What causes a JRE 6 JVM code cache leak?

Question

Since switching to JRE 6, my server's code cache usage (non-heap) keeps growing indefinitely. My application creates a lot of classes at runtime, BUT these classes are successfully unloaded during the GC process. I can see these classes getting unloaded in the gc logs and also the permGen usage stays constant. I specifically make sure in my code that these classes are orphaned once I am finished with them and so they correctly get garbage collected from permGen.

The code cache however keeps growing. I only became aware of the code cache after switching to JRE 6. So I guess my questions are:

1. Does GC include the code cache?
2. What could cause a code cache memory leak, specifically.
3. Is there a bug in JDK 6 in this area?

Answer 1

You may want to look through this discussion and just go backwards to see what may be helpful in trying to narrow this down: http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2009-January/000530.html

This one involves JDK5 but may be helpful: http://www.nabble.com/Java-code-cache-memory-td22202283.html

Are you using this to compile jsp pages, or something similar? If not, what is being compiled after the application starts up? Are you using AspectJ with runtime weaving?

It would help to know what you are doing to get a better idea as to how to help.

Also, when the code cache is exhausted, does it just stop compiling anew or does the jvm crash? I would expect the former.

Are you using Sun's JDK? I am guessing you are since I doubt the others are listed as version 6, but it doesn't hurt to ask.

Answer 2

Two possible places i would look for the problem:Leaked Loaders and Massive String interning.

From your app description, i would more likely verify whether old generated classes are being unloaded by the GC.

I dont think jConsole has a way of visualizing this, but if you can get a copy of YourKit, it has a graphical way of detecting both the above problems.