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

[Dario Faggioli <dario.faggioli@citrix.com>]

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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
follows:
 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.

Regards,
Dario

-- 
<<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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