private ResizableBlockCache toResizableBlockCache(BlockCache blockCache) {
    if (blockCache instanceof CombinedBlockCache) {
      return (ResizableBlockCache) ((CombinedBlockCache) blockCache).getFirstLevelCache();
    } else {
      return (ResizableBlockCache) blockCache;
    }
  }
Manages tuning of Heap memory using <code>HeapMemoryTuner</code>. Most part of the heap memory is
split between Memstores and BlockCache. This manager helps in tuning sizes of both these
dynamically, as per the R/W load on the servers.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HeapMemoryManager.java