There are two parameters controlling fair CPU scheduler in OpenVZ: cpuunits and cpulimit.
cpuunits
Cpuunits are set via
vzctl set 101 --cpuunits 1000 --save
For example. If you set a cpuunit for one container to a value and set a cpuunit on another container to a different value, the time allotted to each of the containers will be the ratio of the two units. Let's use a real example.
We did the following:
vzctl set 101 --cpuunits 1000 --save
vzctl set 102 --cpuunits 2000 --save
vzctl set 103 --cpuunits 3000 --save
If we started a CPU intensive application on each CT, then 103 would be given 3 times as much cpu time as 101 and 102 would get twice as much as 101, but some fraction of what 103 got. Here's how to determine what the real ratios are.
Add the three units, 1000+2000+3000 = 6000
101 gets 1000/6000 or 1/6th of the time. (16%) 102 gets 2000/6000 or 1/3rd of the time. (34%) 103 gets 3000/6000 or 1/2 of the time. (50%)
cpulimit
The cpulimit parameter sets the absolute maximum limit for a container to a percent value. For instance:
vzctl set 101 --cpulimit 25 --save
says that container 101 cannot ever have more than 25 percent of a CPU even if the CPU is idle for the other 75% of the time. The limit is calculated as a percentage of a single CPU, not as a percentage of the server's CPU resources as a whole. In other words, if you have more than one CPU, you can set a cpulimit > 100. In a quad-core server, setting cpulimit to 100 permits a container to consume one entire core (and not 100% of the server).
CPU limits are only available in rhel5-based and rhel6-based kernels and they behave a bit differently in them.
In the rhel5 kernel the limit has a container-wide meaning. That said if you have e.g. a container of 2 CPUS with the 100% cpulimit set, this container's usage of CPUs can be 100%/0% or 50%/50% or any other values, whose sum is 100%.
In the rhel6 kernel the applied limit is divided between onlince CPUs proportionally and a busy CPU cannot borrow time from an idle one. I.e. with a 2 CPUs container and 100% limit set the usage of each CPU cannot exceed 50% in any case
(http://wiki.openvz.org/Resource_shortage)
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