[
https://issues.apache.org/jira/browse/HBASE-5979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14050511#comment-14050511
]
Vladimir Rodionov commented on HBASE-5979:
------------------------------------------
{quote}
The advantage of pread is that it doesn't hold any locks, so multiple gets can
proceed at the same time.
{quote}
Interesting .. This is th code snippet from FSInputStream (Hadoop 2.2)
{code}
@Override
public int read(long position, byte[] buffer, int offset, int length)
throws IOException {
synchronized (this) {
long oldPos = getPos();
int nread = -1;
try {
seek(position);
nread = read(buffer, offset, length);
} finally {
seek(oldPos);
}
return nread;
}
}
{code}
*Positional read is implemented with a lock*. DFSInputStream extends the above
class and does not override the default implementation. You have no parallelism
even for simple gets.
> 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.
--
This message was sent by Atlassian JIRA
(v6.2#6252)
