HBASE-25528: Dedicated merge dispatch threadpool on master (#2904)

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.

Signed-off-by: Wellington Chevreuil <wchevreuil@apache.org>
Signed-off-by: Viraj Jasani <vjasani@apache.org>

Author: bharathv

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

@@ -1605,6 +1605,13 @@ public enum OperationStatusCode {
       "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/executor/EventType.java

@@ -152,7 +152,7 @@ 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-server/src/main/java/org/apache/hadoop/hbase/executor/ExecutorType.java

@@ -35,6 +35,7 @@ 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-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java

@@ -1324,6 +1324,9 @@ private void startServiceThreads() throws IOException {
       HConstants.MASTER_LOG_REPLAY_OPS_THREADS, HConstants.MASTER_LOG_REPLAY_OPS_THREADS_DEFAULT));
     this.executorService.startExecutorService(ExecutorType.MASTER_SNAPSHOT_OPERATIONS, conf.getInt(
       SnapshotManager.SNAPSHOT_POOL_THREADS_KEY, SnapshotManager.SNAPSHOT_POOL_THREADS_DEFAULT));
+    this.executorService.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

@@ -209,14 +209,21 @@ public List<NormalizationPlan> computePlansForTable(final TableName table) {
       ctx.getTableRegions().size());
 
     final List<NormalizationPlan> plans = new ArrayList<>();
+    int splitPlansCount = 0;
     if (proceedWithSplitPlanning) {
-      plans.addAll(computeSplitNormalizationPlans(ctx));
+      List<NormalizationPlan> splitPlans = computeSplitNormalizationPlans(ctx);
+      splitPlansCount = splitPlans.size();
+      plans.addAll(splitPlans);
     }
+    int mergePlansCount = 0;
     if (proceedWithMergePlanning) {
-      plans.addAll(computeMergeNormalizationPlans(ctx));
+      List<NormalizationPlan> mergePlans = computeMergeNormalizationPlans(ctx);
+      mergePlansCount = mergePlans.size();
+      plans.addAll(mergePlans);
     }
 
-    LOG.debug("Computed {} normalization plans for table {}", plans.size(), table);
+    LOG.debug("Computed normalization plans for table {}. Total plans: {}, split plans: {}, " +
+        "merge plans: {}", table, plans.size(), splitPlansCount, mergePlansCount);
     return plans;
   }