[CentOS] cgroups not working?

Hi, I'm trying to use cgroups to limit the apache webserver but this doesn't seem to work as far as I can tell. I modified cgconfig.conf like this:

group webserver { cpu { cpu.shares="250"; } }

and addes this to cgrules.conf: *:httpd * webserver

Looking into the "webserver" cgroup shows that the pid's of the httpd processes are correctly assigned: $ cat /cgroup/cpu/webserver/tasks 6191 6193 6194 6195 6196 6197 6207

Yet when I run a php file with a dummy loop I get a process with 100% cpu usage until the script times out which is not what I expect. Am I doing something wrong or why doesn't the cpu.shares setting seem to take?

Regards, Dennis