Todd Lipcon created HBASE-5979:
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Summary: Non-pread DFSInputStreams should be associated with
scanners, not HFile.Readers
Key: HBASE-5979
URL: https://issues.apache.org/jira/browse/HBASE-5979
Project: HBase
Issue Type: Improvement
Components: performance, regionserver
Reporter: Todd Lipcon
Currently, every HFile.Reader has a single DFSInputStream, which it uses to
service all gets and scans. For gets, we use the positional read API (aka
"pread") and for scans we use a synchronized block to seek, then read. The
advantage of pread is that it doesn't hold any locks, so multiple gets can
proceed at the same time. The advantage of seek+read for scans is that the
datanode starts to send the entire rest of the HDFS block, rather than just the
single hfile block necessary. So, in a single thread, pread is faster for gets,
and seek+read is faster for scans since you get a strong pipelining effect.
However, in a multi-threaded case where there are multiple scans (including
scans which are actually part of compactions), the seek+read strategy falls
apart, since only one scanner may be reading at a time. Additionally, a large
amount of wasted IO is generated on the datanode side, and we get none of the
earlier-mentioned advantages.
In one test, I switched scans to always use pread, and saw a 5x improvement in
throughput of the YCSB scan-only workload, since it previously was completely
blocked by contention on the DFSIS lock.
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