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

[George Dunlap <george.dunlap@eu.citrix.com>]

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
> 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.

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.

  -George