private void flushAllBPWrappers() {
    for (BPWrapper bpWrapper : bpWrappers.values()) {
      bpWrapper.lock.lock();
      try {
        bpWrapper.flush();
      } finally {
        bpWrapper.lock.unlock();
      }
    }
  }
Flushes all BPWrappers which will in turn clear IO-Threads' threadlocal buffers.
llap-server/src/java/org/apache/hadoop/hive/llap/cache/LowLevelLrfuCachePolicy.java