[jira] [Assigned] (HADOOP-6502) DistributedFileSystem#listStatus is very slow when listing a directory with a size of 1300

["Todd Lipcon (Assigned) (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HADOOP-6502?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Todd Lipcon reassigned HADOOP-6502:
-----------------------------------

    Assignee: Todd Lipcon

Assigning to myself to get this committed - will reassign to Hairong upon commit since she's the contributor, but this way it'll stay on my queue :)
                
> 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
>
>
> 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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