Sunday, December 23, 2012

Why is memory management so visible in Java VM?

Java gives you a bit more control about memory -- strike one for people wanting to apply that control there, vs Ruby, Perl, and Python, which give you less control on that. Java's typical implementation is also very memory hungry (because it has a more advanced garbage collection approach) wrt the typical implementations of the dynamic languages... but if you look at JRuby or Jython you'll find it's not a language issue (when these different languages use the same underlying VM, memory issues are pretty much equalized). I don't know of a widespread "Perl on JVM" implementation, but if there's one I'm willing to bet it wouldn't be measurably different in terms of footprint from JRuby or Jython!

Python/Perl/Ruby allocate their memory with malloc() or an optimization thereof. The limit to the heap space is determined by the operating system rather than the VM, so there's no need for options like -Xmxn. Also, the garbage collection is simpler, based mostly on reference counting. So there's a lot less to fine-tune.

Furthermore, dynamic languages tend to be implemented with bytecode interpreters rather than JIT compilers, so they aren't used for performance-critical code anyway.

"So why does the JVM have (need?) a ceiling at all? Why can't it be flexible enough to request more memory from the OS when the need arises?" The Sun JVM can be easily configured to do just that. It's not the default because you have to be careful that your process doesn't cause the OS to thrash

Changing JVM is not a panacea. You can get new unexpected issues (e.g. see an article about launching an application under 4 different JVM).

You can have a class leak (e.g. via classloaders) that mostly often happen on redeploy. Frankly, I've never saw working hot redeploy on Tomcat (hope to see one day).
You can have incorrect JVM paramaters (e.g. for Sun JDK 6 64 bits -XX:+UseParNewGC switch leads to leak PermGen segment of memory. If you add additional switches: -XX:+UseConcMarkSweepGC -XX:+CMSClassUnloadingEnabled-XX:+CMSPermGenSweepingEnabled the situation will be resolved. Funny, but I never met above mentioned leak with Sun JDK 6 32 bits). Link to an article "Tuning JVM Garbage Collection for Production Deployments".
PermGen chunk can be not enough to load classes and related information (actually that most often happens after redeploy under Tomcat, old classes stay in memory and new ones are loading) -- Great Article!!

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