RE: [Xen-users] XP HVM performance

James Harper
Thu, 25 Jun 2009 07:37:19 -0000

>
> I recently reinstalled an XP over my xen 3.2.1 (my conf is at the end
of
> the mail).
>
> And it''s working but I find it a bit (well a big bit ...) too slow
and
> use a lot of CPU.
>
> For instance just having a taskmanager running use between 10/15% of
CPU
> as reported by taskmanager and xm top.
>
> When I run iozone the CPU usage is almost 95% whereas on a real XP
> workstation it''s only a few percents.
>
> The server hosting this XP is a 4 core Xeon E5320  @ 1.86GHz and I
have
> only dom0 with 2 VCPU and 1 VCPU in XP.
>
> I know that running XP in XEN will bring a penalty but I wasn''t
> expecting such penalty.
>
> Do I miss something ?
>
> I also use the latest GPL PV drivers and found with iozone that the
> throughput is 1/3 of the raw capacity of the disk (as measured in
dom0).
> Does this number seems normal to you or once more am I missing
something ?
>
> Finally I also did some tests with more than one VCPU but if the whole
> system load reduced somehow (which is logical) the IO perf where even
> worse. Is it a good idea to use more than one VCPU in HVM ?
>

Please try putting /PATCHTPR in your boot.ini (make a copy of the
existing line in case it causes a problem). That will get GPLPV to patch
the windows kernel to stop using one of the hardware APIC registers so
often (intel) or at all (amd). Let me know if that affects the speed in
any way.

On an Intel processor you will get about 50% of the performance
improvement that you would with an AMD processor, but it still makes a
big difference. I''m not sure but I think that later versions of Xen
include a hypervisor-side speedup for Intel which would make /PATCHTPR
unnecessary.

Anyway, please try it and let me know.

James