Re: Performance difference between Xen versions

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

On 04/29/11 15:28, Keir Fraser wrote:
> 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.
Already done :-), so yes, I am sure the tsc rate is the same. The debug key
's' (softTSC stats) shows that no tsc is emulated.

BTW: different tsc rate is improbable as the memory access loop shows
nearly the same tsc difference...

Juergen

>   -- Keir
>
> On 29/04/2011 13:32, "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).
>>
>>
>> Juergen
-- 
Juergen Gross                 Principal Developer Operating Systems
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