Re: Performance difference between Xen versions

["Jan Beulich" <JBeulich@novell.com>]

>>> On 29.04.11 at 14:32, Juergen Gross <juergen.gross@ts.fujitsu.com> 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.

> 
> 
> Juergen