@Override
  public void waitForFreeSlot(long id, int periodToTrigger, Consumer<Long> trigger)
    throws InterruptedIOException {
    waitForMaximumCurrentTasks(maxTotalConcurrentTasks - 1, id, periodToTrigger, trigger);
  }
The number of task simultaneously executed on a single region server.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/SimpleRequestController.java