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

[Keir Fraser <keir.xen@xxxxxxxxx>]

Are you sure TSC runs at the same rate in the guest on both hypervisor
versions? Xen 4.0 might trap and emulate a more consistent but slower rate
TSC by default. 'tsc_mode=2' in your domain config file on 4.0 might be a
quick fix.

 -- Keir

On 29/04/2011 13:32, "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).
> 
> 
> Juergen



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