Thursday, March 28, 2013

Java JIT and code cache issues

If you have a lot of Java code, you may run out of code cache used by JIT to compile frequently used code. When this happens, you can get an OutOfMemoryError or (if you're lucky) just a message in the console that JIT is stopped and the rest of the newly loaded code will be interpreted, so, slow.

You can increase the code cache size by specifying -XX:ReservedCodeCacheSize=xxx, but you'll run into the same problem after you add more code.

You can enable -XX:+UseCodeCacheFlushing to deal with that. It's supposed to make some cleanup when the code cache gets full, to free space for new method compilation.

Today I learned that it's not as good as it sounds, by reading some 25M of JIT logs on IntelliJ IDEA process (JDK 1.7 64-bit). The code cache got full, it was flushed, which really freed up some decent amount of code cache space. That's OK. There was a problem though. After this event all the JIT compilation was stopped. So the compiled code that happened to be flushed away was interpreted now, no matter how hot-spottish it really is.

This is quite noticeable. For example, code completion and goto-class navigation suddenly become very slow after a day of work. They match lots of names against your typed prefix, and unfortunately some of this matching code gets deoptimized and becomes visible in the CPU snapshots.

What to do? Wait for Oracle to fix this. And for now we increased code cache size for 64-bits JVMs.

Maybe we should also take a closer look at the logs and try to reduce the hotspot count. Just look at the methods that are compiled at runtime and try calling them less frequently to prevent them occupying code cache at all. This could also improve the overall performance.