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

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@xxxxxxxxxx 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?

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