jvm - Java seems to ignore -Xms and -Xmx options

Question

I'd like to run a very simple bot written in java on my VPS. I want to limit jvm memory to let's say 10MB (I doubt it would need any more).

I'm running the bot with the following command:

java -Xms5M -Xmx10M -server -jar IrcBot.jar "/home/jbot"

But top shows that actual memory reserved for java is 144m (or am I interpreting things wrong here?).

13614 jbot 17 0 144m 16m 6740 S 0.0 3.2 0:00.20 java

Any ideas what can be wrong here?

Java version "1.6.0_20" Java(TM) SE Runtime Environment (build 1.6.0_20-b02) Java HotSpot(TM) Client VM (build 16.3-b01, mixed mode)

BTW. I'm running CentOS - if it matters.

EDIT: Thank you for your answers.

I can't really accept any of them, since it turns out the problem lies within the language i choose to write the program, not the JVM itself.

Answer 1

-Xmx specifies the max Java heap allocation (-Xms specifies the min heap allocation). The Java process has its own overhead (the actual JVM etc), plus the loaded classes and the perm gen space (set via -XX:MaxPermSize=128m) sits outside of that value too.

Think of your heap allocation as simply Java's "internal working space", not the process as a whole.

Try experimenting and you'll see what I mean:

java -Xms512m -Xmx1024m ...

Also, try using a tool such as JConsole or JVisualVM (both are shipped with the Sun / Oracle JDK) and you'll be able to see graphical representations of the actual heap usage (and the settings you used to constrain the size).

Finally, as @Peter Lawrey very rightly states, the resident memory is the crucial figure here - in your case the JVM is only using 16 MiB RSS (according to 'top'). The shared / virtual allocation won't cause any issues as long as the JVM's heap isn't pushed into swap (non-RAM). Again, as I've stated in some of the comments, there are other JVM's available - "Java" is quite capable of running on low resource or embedded platforms.