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Date: Tue, 16 Feb 2010 15:31:28 +0000 (UTC)
From: "Jay Booth (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Subject: [jira] Commented: (HDFS-918) Use single Selector and small thread
 pool to replace many instances of BlockSender for reads
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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 ] 

Jay Booth commented on HDFS-918:
--------------------------------

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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