Java using up far more memory than allocated with -Xmx

Question

I have a project I\'m writing (in Java) for a class where the prof says we\'re not allowed to use more than 200m I limit the stack memory to 50m (just to be absolutely sure)

Answer 1

It's important to note that "total memory used" (RSS in Linux land) includes JDK heap (+ other JDK areas) as well as any "native memory" allocated.

For instance, these people found that allocating too many jaxbcontexts (which have associated native memory) between GC's could cause it to use a lot of extra RAM. Another common one is apparently ZipInflater if you don't call close on it (or GZipStream, etc.)

http://sleeplessinslc.blogspot.com/2014/08/jvm-native-memory-leak.html

His final workaround/fix was to either GC "more often" (by using GC1 garbage collector, or specifying a smaller [ironically] -Xmx setting) or by cacheing the JaxBContext objects (since they have no close method so you can't control the leak).

Also note that sometimes you can find memory culprits by just examing jstack: http://javaeesupportpatterns.blogspot.com/2011/09/jaxbcontext-performance-problem-case.html

It's also sometimes possible to "miss" closing for instance GZipStreams accidentally http://kohsuke.org/2011/11/03/quiz-time-memory-leak-in-java