Re: Performance difference between Xen versions

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

On 04/29/11 18:10, Jan Beulich wrote:
>>>> 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.
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

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Juergen Gross                 Principal Developer Operating Systems
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