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https://issues.apache.org/jira/browse/HDFS-918?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12828009#action_12828009
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Jay Booth commented on HDFS-918:
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I haven't had a chance to run benchmarks yet, but I think that under lots of
connections, the thread-per-connection model will spend more time swapping
compared to getting work done, plus it has a few places where they "hot block"
by doing while (buff.hasRemaining()) { write() }. Only selecting the currently
writeable connections and scheduling them sidesteps both issues while being
less of a resource footprint - assuming it delivers on the performance. As
soon as I get a chance, I'll write some benchmarks.
If anyone wants to take a look at the code in the meantime, I think this patch
is pretty easy to set up -- just enable MultiplexBlockSender.LOG for TRACE and
run tests, and you can see how each packet is built and sent. 'ant compile
eclipse-files' will set up the extra dependencies on commons-pool and
commons-math.
> Use single Selector and small thread pool to replace many instances of
> BlockSender for reads
> --------------------------------------------------------------------------------------------
>
> Key: HDFS-918
> URL: https://issues.apache.org/jira/browse/HDFS-918
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: data-node
> Reporter: Jay Booth
> Fix For: 0.22.0
>
> Attachments: hdfs-918-20100201.patch, hdfs-multiplex.patch
>
>
> Currently, on read requests, the DataXCeiver server allocates a new thread
> per request, which must allocate its own buffers and leads to
> higher-than-optimal CPU and memory usage by the sending threads. If we had a
> single selector and a small threadpool to multiplex request packets, we could
> theoretically achieve higher performance while taking up fewer resources and
> leaving more CPU on datanodes available for mapred, hbase or whatever. This
> can be done without changing any wire protocols.
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