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Re: [Xen-devel] Virt overehead with HT [was: Re: Xen 4.5 development update]

On Mon, 2014-07-14 at 17:32 +0100, Gordan Bobic wrote:
> On 07/14/2014 05:12 PM, Dario Faggioli wrote:
> > Elapsed(stddev)   BAREMETAL             HVM
> > kernbench -j4     31.604 (0.0963328)    34.078 (0.168582)
> > kernbench -j8     26.586 (0.145705)     26.672 (0.0432435)
> > kernbench -j      27.358 (0.440307)     27.49 (0.364897)
> >
> > With HT disabled in BIOS (which means only 4 CPUs for both):
> > Elapsed(stddev)   BAREMETAL             HVM
> > kernbench -j4     57.754 (0.0642651)    56.46 (0.0578792)
> > kernbench -j8     31.228 (0.0775887)    31.362 (0.210998)
> > kernbench -j      32.316 (0.0270185)    33.084 (0.600442)
BTW, there's a mistake here. The three runs, in the no-HT case are as
 kernbench -j2
 kernbench -j4
 kernbench -j

I.e., half the number of VCPUs, as much as there are VCPUs and
unlimited, exactly as for the HT case.

The numbers are the right one.

> Just to make sure I'm reading this right - _disabling_ HT causes a near 
> 50% performance drop?
For kernbench, and if you consider the "-j <half_of_nr_cpus>" run, yes,
nearly. And that is both for baremetal and HVM guest. And with
baremetal, I mean just bare Linux, no Xen at all involved.

Doesn't this make sense? Well, perhaps the wrong indication I gave about
the actual number of jobs used was misleading... better now?

BTW, the idea here was to compare perf between baremetal and HVM, and
they appear to be consistent.


<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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