Re: [RFC 0/3] Experimental patchset for CPPC

On Fri, Aug 15, 2014 at 10:37:32AM -0400, Ashwin Chaugule wrote:
> Hi Peter,
> 
> On 15 August 2014 10:07, Peter Zijlstra <[email protected]> wrote:
> > On Fri, Aug 15, 2014 at 09:08:50AM -0400, Ashwin Chaugule wrote:
> >> If the OS only looks at Highest, Lowest, Delivered registers and only
> >> writes to Desired, then we're not really any different than how we do
> >> things today in the CPUFreq layer.
> >
> > The thing is; we're already struggling to make 'sense' of x86 as it
> > stands today. And it looks like this CPPC stuff makes the behaviour even
> > less certain.
> 
> I think its still better than the "p-state" thing we have going today,
> where the algorithms are making their decisions based on the incorrect
> assumption that the CPU got what it requested for. (among other things
> listed earlier.) CPPC at least gives you a guarantee that the
> delivered performance will be within a range you requested. It can
> even force the platform to deliver a specific performance value if you
> choose over a specific time window.

Maybe; the guarantee and interrupt on change might be useful indeed. But
which ever way we need aperf/mperf ratios somewhere.

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