Java Server Application takes more RAM than allocated maximum!

Question

I'm hosting a java server application on a Linux Root-Server.

My problem is, that sometimes the server takes more RAM than the allocated maximum, which leads to an automatic process kill (crash).

That's my start-script:

nohup java -Xmx900m -Xms900m -XX:MaxPermSize=128m -jar -Xss1m jar-file.jar &

As you can see, I also set the minimum and maximum RAM (-Xms900, -Xmx900m), so there shouldn't be a problem.

Does anyone know what I'm doing wrong? If yes, please tell me and show me an "improved start script".

Best wishes, Ishidres

Answer 1

In fact Xms and Xmx determine the lmit of the heap size but there other memory regions : Stack (Xss x Number of Threads) + PermGen + CodeCahe (48MB by default in Java 7 ) + Direct Memory.

In general,those add approximately 1/2 GB if there no specifications.