Re: [Xen-devel] [PATCH 0/9] Porting the intel_pstate driver to Xen

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.04.15 at 07:12, <wei.w.wang@xxxxxxxxx> wrote:
> On 23/04/2015 22:09, Jan Beulich wrote:
>> >>> On 23.04.16 at 15:31, <wei.w.wang@xxxxxxxxx> wrote:
>> > The intel_pstate.c file under xen/arch/x86/acpi/cpufreq/ contains all
>> > the logic for selecting the current P-state. It follows its
>> > implementation in the kernel. Instead of using the traditional cpufreq
>> > governors, intel_pstate implements its internal governor in the
>> > "setpolicy()".
>> 
>> And this internal governor behaves how? Like ondemand, powersave,
>> peerformance, or yet something else? And how would its behavior be
>> changed?
> 
> In the kenel intel_pstate implementation, they have two internal governors: 
> Powersave and Performance.
> Powersave is similar to the old (cpufreq) ondemand governor. A timer 
> function is periodically invoked to sample the CPU busy info (e.g. will get 
> increased due to the running of a CPU-intensive workload). However, the final 
> calculated target value is clamped into the [min_pct, max_pct] limit 
> interval.
> The Performance governor is actually a special case of Powersave, when the 
> min_pct= max_pct=100%. This is the same as the old performance governor.

So a true powersave one would then be accomplished by setting
min_pct = max_pct = <some value smaller than 100>%. Is there
a limit on the valid percentages to be specified here?

Also, you calling "powersave" what supposedly is "ondemand"
makes me nervous about it not immediately raising the CPU freq
when load increases, yet imo that's a fundamental requirement
for server kind loads where you don't want to run in
"performance" mode. Can you clarify the behavior here?

> Here in the ported version, the limit interval can be set via the new added 
> interfaces in xenpm. I think we can make use of only the Powersave governor, 
> and the Performance governor can actually be simply achieved by setting 
> min_pct= max_pct=100%. 
> 
> If all of you are agree, I will remove the Performance governor related code 
> in the next version of patchset.  

Yes, this certainly seems to make sense with the above.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel