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https://issues.apache.org/jira/browse/HDFS-918?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12834252#action_12834252
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Jay Booth commented on HDFS-918:
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Yeah, I'll do my best to get benchmarks by the end of the weekend, kind of a
crazy week this week and I moved this past weekend, so I don't have a ton of
time. Todd, if you feel like blasting a couple stream-of-consciousness
comments to me via email, go right ahead, otherwise I'll run the benchmarks
this weekend and wait for the well-written version :).
Zlatin, I originally had a similar architecture to what you're describing,
using a BlockingQueue to funnel the actual work to a threadpool, but I had some
issues with being able to get the locking quite right, either I wasn't getting
things into the queue as fast as possible, or I was burning a lot of empty
cycles in the selector thread. Specifically, I can't cancel a SelectionKey and
then re-register with the same selector afterwards, it leads to exceptions, so
my Selector thread was spinning in a tight loop verifying that, yes, all of
these writable SelectionKeys are currently enqueued for work, whenever anything
was being processed. But that was a couple iterations ago, maybe I'll have
better luck trying it now. What we really need is a libevent-like framework,
I'll spend a little time reviewing the outward facing API for that and
scratching my noggin.
Ultimately, only so much I/O can actually happen at a time before the disk is
swamped, so it might be that a set of, say, 32 selector threads gets the same
performance as 1024 threads. In that case, we'd be taking up fewer resources
for the same performance. At any rate, I need to benchmark before speculating
further.
> 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-918-20100211.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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