Xmx set to 256m, but getRuntime().totalMemory() returns 900MB after 1 day

We use Websphere Application Server 4, and set -Xmx256m and -Xms256m to our application.

After 1 day, we found that the value returned by Runtime.getRuntime().totalMemory() is 900MB.

I just wonder why the heapSize will be 3 times more than we set at -Xmx ?

Is it possible the bugs of JVM or WAS4 ?

No outOfMemory is throwed even it is >> 256M

[380 byte] By [chanronny01a] at [2007-11-26 15:19:42]