VM Perm Out of memory fix

With Coldfusion and Railo applications making Java so easy it is not unusual that you can have 1,000’s of underlying java classes created and this can cause jvm OutOfMemoryError issues relating to the permanent generation space. The perm is like a big database of classes that are loaded in the JVM. Recently we have seen this happen on Railo a number of times, the problem is that by default a JVM garbage collection cycle does not collect permanent generation so it just keeps growing and eventually runs out of memory.

The Fix:

We need to add some arguments to the jvm.config file of the application server.

The standard garbage collector can’t collect in the permanent generation, but the concurrent collector can. So the first thing we need to do is to make sure that the jvm uses the concurrent garbage collector

-XX:+UseConcMarkSweepGC

But this is not enough.

We must also specifically tell it to collect in the permanent generation, and this is done with this command line argument:

-XX:+CMSPermGenSweepingEnabled

Good, now the concurrent collector will take the permantent generation under its wings.

But wait! Classes are special, and the jvm is reluctant to let go of them, so we must also explicitly allow classes to be unloaded:

-XX:+CMSClassUnloadingEnabled

Don’t forget to also give your perm memory a decent allocation of memory

-XX:MaxPermSize=256m

Thanks to Christian Vest Hansen for figuring this out.