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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3672) support for persistent connections to improve pread() performance.
Date Tue, 01 Jul 2008 17:56:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3672?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12609652#action_12609652
] 

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

Both pread() and seek();read() would benefit greatly from a connection cache.  We'll probably
want to both limit the number of cached connections (so that we don't exhaust file handles)
and time-out idle connections.  A single connection to a datanode would ideally be used for
all client communications with that datanode.  For example, two threads reading different
blocks from the same datanode should multiplex their reads over a shared connection.

All this would be done for free if we used RPC to read from datanodes, no?  It would be good
to benchmark the performance of that approach.


> support for persistent connections to improve pread() 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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