@Override
  public void incTaskCounters(Collection<byte[]> regions, ServerName sn) {
    tasksInProgress.incrementAndGet();

    computeIfAbsent(taskCounterPerServer, sn, AtomicInteger::new).incrementAndGet();

    regions
      .forEach((regBytes) -> computeIfAbsent(taskCounterPerRegion, regBytes, AtomicInteger::new)
        .incrementAndGet());
  }
The number of task simultaneously executed on a single region server.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/SimpleRequestController.java