Understanding Java's "Perm Gen": MaxPermSize, heap space, etc.


Thu Jan 29 22:00:00 2009 -0800

During my travels at work, I’ve come across a few interesting memory management issues in Java. My team has deployed several large web-applications in a single instance of Apache Tomcat. The Linux box running these applications only has about 2GB of physical memory available. Once the apps are deployed, about 1.8 GB of system memory is consumed by Java alone. Clearly, we need to improve our memory management a bit.

However, I took a few minutes to do some digging on Java’s Permanent Generation (Perm Gen) and how it relates to the Java heap. Here are some distilled notes from my research that you may find useful when debugging memory management issues in Java:

JVM argument -Xmx defines the maximum heap size. The arg -Xms defines the initial heap size. For example:

-Xmx4g -Xms512m

In Tomcat land, these settings would go in your startup.sh or init script, depending on how you start and run Tomcat. With regards to the MaxPermSize, this argument adjusts the size of the “permanent generation.” As I understand it, the perm gen holds information about the “stuff” in the heap. So, the heap stores the objects and the perm gen keeps information about the “stuff” inside of it. Consequently, the larger the heap, the larger the perm gen needs to be.

Here is an example showing how you might use MaxPermSize:


Additional Notes

  • Use the JVM options -XX:+TraceClassLoading and -XX:+TraceClassUnloading to see what classes are loaded/un-loaded in real-time. If you have doubts about excessive class loading in your app; this might help you find out exactly what classes are loaded and where.
  • Use -XX:+UseParallelGC to tell the JVM to use multi-threaded, one thread per CPU, garbage collection. This might improve GC performance since the default garbage collector is single-threaded. Define the number of GC threads to use with the -XX:ParallelGCThreads=N option where N is the number of GC threads you wish to consume.
  • Never call System.gc() in your code. The application doesn’t know the best time to garbage-collect, only the JVM really does.
  • The JVM option -XX:+AggressiveHeap inspects the machine resources (size of memory and number of processors) and attempts to set various heap and memory parameters to be optimal for long-running, memory allocation-intensive jobs.