private LlapCacheableBuffer evictFromHeapUnderLock(long time) {
    while (true) {
      if (heapSize == 0) {
        return null;
      }
      LlapCacheableBuffer result = evictHeapElementUnderLock(time, 0);
      if (result != null) {
        return result;
      }
    }
  }
Flushes all BPWrappers which will in turn clear IO-Threads' threadlocal buffers.
llap-server/src/java/org/apache/hadoop/hive/llap/cache/LowLevelLrfuCachePolicy.java