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https://issues.apache.org/jira/browse/HDFS-918?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12829293#action_12829293
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ryan rawson commented on HDFS-918:
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I have done some thinking about HBase performance in relation to HDFS, and
right now we are currently bottlenecking reads to a single file. By moving to a
re-entrant API (pread) we are looking to unleash the parallelism. This is
important I think, because we want to push as many parallel reads from our
clients down into Datanode then down into the kernel to benefit from the IO
scheduling in the kernel & hardware.
This could mean we might expect literally dozens of parallel reads per node on
a busy cluster. Perhaps even hundreds! Per node. To ensure scalbility we'd
probably want to get away from the xciever model, for more than 1 reason... If
I remember correctly, xcivers not only consume threads (hundreds of threads is
OK but non ideal) but it also consumes epolls, and there is just so many epolls
available. So I heartily approve of the direction of this JIRA!
> 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-918-20100203.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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