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Re: [Xen-devel] Re: Poor performance on HVM (kernbench)

  • To: "Gianluca Guida" <gianluca.guida@xxxxxxxxxxxxx>
  • From: "George Dunlap" <dunlapg@xxxxxxxxx>
  • Date: Thu, 11 Sep 2008 19:07:56 +0100
  • Cc: Muli Ben-Yehuda <MULI@xxxxxxxxxx>, deshantm@xxxxxxxxx, Anthony Liguori <aliguori@xxxxxxxxxx>, xen-devel mailing list <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Thu, 11 Sep 2008 11:08:26 -0700
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So, the problem appears to be with a ton of brute-force searches to
remove writable mappings, both during resync and promotion.  My
analysis tool is reporting that of the 30 seconds or so in the trace
from xen-unstable, the guest spent a whopping 67% in the hypervisor:
 * 26% doing resyncs as a result of marking another page out-of-sync
 * 9% promoting pages
 * 27% resyncing as a result of cr3 switches
And almost the entirety of all of those can be attributed to
brute-force searches to remove writable mappings.

(Caveat emptor: My tool was designed to analyze XenServer product
traces, which have a different trace file format than xen-unstable.
I've just taught it to read the xen-unstable trace formats, so the
exact percentages may be incorrect still.  But the preponderance of
brute-force searches is unmistakable.)

The good news is that if we can finger the cause of the brute-force
searches, we should be able to reduce all those numbers down to
respectable levels; my guess is totaling not more than 5%.


On Thu, Sep 11, 2008 at 6:25 PM, Gianluca Guida
<gianluca.guida@xxxxxxxxxxxxx> wrote:
> Todd Deshane wrote:
>> Can you also run kernbench on native for comparison?
> Here tehy are, with a two CPUs dom0.
> Thu Sep 11 13:03:57 EDT 2008
> Average Optimal load -j 8 Run (std deviation):
> Elapsed Time 181.51 (0.550318)
> User Time 300.494 (0.965572)
> System Time 54.198 (0.784391)
> Percent CPU 195 (0.707107)
> Context Switches 26611.8 (205.029)
> Sleeps 29778.8 (330.637)
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