public boolean isTerminated() {
    return workerPool.isTerminated();
  }
Select some subtrees to flush. The subtrees are selected from the MTreeStore by the sequence of
the regionIds. The number of subtrees to flush is determined by parameter {@link
Scheduler#BATCH_FLUSH_SUBTREE}. It will return asynchronously. If worker pool is full, the task
will be discarded directly.

@param regionIds determine the MTreeStore to select subtrees, the head of the list is the first
    MTreeStore to select subtrees
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/pbtree/flush/Scheduler.java