Cpu usage between "request" and "limit"

General Discussions
#1

If the cpu request is 500 and the cpu limit is 4000. What happens when the pod (there is only 1 container in the pod) tries to use more than 500 cpu?

By “what happens”, I mean to ask if there is some throttling, etc. involved?

Lets say the node has ample free cpu. Then, will there be any difference in performance between the foll.:
(a) request=500 & limit=4000. Actual cpu usage 2000.
(b) request=3000 & limit=4000. Actual cpu usage 2000.

Basically, the moment actual cpu usage crosses 500, though it will be allowed to use more cpu (upto 4000) but will that be allowed in a step-wise fashion? Will the scheduler play any role?

Thanks

#2

If there is no contention, then there should be no impact.

1 Like
#3

I think limit sets /sys/fs/cgroup/cpu/cpu.cfs_quota_us inside of container and this is hard limit that will not be exceeded.

The request sets /sys/fs/cgroup/cpu/cpu.shares. As long as there is idle CPU on the host it will not make difference but when host that is running those containers will have 100% CPU usage the pod with request 500 will have 6 time less CPU than pod with request 3000.

#4

Thanks @thockin @Adam_Dembek