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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3672) support for persistent connections to improve random read performance.
Date Mon, 14 Jul 2008 23:35:31 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3672?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12613488#action_12613488
] 

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

As final note, I agree completely that we have to improve resource consumption of HDFS read()
and make it behave decently for most users. Right now slow readers inadvertently hold up lot
of resources (threads, connections, kernel buffers etc) and random reads using read() wastes
network bandwidth in addition.


> 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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