThreads);
       }
@@ -1050,6 +1064,25 @@ public class Dispatcher {
       LOG.debug("Disperse Interval sec = " +
           concurrentThreads / BALANCER_NUM_RPC_PER_SEC);
     }
+
+    // Determine the size of each mover thread pool per target
+    int threadsPerTarget = maxMoverThreads/targets.size();
+    if (threadsPerTarget == 0) {
+      // Some scheduled moves will get ignored as some targets won't have
+      // any threads allocated.
+      moverThreadAllocator.setLotSize(1);
+      LOG.warn(DFSConfigKeys.DFS_BALANCER_MOVERTHREADS_KEY + "=" +
+          maxMoverThreads + " is too small for moving blocks to " +
+          targets.size() + " targets. Balancing may be slower.");
+    } else {
+      if  (threadsPerTarget > maxConcurrentMovesPerNode) {
+        threadsPerTarget = maxConcurrentMovesPerNode;
+        LOG.info("Limiting threads per target to the specified max.");
+      }
+      moverThreadAllocator.setLotSize(threadsPerTarget);
+      LOG.info("Allocating " + threadsPerTarget + " threads per target.");
+    }
+
     long dSec = 0;
     final Iterator<Source> i = sources.iterator();
     for (int j = 0; j < futures.length; j++) {