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From "Arun C Murthy (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6502) DistributedFileSystem#listStatus is very slow when listing a directory with a size of 1300
Date Sat, 23 Jan 2010 07:59:17 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12804034#action_12804034
] 

Arun C Murthy commented on HADOOP-6502:
---------------------------------------

I don't see much of a problem with caching negatives, but I'm not sure we need to anything
here at all... I can go either way. *shrug*

> DistributedFileSystem#listStatus is very slow when listing a directory with a size of
1300
> ------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-6502
>                 URL: https://issues.apache.org/jira/browse/HADOOP-6502
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: util
>    Affects Versions: 0.20.0
>            Reporter: Hairong Kuang
>            Priority: Critical
>             Fix For: 0.20.2, 0.21.0, 0.22.0
>
>
> When listing a directory of around 1300 children, it takes hundreds of milliseconds.
It turns out the slowdowness is caused by the change made by HADOOP-4187. The return value
of listStatus is an array of FileStatus. When deserializing each element of the array, ReflectionUtils#newInstance(Class<T>,
Configuration) is called and then calls setConf, which calls setJobConf. SetJobConf checks
if JobConf is on the class path by calling Configuration#getClassByName. Even though Configuration#getClassByName
tries to optimize the lookup using a cached map, but since JobConf is not in the class path,
so it is not in the cache. Every checkup ends up calling Class.ForName which is very expensive.
Deserializing an array of 1300 entries requires calling of Class#ForName 1300 times!

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