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I am transitioning from Xen to KVM, and will be using Fedora 14. I plan to use cgroups to assign priorities to VMs.

My question is as follows: If I place VMs in various cgroups, should I also put all other Domain-0 (physical host) processes in a single cgroup, which I would then assign lower priority? The analogy for Xen would be that all Dom0 processes are batch scheduled when the Dom0 "VM" is scheduled, so by placing them all in the same cgroup, I would be emulating this behavior. However, since KVM doesn't suffer from the same Dom0 IO dependencies as Xen, I could assign the Dom0 cgroup a low priority.

In other words, is there any potential performance degradation from all the background Dom0 processes, like ntpd, yum etc, competing with VMs whose resource allocations are being explicitly controlled via cgroups? What is the best practice here?

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I think you should consider this on a case by case basis (depending on services you are running on your server). For example I wouldn't put ntpd in a low prio group because you rely on it to set correct timing so any delay in executing ntpd code would result in time inaccuracy. Also ntpd has low resource usage so why worry about it?

But it makes sense to limit other processes like yum and other system non-critical processes you might be running like GUI environment, browser, whatever. These tend to be pretty resource hungry usually.

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