Zheng Hu created HDFS-14535:
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Summary: The default 8KB buffer in
requestFileDescriptors#BufferedOutputStream is causing lots of heap allocation
in HBase when using short-circut read
Key: HDFS-14535
URL: https://issues.apache.org/jira/browse/HDFS-14535
Project: Hadoop HDFS
Issue Type: Bug
Reporter: Zheng Hu
Assignee: Zheng Hu
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.
Hope this attached patch can be merged into HDFS trunk, also Hadoop-2.8.x,
HBase will benifit a lot from this.
Thanks.
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