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Date: Tue, 7 Aug 2012 18:10:10 +0000 (UTC)
From: "Costin Leau (JIRA)" <jira@apache.org>
To: common-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HADOOP-8632) Configuration leaking
 class-loaders
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    [ https://issues.apache.org/jira/browse/HADOOP-8632?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13430490#comment-13430490 ] 

Costin Leau commented on HADOOP-8632:
-------------------------------------

@Robert

My issue is not with the cache itself but with the leakage. If a client submits several big jobs, she has to either launch a new JVM for each submission or somehow patch the leak from outside. Or face OOM.
Addressing this in the framework directly obviously is much better.

@Todd
Wrapping the value with a WeakReference probably it's the easiest solution since it doesn't introduce a new library dependency. It can later be upgraded to MapMaker if the pattern occurs often.
                
> Configuration leaking class-loaders
> -----------------------------------
>
>                 Key: HADOOP-8632
>                 URL: https://issues.apache.org/jira/browse/HADOOP-8632
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: conf
>    Affects Versions: 2.0.0-alpha
>            Reporter: Costin Leau
>
> The newly introduced CACHE_CLASSES leaks class loaders causing associated classes to not be reclaimed.
> One solution is to remove the cache itself since each class loader implementation caches the classes it loads automatically and preventing an exception from being raised is just a micro-optimization that, as one can tell, causes bugs instead of improving anything.
> In fact, I would argue in a highly-concurrent environment, the weakhashmap synchronization/lookup probably costs more then creating the exception itself.
> Another is to prevent the leak from occurring, by inserting the loadedclass into the WeakHashMap wrapped in a WeakReference. Otherwise the class has a strong reference to its classloader (the key) meaning neither gets GC'ed.
> And since the cache_class is static, even if the originating Configuration instance gets GC'ed, its classloader won't.

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