On Thu, Mar 3, 2016 at 1:20 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Wed, Mar 02, 2016 at 11:49:48PM +0100, Rafael J. Wysocki wrote: >> >>> + min_f = sg_policy->policy->cpuinfo.min_freq; >> >>> + max_f = sg_policy->policy->cpuinfo.max_freq; >> >>> + next_f = util > max ? max_f : min_f + util * (max_f - min_f) / max; > >> In case a more formal derivation of this formula is needed, it is >> based on the following 3 assumptions: >> >> (1) Performance is a linear function of frequency. >> (2) Required performance is a linear function of the utilization ratio >> x = util/max as provided by the scheduler (0 <= x <= 1). > >> (3) The minimum possible frequency (min_freq) corresponds to x = 0 and >> the maximum possible frequency (max_freq) corresponds to x = 1. >> >> (1) and (2) combined imply that >> >> f = a * x + b >> >> (f - frequency, a, b - constants to be determined) and then (3) quite >> trivially leads to b = min_freq and a = max_freq - min_freq. > > 3 is the problem, that just doesn't make sense and is probably the > reason why you see very little selection of the min freq. It is about mapping the entire [0,1] interval to the available frequency range. I till overprovision things (the smaller x the more), but then it may help the race-to-idle a bit in theory. > Suppose a machine with the following frequencies: > > 500, 750, 1000 > > And a utilization of 0.4, how does asking for 500 + 0.4 * (1000-500) = > 700 make any sense? Per your point 1, it should should be asking for > 0.4 * 1000 = 400. > > Because, per 1, at 500 it runs exactly half as fast as at 1000, and we > only need 0.4 times as much. Therefore 500 is more than sufficient. OK, but then I don't see why this reasoning only applies to the lower bound of the frequency range. Is there any reason why x = 1 should be the only point mapping to max_freq? If not, then I think it's reasonable to map the middle of the available frequency range to x = 0.5 and then we have b = 0 and a = (max_freq + min_freq) / 2. I'll try that and see how it goes. > Note. we all know that 1 is a 'broken' assumption, but lacking anything > better I think its a reasonable one to make. Right. -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
