[jira] Commented: (HADOOP-6133) ReflectionUtils performance regression

["Chris Douglas (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-6133?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12749771#action_12749771 ] 

Chris Douglas commented on HADOOP-6133:
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Won't this prevent classes from being unloaded by retaining a reference to ClassLoaders in the cache? While HADOOP-4187 is explicitly temporary, this solution is unlikely to be removed with the cause of the regression.

I can't think of any places where there would be fierce contention for an explicit lock on the cache requiring a ConcurrentHashMap for the outer map. Would a WeakHashMap suffice, here?

> ReflectionUtils performance regression
> --------------------------------------
>
>                 Key: HADOOP-6133
>                 URL: https://issues.apache.org/jira/browse/HADOOP-6133
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: conf
>    Affects Versions: 0.20.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hadoop-6133-0.20.patch, hadoop-6133-trunk.txt, Test.java
>
>
> HADOOP-4187 introduced extra calls to Class.forName in ReflectionUtils.setConf. This caused a fairly large performance regression. Attached is a microbenchmark that shows the following timings (ms) for 100M constructions of new instances:
> Explicit construction (new Test): around ~1.6sec
> Using Test.class.newInstance: around ~2.6sec
> ReflectionUtils on 0.18.3: ~8.0sec
> ReflectionUtils on 0.20.0: ~200sec
> This illustrates the ~80x slowdown caused by HADOOP-4187.

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