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From "Eli Collins (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1140) Speedup INode.getPathComponents
Date Thu, 27 May 2010 00:51:38 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1140?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12872041#action_12872041
] 

Eli Collins commented on HDFS-1140:
-----------------------------------

Makes sense. I was mostly curious to get your thoughts on how hard it would be to use byte[]
throughout. It's probably not worth refactoring the code to have eg INode#name be an index
into a path byte[].

> Speedup INode.getPathComponents
> -------------------------------
>
>                 Key: HDFS-1140
>                 URL: https://issues.apache.org/jira/browse/HDFS-1140
>             Project: Hadoop HDFS
>          Issue Type: Bug
>            Reporter: Dmytro Molkov
>            Assignee: Dmytro Molkov
>         Attachments: HDFS-1140.2.patch, HDFS-1140.patch
>
>
> When the namenode is loading the image there is a significant amount of time being spent
in the DFSUtil.string2Bytes. We have a very specific workload here. The path that namenode
does getPathComponents for shares N - 1 component with the previous path this method was called
for (assuming current path has N components).
> Hence we can improve the image load time by caching the result of previous conversion.
> We thought of using some simple LRU cache for components, but the reality is, String.getBytes
gets optimized during runtime and LRU cache doesn't perform as well, however using just the
latest path components and their translation to bytes in two arrays gives quite a performance
boost.
> I could get another 20% off of the time to load the image on our cluster (30 seconds
vs 24) and I wrote a simple benchmark that tests performance with and without caching.

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