What? Control Groups aka cgroups – see the docs . Resource control and monitoring. Some examples follow for throttling i/o speed(s) for a process control group (cgroup).
In relatively unscientific tests, it seems moving from PHP5.4 with xcache to PHP 5.6 with Zend’s OpCache can lead to approximately 50% memory reduction. (xcache was set to have a 64mb size, Zend OpCache is also using 64mb of memory).