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

On 07/14/2014 05:44 PM, Dario Faggioli wrote:
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.

Ah -- that's a pretty critical piece of information.

So actually, on native, HT enabled and disabled effectively produce the same exact thing if HT is not actually being used: 31 seconds in both cases. But on Xen, enabling HT when it's not being used (i.e., when in theory each core should have exactly one process running), performance goes from 31 seconds to 34 seconds -- roughly a 10% degradation.


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