-XX:MaxPermSize with or without -XX:PermSize

Question

We've run into a Java.lang.OutOfMemoryError: PermGen space error and looking at the tomcat JVM params, other than the -Xms and -Xmx params we also specify -XX:MaxPermSize=128m. After a bit of profiling I can see occasionally garbage collection happening on the PermGen space saving it from running full.

My question is: other than increasing the -XX:MaxPermSize what would be the difference if I specify as well -XX:PermSize? I know the total memory then would be Xmx+maxPermSize but is there any other reason why -XX:PermSize should not be there when -XX:MaxPermSize is specified?

Please do share if you have real-world experience dealing with these JVM parameters.

ps. The JVM is HotSpot 64bit Server VM build 16.2-b04

Answer 1

-XX:PermSize specifies the initial size that will be allocated during startup of the JVM. If necessary, the JVM will allocate up to -XX:MaxPermSize.

Answer 2

By playing with parameters as -XX:PermSize and -Xms you can tune the performance of - for example - the startup of your application. I haven't looked at it recently, but a few years back the default value of -Xms was something like 32MB (I think), if your application required a lot more than that it would trigger a number of cycles of fill memory - full garbage collect - increase memory etc until it had loaded everything it needed. This cycle can be detrimental for startup performance, so immediately assigning the number required could improve startup.

A similar cycle is applied to the permanent generation. So tuning these parameters can improve startup (amongst others).

WARNING The JVM has a lot of optimization and intelligence when it comes to allocating memory, dividing eden space and older generations etc, so don't do things like making -Xms equal to -Xmx or -XX:PermSize equal to -XX:MaxPermSize as it will remove some of the optimizations the JVM can apply to its allocation strategies and therefor reduce your application performance instead of improving it.

As always: make non-trivial measurements to prove your changes actually improve performance overall (for example improving startup time could be disastrous for performance during use of the application)

Answer 3

If you're doing some performance tuning it's often recommended to set both -XX:PermSize and -XX:MaxPermSize to the same value to increase JVM efficiency.

Here is some information:

1. Support for large page heap on x86 and amd64 platforms
2. Java Support for Large Memory Pages
3. Setting the Permanent Generation Size

You can also specify -XX:+CMSClassUnloadingEnabled to enable class unloading option if you are using CMS GC. It may help to decrease the probability of Java.lang.OutOfMemoryError: PermGen space