Re: [Xen-users] Xen Performance

Adam Wead
Tue, 02 Jun 2009 12:42:56 -0000

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