@Override
  public int getClusterNodeCount() {
    readLock.lock();
    try {
      return activeInstances.getAll().size();
    } finally {
      readLock.unlock();
    }
  }
The difference between this and getTotalResources() is that this only gives currently free
resource instances, while the other lists all the instances that may become available in a
while.
llap-tez/src/java/org/apache/hadoop/hive/llap/tezplugins/LlapTaskSchedulerService.java