Re: [Xen-users] Xen processor time scheduling

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Jul 9, 2012 at 9:40 PM, John Buchanan
<John.Buchanan@xxxxxxxxxxxxxxxxxx> wrote:
> My apologies if this question has been asked/answered elsewhere/previously.
>
> I'm using XCP 1.1, Xeon E7-2860 processors (10 cores with HT).  My question
> is, is it more efficient in terms of scheduling of processor time to
> allocate fewer Vcpu's to a given domU guest?  A year or two ago I had read
> that with a VMWare product (don't recall specifically which, but a type 1
> hypervisor), the way the hypervisor allocated cpu time amongst the guests
> was such that if you allocated > 1 Vcpu's the host needed to have exactly
> that number of physical cpus/cores available at that time, if not then that
> guest got skipped until the next rotation.  Therefore VMWare's best practice
> recommendation was to always start your VM's with 1 Vcpu, only increase if
> necessary.
>
> Also seemed odd to me that they designed it as such because in my production
> environments 1 Vcpu is rarely adequate.
>
> Also, performance peaks at 8 Vcpu allocation, anything beyond that and
> performance steadily declines.


It sounds like you're describing from VMWare what is called "gang
scheduling" (i.e., all the vcpus are scheduled or none).  Xen does not
implement gang scheduling, for exactly the reasons you describe.

It is, however, still a good idea to give a VM only as many vcpus as
it actually needs.  But it sounds like you've established that by
measurement that you do need 8 vcpus, so that seems like the right
plan.

If you're really keen, I did give a talk about this very subject,
which you can see here: http://www.citrix.com/tv/#videos/2930

 -George

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