Low utilization of OFBiz on multi core servers

Hi all,

I'm using OFBiz to study the performance of web service in the cloud.
I found a problem when I deploy OFBiz on a 2 core server, which has
4G memory. The OFBiz I used is the 12.04.03 version. The OS is ubuntu
12.04.

I was using OFBench to send requests to OFBiz. The OFBench is able
to send different kinds of requests, like login, logout, visit main page.
I was testing how many users I set in OFBench will saturate the server.
At some point I found that the server was overloaded when I set 20
users because the mean response time for the "main" request is over
10 seconds. However, the CPU utilization is only 60%. I also did a test
for one core server. When it's overloaded, the utilization is almost 100%.

I was using the default derby database. I monitor the memory of the
2 core server, it was like 2GB used. So I guess memory is not a problem.
The other parameters are default. This is not the first time I notice this
problem. I also found this on other multi-core servers.

Could anyone help me on this please?

Thanks a lot in advance!!!!

Best wishes,
Weikun