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Re: [Xen-users] Xen Performance

  • To: Xen User-List <xen-users@xxxxxxxxxxxxxxxxxxx>
  • From: Adam Wead <awead@xxxxxxxxxxx>
  • Date: Tue, 2 Jun 2009 08:42:56 -0400
  • Delivery-date: Tue, 02 Jun 2009 05:44:04 -0700
  • List-id: Xen user discussion <xen-users.lists.xensource.com>

On Jun 2, 2009, at 1:59 AM, Luke S Crawford wrote:

Adam Wead <awead@xxxxxxxxxxx> 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.


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.


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