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Date: Thu, 4 Feb 2010 01:20:28 +0000 (UTC)
From: "dhruba borthakur (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=12829364#action_12829364 ] 

dhruba borthakur commented on HDFS-918:
---------------------------------------

> because we want to push as many parallel reads from our clients down into Datanode 

Which part of HBase are you referring to here? I can understand that HBase can use pread to read the contents of a file but am not able to understand why the same file be read by multiple region servers at the same time.

> 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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