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From "Jay Booth (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-918) Use single Selector and small thread pool to replace many instances of BlockSender for reads
Date Fri, 12 Mar 2010 18:14:27 GMT

    [ https://issues.apache.org/jira/browse/HDFS-918?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12844596#action_12844596

Jay Booth commented on HDFS-918:

.bq I think it is very important to have separate pools for each partition. Otherwise, each
disk will be accessed only as much as the slowest disk (when DN has enough load).

This would be the case if I were using a fixed-size thread pool and a LinkedBlockingQueue
-- but I'm not, see Executors.newCachedThreadPool(), it's actually bounded at Integer.MAX_VALUE
threads and uses a SynchronousQueue.  If a new thread is needed in order to start work on
a task immediately, it's created.  Otherwise, an existing waiting thread will be re-used.
 (Threads are purged if they've been idle for 60 seconds).  Either way, the underlying I/O
request is dispatched pretty much immediately after the connection is writable.  So I don't
see why separate pools per partition would help anything, the operating system will handle
IO requests as it can and put threads into runnable state as it can regardless of which pool
they're in.

RE: Netty, I'm not very knowledgeable about it beyond the Cliff's Notes version, but my code
dealing with the Selector is pretty small -- the main loop is under 75 lines, and java.util.concurrent
does most of the heavy lifting.  Most of the code is dealing with application and protocol
specifics.  So my instinct in general is that adding a framework may actually increase the
amount of code, especially if there's any mismatches between what we're doing and what it
wants us to do (the packet-header, sums data, main data format is pretty specific to us).
 Plus, as Todd said, we can't really change the blocking IO nature of the main accept() loop
in DataXceiverServer without this becoming a much bigger patch, although I agree that we should
go there in general.  That being said, better is better, so if a Netty implementation took
up fewer lines of code and performed better, then that speaks for itself.

> Use single Selector and small thread pool to replace many instances of BlockSender for
> --------------------------------------------------------------------------------------------
>                 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-918-20100228.patch, hdfs-918-20100309.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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