Java using up far more memory than allocated with -Xmx
java, memory, memory-management, stack
Solution
Top command reflects the total amount of memory used by the Java application. This includes among other things:
- A basic memory overhead of the JVM itself
- the heap space (bounded with -Xmx)
- The permanent generation space (-XX:MaxPermSize - not standard in all JVMs)
- threads stack space (-Xss per stack) which may grow significantly depending on the number of threads
- Space used by native allocations (using ByteBufer class, or JNI)
Problem
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) with -Xmx50m but according to top, it's still using 300m I tried running Eclipse Memory Analyzer and it reports only 26m Could this all be memory on the stack?, I'm pretty sure I never go further than about 300 method calls deep (yes, it is a recursive DFS search), so that would have to mean every stack frame is using up almost a megabyte which seems hard to believe. The program is single-threaded. Does anyone know any other places in which I might reduce memory usage? Also, how can I check/limit how much memory the stack is using? UPDATE: I'm using the following JVM options now with no effect (still about 300m according to top): `-Xss104k -Xms40m -Xmx40m -XX:MaxPermSize=1k` Another UPDATE: Actually, if I let it run a little bit longer (with all these options) about half the time it suddenly drops to 150m after 4 or 5 seconds (the other half it doesn't drop). What makes this really strange is that my program has no stochastic (and as I said it's single-threaded) so there's no reason it should behave differently on different runs Could it have something to do with the JVM I'm using? ``` java version "1.6.0_27" OpenJDK Runtime Environment (IcedTea6 1.12.3) (6b27-1.12.3-0ubuntu1~10.04) OpenJDK 64-Bit Server VM (build 20.0-b12, mixed mode) ``` According to java -h, the default JVM is -server. I tried adding -cacao and now (with all the other options) it's only 59m. So I suppose this solves my problem. Can anyone explain why this was necessary? Also, are there any drawbacks I should know about? One more update: cacao is really really slow compared to server. This is an awful option