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Re: [Xen-devel] [Patch 0 of 2]: PV-domain SMP performance

  • To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
  • From: Juergen Gross <juergen.gross@xxxxxxxxxxxxxxxxxxx>
  • Date: Thu, 18 Dec 2008 13:06:54 +0100
  • Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Thu, 18 Dec 2008 04:07:30 -0800
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Keir Fraser wrote:
> On 17/12/2008 14:20, "Juergen Gross" <juergen.gross@xxxxxxxxxxxxxxxxxxx>
> wrote:
>> About real world applications:
>> Again 4 vcpus pinned to one physical cpu, 3 files copied via scp to the test
>> machine at the same time, each file about 50 MB.
>> Linux-xen from xensource: about 1:50 elapsed time for each job
>> My modified Linux:        about 0:50 elapsed time
> So this provides great wins for those who run multi-vcpu VMs on a single
> physical CPU? ;-)

Absolutely! :-)
I thought this would be an easy way to force vcpu scheduling without having to
use multiple domains.
The picture would be quite similar (may be not so extreme) on a machine with
several multi-vcpu domains under heavy load.

> Actually getting a speedup on this benchmark even in that
> configuration is a surprise I will admit -- I'd expect most time to be spent
> in sshd in user space. By 0:50 for each job you mean 0:50 for 50MB? That's
> 10Mbps and I wouldn't even expect a single CPU working alone to be breaking
> a sweat. Weird...

I think the problem here are the network interrupts which will occur in a
round robin fashion on all vcpus. And those are serialized by the scheduler...


Juergen Gross                             Principal Developer
IP SW OS6                      Telephone: +49 (0) 89 636 47950
Fujitsu Siemens Computers         e-mail: juergen.gross@xxxxxxxxxxxxxxxxxxx
Otto-Hahn-Ring 6                Internet: www.fujitsu-siemens.com
D-81739 Muenchen         Company details: www.fujitsu-siemens.com/imprint.html

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