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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3672) support for persistent connections to improve random read performance.
Date Mon, 14 Jul 2008 21:42:31 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3672?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12613464#action_12613464
] 

Doug Cutting commented on HADOOP-3672:
--------------------------------------

Note that it should be possible to implement such a send/select API entirely in the client:
the wire protocol is already asynchronous in that responses can arrive out of order.  We mostly
just need to subclass Client.Call, overriding callComplete() to maintain a table of completed
calls.


> 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
>         Attachments: pread_test.java
>
>
> 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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