Garbage Collection Overhead Help Needed

Question

I am looking for a little bit of guidance in trying to diagnose a GC related issue.

We are testing on Solaris with WebSphere Portal and my current environment has a Garbage Collection Overhead of 7% (This was calculated with verbose GC and running the log through PMAT) I am supposed to compare this value with another environment which is running at an avg of 4.5%. The env are on the exact same version of WebSphere Portal, same JVM sizes,parameters,custom varibles,etc. My JVMs had 20 more Allocation Failures, 2 more full GC's, a 2 sec higher mean pause time during GCs than their environment during a 1 hr performance testing period.

Can you give me any advice on what could be causing this issue with all the same configuration values and the same exact 1 hr performance test? Or anything else to review?

Thanks

Answer 1

The first thing I'd check is whether the process is ever being pushed into swap / "virtual" memory. This would have a significant impact on your GC overhead.

With identical Java configurations and load etc., the most obviously difference would be the amount of physical RAM on each machine. Assuming they're the same, see what other processes / services are running on each machine - unless they're both fresh installs of the same OS, I'd expect there to be some (potentially significant) differences.

Post the output from top and iostat (or your preferred tools) and we'll see if there's anything obvious.