Recently did some perf testing of a process running in a docker container. CPU usage was gathered via the cpuacct.stat file found in the /sys/fs container directory. When monitoring the process by hand, using a tool such as top, I noticed that the cgroup CPU usage was always less than that shown by top, sometimes by a factor of four.
I wrote a quick script to track the ticks used by the process in the container as reported by both cpuacct.stat and /proc//stat. They aren't even close. The proc stats are much larger, which doesn't make any sense; it's like saying the container is smaller than what's in it.
The only reference to any of this I could find was a kernel comment saying that cpuacct.stat might be a little inaccurate once in a while. This is more than a rounding error.
Anyone have any experience or knowledge about this? It kind of throws all of my CPU usage metrics into doubt.
Linux 3.10.0-327.18.2.el7.x86_64 Centos on an 8 cpu box.
