Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [RFC][PATCH 7/7] cpufreq: intel_pstate: Change P-state selection algorithm for Core | Date | Tue, 23 Aug 2016 19:30:46 +0200 |
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On Tuesday, August 23, 2016 12:50:14 PM Wanpeng Li wrote: > 2016-08-23 12:08 GMT+08:00 Srinivas Pandruvada > <srinivas.pandruvada@linux.intel.com>: > > On Tue, 2016-08-23 at 11:48 +0800, Wanpeng Li wrote: > >> Hi Rafael, > >> 2016-08-01 7:38 GMT+08:00 Rafael J. Wysocki <rjw@rjwysocki.net>: > >> > > >> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com> > >> > > >> > The PID-base P-state selection algorithm used by intel_pstate for > >> > Core processors is based on very weak foundations. Namely, its > >> > decisions are mostly based on the values of the APERF and MPERF > >> > feedback registers and it only estimates the actual utilization to > >> > check if it is not extremely low (in order to avoid getting stuck > >> > in the highest P-state in that case). > >> > > >> > Since it generally causes the CPU P-state to ramp up quickly, it > >> > leads to satisfactory performance, but the metric used by it is > >> > only > >> > really valid when the CPU changes P-states by itself (ie. in the > >> > turbo > >> > range) and if the P-state value set by the driver is treated by the > >> > CPU as the upper limit on turbo P-states selected by it. > >> > >> Do you mean CPU will ignore the request value which is submitted by > >> intel_pstate driver in the turbo range, but respect the upper limit > >> on > >> the turbo P-states which is submitted by intel_pstate driver? > > Any request to request a P-state even upper limit can be totally > > ignored in turbo range. > > How to understand Rafael's description "if the P-state value set by > the driver is treated by the CPU as the upper limit on the turbo > P-states selected by it"? The upper limit will be totally ignored in > turbo range.
It wasn't ignored, though, by some older CPU models.
Thanks, Rafael
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