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From "Tomasz Nykiel (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2477) Optimize computing the diff between a block report and the namenode state.
Date Tue, 25 Oct 2011 20:20:32 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13135393#comment-13135393

Tomasz Nykiel commented on HDFS-2477:

Thanks Konstantin for your comments, you got it exactly right, so I incorporated all your

The changes aren't huge code-wise, but the moveToHead() call is happening for (almost) every
block in every block report. 
With this changes I noticed quite substantial speed-up of reportDiff itself.

I updated the patch.
> Optimize computing the diff between a block report and the namenode state.
> --------------------------------------------------------------------------
>                 Key: HDFS-2477
>                 URL: https://issues.apache.org/jira/browse/HDFS-2477
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: name-node
>            Reporter: Tomasz Nykiel
>            Assignee: Tomasz Nykiel
>         Attachments: reportDiff.patch, reportDiff.patch-2, reportDiff.patch-3, reportDiff.patch-4
> When a block report is processed at the NN, the BlockManager.reportDiff traverses all
blocks contained in the report, and for each one block, which is also present in the corresponding
datanode descriptor, the block is moved to the head of the list of the blocks in this datanode
> With HDFS-395 the huge majority of the blocks in the report, are also present in the
datanode descriptor, which means that almost every block in the report will have to be moved
to the head of the list.
> Currently this operation is performed by DatanodeDescriptor.moveBlockToHead, which removes
a block from a list and then inserts it. In this process, we call findDatanode several times
(afair 6 times for each moveBlockToHead call). findDatanode is relatively expensive, since
it linearly goes through the triplets to locate the given datanode.
> With this patch, we do some memoization of findDatanode, so we can reclaim 2 findDatanode
calls. Our experiments show that this can improve the reportDiff (which is executed under
write lock) by around 15%. Currently with HDFS-395, reportDiff is responsible for almost 100%
of the block report processing time.

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