HBASE-25528: Dedicated merge dispatch threadpool on master (#2909)

Adds "hbase.master.executor.merge.dispatch.threads" and defaults to 2.

Also adds additional logging that includes the number of split plans
and merge plans computed for each normalizer run.

(cherry picked from commit 36b4698cea)

Signed-off-by: Andrew Purtell <apurtell@apache.org>
Signed-off-by: Michael Stack <stack@apache.org>

hbase-client/src/main/java/org/apache/hadoop/hbase/executor/EventType.java

@@ -149,7 +149,7 @@ public enum EventType {
    * C_M_MERGE_REGION<br>
    * Client asking Master to merge regions.
    */
-  C_M_MERGE_REGION          (30, ExecutorType.MASTER_TABLE_OPERATIONS),
+  C_M_MERGE_REGION          (30, ExecutorType.MASTER_MERGE_OPERATIONS),
   /**
    * Messages originating from Client to Master.<br>
    * C_M_DELETE_TABLE<br>

hbase-client/src/main/java/org/apache/hadoop/hbase/executor/ExecutorType.java

@@ -37,6 +37,7 @@ public enum ExecutorType {
   MASTER_META_SERVER_OPERATIONS (6),
   M_LOG_REPLAY_OPS           (7),
   MASTER_SNAPSHOT_OPERATIONS (8),
+  MASTER_MERGE_OPERATIONS (9),
 
   // RegionServer executor services
   RS_OPEN_REGION             (20),

hbase-common/src/main/java/org/apache/hadoop/hbase/HConstants.java

@@ -1397,6 +1397,13 @@ public final class HConstants {
       "hbase.master.executor.serverops.threads";
   public static final int MASTER_SERVER_OPERATIONS_THREADS_DEFAULT = 5;
 
+  /**
+   * Number of threads used to dispatch merge operations to the regionservers.
+   */
+  public static final String MASTER_MERGE_DISPATCH_THREADS =
+      "hbase.master.executor.merge.dispatch.threads";
+  public static final int MASTER_MERGE_DISPATCH_THREADS_DEFAULT = 2;
+
   public static final String MASTER_META_SERVER_OPERATIONS_THREADS =
       "hbase.master.executor.meta.serverops.threads";
   public static final int MASTER_META_SERVER_OPERATIONS_THREADS_DEFAULT = 5;

hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java

@@ -1269,6 +1269,9 @@ public class HMaster extends HRegionServer implements MasterServices, Server {
       HConstants.MASTER_LOG_REPLAY_OPS_THREADS, HConstants.MASTER_LOG_REPLAY_OPS_THREADS_DEFAULT));
     this.service.startExecutorService(ExecutorType.MASTER_SNAPSHOT_OPERATIONS, conf.getInt(
       SnapshotManager.SNAPSHOT_POOL_THREADS_KEY, SnapshotManager.SNAPSHOT_POOL_THREADS_DEFAULT));
+    this.service.startExecutorService(ExecutorType.MASTER_MERGE_OPERATIONS, conf.getInt(
+        HConstants.MASTER_MERGE_DISPATCH_THREADS,
+        HConstants.MASTER_MERGE_DISPATCH_THREADS_DEFAULT));
 
    // We depend on there being only one instance of this executor running
    // at a time.  To do concurrency, would need fencing of enable/disable of

hbase-server/src/main/java/org/apache/hadoop/hbase/master/normalizer/SimpleRegionNormalizer.java

@@ -183,6 +183,8 @@ public class SimpleRegionNormalizer implements RegionNormalizer {
     LOG.debug("Table " + table + ", average region size: " + avgRegionSize);
 
     int candidateIdx = 0;
+    int splitCount = 0;
+    int mergeCount = 0;
     while (candidateIdx < tableRegions.size()) {
       HRegionInfo hri = tableRegions.get(candidateIdx);
       long regionSize = getRegionSize(hri);
@@ -193,6 +195,7 @@ public class SimpleRegionNormalizer implements RegionNormalizer {
           LOG.info("Table " + table + ", large region " + hri.getRegionNameAsString() + " has size "
               + regionSize + ", more than twice avg size, splitting");
           plans.add(new SplitNormalizationPlan(hri, null));
+          splitCount++;
         }
       } else {
         if (candidateIdx == tableRegions.size()-1) {
@@ -206,6 +209,7 @@ public class SimpleRegionNormalizer implements RegionNormalizer {
               + " plus its neighbor size: " + regionSize2
               + ", less than the avg size " + avgRegionSize + ", merging them");
             plans.add(new MergeNormalizationPlan(hri, hri2));
+            mergeCount++;
             candidateIdx++;
           }
         }
@@ -217,6 +221,10 @@ public class SimpleRegionNormalizer implements RegionNormalizer {
       return null;
     }
     Collections.sort(plans, planComparator);
+    if (LOG.isDebugEnabled()) {
+      LOG.debug(String.format("Computed normalization plans for table %s. Total plans %d, split " +
+          "plans %d, merge plans %d", table, plans.size(), splitCount, mergeCount));
+    }
     return plans;
   }