[CentOS-virt] what scheduling algorithm does KVM use?

Nick oinksocket at letterboxes.org
Fri Nov 19 05:36:04 EST 2010

On 19/11/10 00:11, Kenni Lund wrote:
> I think cgroups is the solution, if you want to guarantee resources to some 
> guests. I haven't tested it with KVM, but perhaps "nice" and "ionice" can be 
> useful as well...the guests are just Linux processes after all.

Just to clarify, it isn't that *I* want to guarantee equal time sharing, it's
that the entire premise of co-scheduling implies that the guest VM's OS may
malfunction if the system doesn't guarantee it, or some approximation of it.

To quote from http://communities.vmware.com/docs/DOC-4960:

> Without coscheduling, the VCPUs associated with an SMP VM would be scheduled
> independently, breaking the guest's assumptions regarding uniform progress.
> We use the term "skew" to refer to the difference in execution rates between
> two or more VCPUs associated with an SMP VM.
> Inter-VCPU skew violates the assumptions of guest software. Non-trivial skew 
> can result in severe performance problems, and may even induce failures when 
> the guest expects inter-VCPU operations to complete quickly. Let's first 
> consider the performance implications of skew. Guest OS kernels typically
> use spin locks for interprocessor synchronization. If the VCPU currently
> holding a lock is descheduled, then the other VCPUs in the same VM will waste
> time busy-waiting until the lock is released. Similar performance problems
> can also occur in multi-threaded user-mode applications, which may also 
> synchronize using locks or barriers. Unequal VCPU progress will also confuse 
> the guest OS cpu scheduler, which attempts to balance load across VCPUs.


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