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Date: Sat, 13 Sep 2008 09:06:44 -0700 (PDT)
From: "Chris Lu (JIRA)" <jira@apache.org>
To: java-dev@lucene.apache.org
Subject: [jira] Commented: (LUCENE-1383) Work around ThreadLocal's "leak"
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    [ https://issues.apache.org/jira/browse/LUCENE-1383?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12630789#action_12630789 ] 

Chris Lu commented on LUCENE-1383:
----------------------------------

The test is repeatedly close and open the index. 

After the repeat close/open test run finished, I closed the RAMDirectory based searcher/reader, and switched to FSDirectory, the RAMDirectory(which is fairly large) was left in the memory. I did several rounds of GC, but the RAMDirectory was not cleaned. At this time, I tried to do a memory dump, but an OOM occured.


> 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
>
>
> 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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