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https://issues.apache.org/jira/browse/HADOOP-6502?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Hairong Kuang updated HADOOP-6502:
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Description: 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! (was: 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 serializing 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!)
> 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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