Re: [Xen-devel] Performance difference between Xen versions

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

On 04/29/11 18:10, Jan Beulich wrote:
> > > > On 29.04.11 at 14:32, Juergen Gross<juergen.gross@xxxxxxxxxxxxxx>  wrote:
> > Hi,
> >
> > comparing performance of different Xen versions with BS2000 as HVM guest
> > showed some weird data I'd like to understand.
> >
> > All measurements were done on an Intel Xeon E7220 box. We used a disk-
> > benchmark and found the cpu utilization was much higher with Xen 4.0
> > compared
> > to Xen 3.3. I did some more investigation and narrowed things down to calls
> > of
> > the hypervisor (implicit or explicit).
> >
> > Following is a table with timing data for different low-level functions, all
> > timing values are tsc ticks obtained via rdtsc:
> >
> > Xen 3.3     Xen 4.0      Function
> >         88        165      just the measurement overhead
> >        176        330      rdtsc-instruction + cli/sti
> >       5896      11044      lapic timer query
> >       7381      13519      setting lapic timer
> >       4653       8987      reload of cr3
> >       3124       5709      invlpg instruction
> >     792253     792264      wbinvd instruction
> >        748       1375      int + iret
> >       5203       9317      hypervisor yield call
> > 12598102   12597882      memory access loop
> >
> > All operations involving the hypervisor take nearly twice the time on 4.0.
> > Operations not involving the hypervisor (wbinvd and memory access loop) are
> > the same on both systems (this rules out the possibility of different rdtsc
> > behavior).
> >
> > Is there any easy explanation for this? Both Xen versions are from SLES
> > (SLES11 or SLES11 SP1).
> I think cpufreq handling was off by default in 3.3, and is on by
> default on 4.0. Try turning this off, or using the performance
> governor.
Jan, you got it! With cpufreq=none Xen 4.0 has more or less the same numbers
as 3.3. Now I wonder why the default is so much slower. I looks as if the
hypervisor would run at a lower speed. I can't believe it should behave like 
that!


Juergen

--
Juergen Gross                 Principal Developer Operating Systems
TSP ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
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