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From "Remko Popma (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LOG4J2-819) PermGen OutOfMemoryError when reloading webapp on Tomcat 6
Date Sun, 14 Sep 2014 00:41:34 GMT

    [ https://issues.apache.org/jira/browse/LOG4J2-819?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14133039#comment-14133039
] 

Remko Popma commented on LOG4J2-819:
------------------------------------

Ralph, since LOG4J2-628 the Log4JLogEvent Clock is actually used for all (non-async logger)
event timestamps. Please see the [constructor|http://logging.apache.org/log4j/2.x/log4j-core/xref/org/apache/logging/log4j/core/impl/Log4jLogEvent.html#145]
on line 145-155.

I don't oppose your solution, but there is something missing:
The Coarse/CachedClock classes are loaded because of the way ClockFactory is coded. This has
an undesired side effect that Tomcat ends up creating the background threads for *both* CoarseCachedClock
and CacheClock after the web app is stopped. This happens *even* if Log4J uses SystemClock
to create timestamps.

So just managing a central Log4j clock instance in LoggerContext is not enough. I propose
we modify ClockFactory such that the class loader no longer loads Coarse/CachedClock by accident.
That would solve this Jira.

That leaves the possibility that the user _intentionally_ configures CachedClock in a web
application (which would also cause a memory leak). Ralph's proposal to make Clock part of
the LoggerContext life cycle would address that concern.

I don't oppose that, but that will be significant work. We don't have a user requesting this
feature, so before we spend a lot of time on it I just want to point out an alternative:
What about simply documenting on the site that CachedClock should not be used by web applications?
There is only one brief [mention here|http://logging.apache.org/log4j/2.x/manual/async.html#SysPropsAllAsync]
that CachedClock exists. CoarseCachedClock is never mentioned on the site. So I doubt that
this would inconvenience any users.


> PermGen OutOfMemoryError when reloading webapp on Tomcat 6
> ----------------------------------------------------------
>
>                 Key: LOG4J2-819
>                 URL: https://issues.apache.org/jira/browse/LOG4J2-819
>             Project: Log4j 2
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 2.0.2
>         Environment: Tomcat 6
>            Reporter: Costa Theodosiou
>            Priority: Critical
>         Attachments: demo.zip, demorun-tomcat6-with-reload.zip, demorun.zip, gg-log4j2-clocks-interrupts.patch,
gg-log4j2-clocks-v2.patch
>
>
> When reloading an application 3 or 4 times in Tomcat 6, the application crashes with
a "java.lang.OutOfMemoryError: PermGen space" exception.
> After some investigation using the "When all else fails" section of https://wiki.apache.org/tomcat/OutOfMemory
in conjunction with Java VisualVM, I have narrowed down the problem to the Thread created
within org.apache.logging.log4j.core.util.CoarseCachedClock.
> When a Thread is created, it contains a reference to the classloader that it was created
with. In this case, the Thread's contextClassLoader field contains a reference to the WebappClassLoader.
When Tomcat attempts to unload the webapp, the Thread still holds onto this reference which
prevents WebappClassLoader from being freed.
> Perhaps the Log4jServletContextListener (Log4jWebInitializerImpl) can be made to stop
the CoarseCachedClock thread.
> I believe this is not an obvious issue on Tomcat 7 due to https://wiki.apache.org/tomcat/MemoryLeakProtection.



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