On Wed, Mar 9, 2016 at 5:39 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Tue, Mar 08, 2016 at 09:05:50PM +0100, Rafael J. Wysocki wrote: >> >> This means that on platforms where the utilization is frequency >> >> invariant we should use >> >> >> >> next_freq = a * x >> >> >> >> (where x is given by (2) above) and for platforms where the >> >> utilization is not frequency invariant >> >> >> >> next_freq = a * x * current_freq / max_freq >> >> >> >> and all boils down to finding a. >> > >> > Right. >> >> However, that doesn't seem to be in agreement with the Steve's results >> posted earlier in this thread. > > I could not make anything of those numbers. > >> Also theoretically, with frequency invariant, the only way you can get >> to 100% utilization is by running at the max frequency, so the closer >> to 100% you get, the faster you need to run to get any further. That >> indicates nonlinear to me. > > I'm not seeing that, you get that by using a > 1. No need for > non-linear. OK >> >> Now, it seems reasonable for a to be something like (1 + 1/n) * >> >> max_freq, so for non-frequency invariant we get >> >> >> >> nex_freq = (1 + 1/n) * current_freq * x (*) (see below) >> > This seems like a big leap; where does: >> > >> > (1 + 1/n) * max_freq >> > >> > come from? And what is 'n'? > >> a = max_freq gives next_freq = max_freq for x = 1, > > next_freq = a * x * current_freq / max_freq > > [ a := max_freq, x := 1 ] -> > > = max_freq * 1 * current_freq / max_freq > = current_freq > > != max_freq > > But I think I see what you're saying; because at x = 1, > current_frequency must be max_frequency. Per your earlier point. Correct. >> but with that choice of a you may never get to x = 1 with frequency >> invariant because of the feedback effect mentioned above, so the 1/n >> produces the extra boost needed for that (n is a positive integer). > > OK, so that gets us: > > a = (1 + 1/n) ; n > 0 > > [ I would not have chosen (1 + 1/n), but lets stick to that ] Well, what would you choose then? :-) > So for n = 4 that gets you: a = 1.25, which effectively gets you an 80% > utilization tipping point. That is, 1.25 * .8 = 1, iow. you'll pick the > next frequency (assuming RELATION_L like selection). > > Together this gets you: > > next_freq = (1 + 1/n) * max_freq * x * current_freq / max_freq > = (1 + 1/n) * x * current_freq That seems to be what I said above (*), isn't it? > Again, with n = 4, x > .8 will result in a next_freq > current_freq, and > hence (RELATION_L) pick a higher one. OK >> Quite frankly, to me it looks like linear really is a better >> approximation for "raw" utilization. That is, for frequency invariant >> x we should take: >> >> next_freq = a * x * max_freq / current_freq > > (its very confusing how you use 'x' for both invariant and > non-invariant). > > That doesn't make sense, remember: > > util = \Sum_i u_i * freq_i / max_freq (1) > > Which for systems where freq_i is constant reduces to: > > util = util_raw * current_freq / max_freq (2) > > But you cannot reverse this. IOW you cannot try and divide out > current_freq on a frequency invariant metric. I see. > So going by: > > next_freq = (1 + 1/n) * max_freq * util (3) I think that should be next_freq = (1 + 1/n) * max_freq * util / max (where max is the second argument of cpufreq_update_util) or the dimensions on both sides don't match. > if we substitute (2) into (3) we get: > > = (1 + 1/n) * max_freq * util_raw * current_freq / max_freq > = (1 + 1/n) * current_freq * util_raw (4) > > Which gets you two formula with the same general behaviour. As (2) is > the only approximation of (1) we can make. OK So since utilization is not frequency invariant in the current mainline (or linux-next for that matter) AFAIC, I'm going to use the following in the next version of the schedutil patch series: next_freq = 1.25 * current_freq * util_raw / max where util_raw and max are what I get from cpufreq_update_util(). 1.25 is for the 80% tipping point which I think is reasonable. -- 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
