RE: [Xen-devel] Xen CPU limit?

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> -----Original Message-----
> From: Pasi Kärkkäinen [mailto:pasik@xxxxxx]
> Sent: Tuesday, March 30, 2010 1:53 AM
> To: Jan Beulich
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Martin Lukasik
> Subject: Re: [Xen-devel] Xen CPU limit?
> 
> On Tue, Mar 30, 2010 at 08:16:26AM +0100, Jan Beulich wrote:
> > >>> Pasi Kärkkäinen<pasik@xxxxxx> 29.03.10 17:49 >>>
> > >I think the number of virtual cpus (vcpus) per guest is (was) 32,
> > >and now in Xen 4.0.0 you can have up to 64 vcpus per guest.
> >
> > I don't think the tools are up to anything beyond 32 yet.
> 
> Hmm.. what's actually missing from the tools to handle >32 vcpus?
> 
> > The (64-bit) hypervisor allows up to 8192 vCPU-s iirc (but that's a
> truly
> > theoretical limit, as Dom0 or a guest likely won't be able to bring
> up
> > that many due to there only being 4096 event channels; current
> > Linux requires 5-6 of them per vCPU for IPIs and timer vIRQ).
> >
> 
> Wow.. I wasn't aware the (theoretical) limit is that big :)

But, if I am reading this correctly, 4096 event channels
is the next scalability barrier.  If, on a "big" machine, one
tries to run 64 (nearly always idle) guests each configured
with 16 vcpus (because they run a busy database load when
they are not idle), it won't work due to the event channel limit?

This scenario seems quite possible in a cloud/hosting environment.

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