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Date: Tue, 1 Jul 2008 15:52:45 -0700 (PDT)
From: "Raghu Angadi (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-3672) support for persistent connections
 to improve random read performance.
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    [ https://issues.apache.org/jira/browse/HADOOP-3672?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12609740#action_12609740 ] 

Raghu Angadi commented on HADOOP-3672:
--------------------------------------

What is the avg size of pread()? Could you attach your benchmark to the jira? 

> support for persistent connections to improve random read performance.
> ----------------------------------------------------------------------
>
>                 Key: HADOOP-3672
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3672
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: dfs
>    Affects Versions: 0.17.0
>         Environment: Linux 2.6.9-55  , Dual Core Opteron 280 2.4Ghz , 4GB memory
>            Reporter: George Wu
>
> preads() establish new connections per request. yourkit java profiles show that this connection overhead is pretty significant on the DataNode. 
> I wrote a simple microbenchmark program which does many iterations of pread() from different offsets of a large file. I hacked DFSClient/DataNode code to re-use the same connection/DataNode request handler thread. The performance improvement was 7% when the data is served from disk and 80% when the data is served from the OS page cache.

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