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From "robert engels (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1383) Work around ThreadLocal's "leak"
Date Wed, 01 Oct 2008 16:05:44 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1383?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12636075#action_12636075
] 

robert engels commented on LUCENE-1383:
---------------------------------------

It doesn't need to be "fixed".

It works fine, you need to understand how to use it properly which may require calling remove()
if your threads are long-lived.

Also, remove() was only added in 1.5, and that is why that technique is not valid for Lucene.
It is also not valid, since you need to clear all values across all threads, remove() only
clears the entry for the calling thread.

The current patch solves the problem suitably for Lucene, at the expense of some performance
degradation.



> Work around ThreadLocal's "leak"
> --------------------------------
>
>                 Key: LUCENE-1383
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1383
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Index
>    Affects Versions: 1.9, 2.0.0, 2.1, 2.2, 2.3, 2.3.1, 2.3.2
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>             Fix For: 2.4
>
>         Attachments: LUCENE-1383.patch, ScreenHunter_01 Sep. 13 08.40.jpg, ScreenHunter_02
Sep. 13 08.42.jpg, ScreenHunter_03 Sep. 13 08.43.jpg, ScreenHunter_07 Sep. 13 19.13.jpg
>
>
> Java's ThreadLocal is dangerous to use because it is able to take a
> surprisingly very long time to release references to the values you
> store in it.  Even when a ThreadLocal instance itself is GC'd, hard
> references to the values you had stored in it are easily kept for
> quite some time later.
> While this is not technically a "memory leak", because eventually
> (when the underlying Map that stores the values cleans up its "stale"
> references) the hard reference will be cleared, and GC can proceed,
> its end behavior is not different from a memory leak in that under the
> right situation you can easily tie up far more memory than you'd
> expect, and then hit unexpected OOM error despite allocating an
> extremely large heap to your JVM.
> Lucene users have hit this many times.  Here's the most recent thread:
>   http://mail-archives.apache.org/mod_mbox/lucene-java-dev/200809.mbox/%3C6e3ae6310809091157j7a9fe46bxcc31f6e63305fcdc%40mail.gmail.com%3E
> And here's another:
>   http://mail-archives.apache.org/mod_mbox/lucene-java-dev/200807.mbox/%3CF5FC94B2-E5C7-40C0-8B73-E12245B91CEE%40mikemccandless.com%3E
> And then there's LUCENE-436 and LUCENE-529 at least.
> A google search for "ThreadLocal leak" yields many compelling hits.
> Sun does this for performance reasons, but I think it's a terrible
> trap and we should work around it with Lucene.

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