Re: [Xen-devel] Priority for SMP VMs

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

Hi Gabriel,

I'm not particularly familiar with the credit scheduler but I'll do my best to 
help clarify things a bit (I hope!).

On Thursday 03 July 2008, Gabriel Southern wrote:
> Hi,
>
> I'm working a project with SMP VMs and I noticed something about the
> behavior of the credit scheduler that does not match my understanding
> of the documentation about the credit scheduler.  It seems like
> assigning more VCPUs to a VM increases the proportion of total system
> CPU resources the VM will receive, whereas the documentation indicates
> that this should be controlled by the weight value.
>
> For example when running a CPU intensvie benchmark with some VMs
> configured with 1-VCPU and other VMs configured with 8-VCPUs, the
> benchmark took 37% longer to complete on the VMs with 1-VCPU than the
> ones with 8-VCPUs.  Unfortunately I did not record the exact values
> for CPU time that each VM received; however, I think that the 8-VCPU
> VMs did receive around 30% more CPU time than the 1-VCPU VMs.  These
> tests were performed with the default weight of 256 for all VMs and no
> cap configured.

You need to tell us a bit more about how you did your benchmarking...  Were 
the SMP and UP guests running concurrently and competing for CPU time?  Or 
were they run separately?  Was the benchmark able to take advantage of 
multiple CPUs itself?

> I don't think that this is the behavior that the scheduler should
> exhibit based on the documentation I read.  I admit the tests I was
> doing were not really practical use cases for real applications.  But
> I'd be curious if anyone knows if this is a limitation of the design
> of the credit scheduler, or possibly due to a configuration problem
> with my system.  I running Xen 3.2.0 compiled from the official source
> distribution tarball, and the guest VMs are also using the 3.2.0
> distribution with the 2.6.18 kernel.  Any ideas anyone has about why
> my system is behaving this way are appreciated.

Without knowing more about your setup there are lots of things that could be 
happening...

If you're not using caps then there's no reason why the SMP guests shouldn't 
get more CPU time if they're somehow able to consume more slack time in the 
system.  SMP scheduling makes things pretty complicated!

If you reply with more details, I can try and offer my best guess as to what 
might be happening.  If you don't get a response within a day or two, please 
feel free to poke me directly.

Cheers,
Mark

>
> Thanks,
>
> Gabriel
>
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