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

[Gordan Bobic <gordan@bobich.net>]

On 07/14/2014 05:12 PM, Dario Faggioli wrote:
> [Sorry to the ones that will receive this mail twice, but I managed to
> drop xen-devel when replying the first time :-(]
>
> On mar, 2014-07-01 at 12:43 -0400, konrad.wilk@oracle.com wrote:
>> == x86 ==
>
>> *  HT enabled, virtualization overhead is high (Xen 4.4) (none)
>>     kernbench demonstrated it
>>     looking and tracing it
>>    -  Dario Faggioli
>>
> I spent a few time running kernbench on different boxes and with
> different configurations. After all this, here's what I found.
>
> So, on a non-NUMA, HT and EPT capable box, both BAREMETAL and HVM case
> were using 8G RAM and 8 CPUs/VCPUs. HT was enabled in BIOS:
>
> 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)

Just to make sure I'm reading this right - _disabling_ HT causes a near 
50% performance drop?