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From Christopher Schultz <ch...@christopherschultz.net>
Subject Re: java.lang.OutOfMemoryError: PermGen space
Date Thu, 20 Oct 2016 16:40:23 GMT
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Jon,

On 10/20/16 6:00 AM, Moore, Jon, Vodafone UK wrote:
> I have a problem where our customers application server stops
> working intermittently and when we check the Tomcat logs we get the
> message " java.lang.OutOfMemoryError: PermGen space" but the Tomcat
> service is still running, and we have to restart the Tomcat
> service. I am a complete novice when it comes to Tomcat but am
> expected to resolve this issue as this server is part of the Avaya
> telephony solution.
> 
> I have 2 questions please.
> 
> 1 - What would be causing this.

Are you also a Java novice as well? No judgement... I just want to
adjust my level of snark appropriately :)

Java has a special heap space called the "permanent generation" (a
term which becomes less descriptive as time goes on) but basically all
java.lang.Class objects loaded by the JVM go there. The more libraries
and other stuff that gets loaded, the more space in permgen is used.
The defaults for the size of permgen are often fairly small, and you
may have to raise them. This is especially true for applications that
use large frameworks like Spring.

Raising the permgen space is almost always the right decision under
normal circumstances when you get an OOME:permgen error. I would
double whatever its current value is and monitor the application for a
while to see if that improves things.

The only situation I know of where raising the permgen size is not the
right move is when there is a "classloader-pinning leak". That happens
when you hot-deploy an application many times, but the old versions of
the application are not undeploying cleanly. This situation is quite
the rabbit-hole, so I'll stop there unless you can confirm that you do
hot-deployments without restarting Tomcat and the JVM.

If you instead get OOME for other reasons, it's frequently NOT the
right decision to increase the heap size because it usually indicates
that there is a memory leak in the application and giving it more heap
just means you'll wait longer between failures. Instead, the
application should be fixed to not leak memory :)

> 2 - What are the default Java memory settings for Initial memory 
> pool, maximum memory pool size and thread stack size when the
> fields are blank when you use the "Configure Tomcat" interface on
> Windows. I was wondering if changing one or more of these settings
> would help ?

The default depends upon the JVM and the OS.

> We are running Tomcat 6.0.26 on Windows Server 2008 R2 standard
> and JVM version is 1.6.0_20-b02

Best way to find out for sure[1] is:

C:\> java -XX:+PrintFlagsFinal -version 2>&1 | findstr HeapSize

The two settings you are looking for are MaxHeapSize and
InitialHeapSize. Those values are in bytes, so you'll probably have to
divide by 1024 a couple of times to get at the "real" value in
human-readable terms.

The best way to find the default permgen size is:

C:\> java -XX:+PrintFlagsFinal -version 2>&1  | findstr PermSize

Hope that helps,
- -chris

[1]
http://stackoverflow.com/questions/2915276/what-is-the-default-maximum-h
eap-size-for-suns-jvm-from-java-se-6
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