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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6502) DistributedFileSystem#listStatus is very slow when listing a directory with a size of 1300
Date Fri, 22 Jan 2010 16:55:21 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12803759#action_12803759
] 

Doug Cutting commented on HADOOP-6502:
--------------------------------------

I don't think it's appropriate to cache JVM state in the configuration.

I think we can better fix this by changing Configuration#getClassByName() to cache failures
as well as successes.

> Will we ever have a case where the JobConf isn't in the classpath once, and then later
shows up because the classloader changed?

The cache in Configuration is per-classloader.  So as long as we go through that we should
be safe.


> 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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