Our HBase team are trying to read the blocks from HDFS into pooled offheap ByteBuffers directly (
HBASE-21879), and recently we had some benchmark, found that almost 45% heap allocation from the DFS client. The heap allocation flame graph can be see here: https://issues.apache.org/jira/secure/attachment/12970295/async-prof-pid-25042-alloc-2.svg
After checking the code path, we found that when requesting file descriptors from a DomainPeer, we allocated huge 8KB buffer for BufferedOutputStream, though the protocal content was quite small and just few bytes.
It made a heavy GC pressure for HBase when cacheHitRatio < 60%, which increased the HBase P999 latency. Actually, we can pre-allocate a small buffer for the BufferedOutputStream, such as 512 bytes, it's enough to read the short-circuit fd protocal content. we've created a patch like that, and the allocation flame graph show that after the patch, the heap allocation from DFS client dropped from 45% to 27%, that's a very good thing I think. see: https://issues.apache.org/jira/secure/attachment/12970475/async-prof-pid-24534-alloc-2.svg
Hope this attached patch can be merged into HDFS trunk, also Hadoop-2.8.x, HBase will benifit a lot from this.