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https://issues.apache.org/jira/browse/HADOOP-6502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13208203#comment-13208203
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Aaron T. Myers commented on HADOOP-6502:
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Tiny nit: In "} else {//check already performed on this class name" please put
a space between "{" and "//".
Otherwise the patch looks good to me. +1.
> 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
> Assignee: Todd Lipcon
> Priority: Critical
> Attachments: 6502.patch, 6502_v2.patch, hadoop-6502-trunk.txt,
> hadoop-6502-trunk.txt
>
>
> 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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