Re: [Xen-users] Xen Performance

Pasi Kärkkäinen
Wed, 22 Jul 2009 18:06:25 -0000

On Tue, Jun 02, 2009 at 08:42:56AM -0400, Adam Wead
wrote:
>
>
> On Jun 2, 2009, at 1:59 AM, Luke S Crawford wrote:
>
> >Adam Wead <awead@indiana.edu> writes:
> >
> >>On Jun 1, 2009, at 8:43 PM, Luke S Crawford wrote:
> >>>
> >>>
> >>>Also note, I''ve seen better worst-case performance by
giving each
> >>>VM fewer
> >>>VCPUs, and the xen guys are not kidding about dedicating a core
> >>>to the
> >>>Dom0.   setting cpus="1-7" in your xm config file
(assuming an 8
> >>>core box)
> >>>and giving dom0 only 1 vcpu makes a world of difference on
heavily
> >>>loaded boxes.
> >>>
> >>
> >>This may be a dumb question, but is that any different from or the
> >>same as setting "dom0-cpus 1" in the xend-config.sxp
file?  Are you
> >>specifying "cpus=1-7" in the guest''s config or
the Xen daemon''s
> >>config?
> >
> >>thanks in advance for clearing up my confusion,
> >
> >setting cpus="1-7" in the guest config file is important,
because
> >otherwise
> >the guests will run on all cpus, including the dom0 cpus.
> >
> >I believe the dom0 vcp0 is pinned to cpu0, vcpu1 is pinned to
> >cpu1 .... so
> >setting dom0-cpus 1  will pin the dom0 cpu 0 to 0.
> >
> >just setting dom0-cpus in xend-config.sxp without setting cpus= in
> >the domu
> >configs doesn''t help much, because the guests still trample
over
> >the one
> >cpu the dom0 has.
>
> Thanks!
>
> So it looks like you have to set both, if you want your Dom0 to be
> completely guest free.  The xend-config.sxp file specifies which cpu
> to use, and the guest config file keeps the guests away from using it
> and only all the others.
>
> ...adam
>

Actually the above is not totally correct.. see this recent thread:

http://lists.xensource.com/archives/html/xen-devel/2009-07/msg00873.html
and
http://lists.xensource.com/archives/html/xen-devel/2009-07/msg00875.html

-- Pasi