On Fri, Aug 15, 2014 at 10:37:32AM -0400, Ashwin Chaugule wrote:
Hi Peter,
On 15 August 2014 10:07, Peter Zijlstra peterz@infradead.org 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.