On 10/03/16 00:41, Rafael J. Wysocki wrote: > On Wed, Mar 9, 2016 at 11:15 AM, Juri Lelli <juri.lelli@xxxxxxx> wrote: > > Hi, > > > > sorry if I didn't reply yet. Trying to cope with jetlag and > > talks/meetings these days :-). Let me see if I'm getting what you are > > discussing, though. > > > > On 08/03/16 21:05, Rafael J. Wysocki wrote: > >> On Tue, Mar 8, 2016 at 8:26 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > >> > On Tue, Mar 08, 2016 at 07:00:57PM +0100, Rafael J. Wysocki wrote: > >> >> On Tue, Mar 8, 2016 at 12:27 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > [...] > > > >> a = max_freq gives next_freq = max_freq for x = 1, 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). > >> > >> 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 > >> > >> (and if x is not frequency invariant, the right-hand side becomes a * > >> x). Then, the extra boost needed to get to x = 1 for frequency > >> invariant is produced by the (max_freq / current_freq) factor that is > >> greater than 1 as long as we are not running at max_freq and a can be > >> chosen as max_freq. > >> > > > > Expanding terms again, your original formula (without the 1.1 factor of > > the last version) was: > > > > next_freq = util / max_cap * max_freq > > > > and this doesn't work when we have freq invariance since util won't go > > over curr_cap. > > Can you please remind me what curr_cap is? > The capacity at current frequency. > > What you propose above is to add another factor, so that we have: > > > > next_freq = util / max_cap * max_freq / curr_freq * max_freq > > > > which should give us the opportunity to reach max_freq also with freq > > invariance. > > > > This should actually be the same of doing: > > > > next_freq = util / max_cap * max_cap / curr_cap * max_freq > > > > We are basically scaling how much the cpu is busy at curr_cap back to > > the 0..1024 scale. And we use this to select next_freq. Also, we can > > simplify this to: > > > > next_freq = util / curr_cap * max_freq > > > > and we save some ops. > > > > However, if that is correct, I think we might have a problem, as we are > > skewing OPP selection towards higher frequencies. Let's suppose we have > > a platform with 3 OPPs: > > > > freq cap > > 1200 1024 > > 900 768 > > 600 512 > > > > As soon a task reaches an utilization of 257 we will be selecting the > > second OPP as > > > > next_freq = 257 / 512 * 1200 ~ 602 > > > > While the cpu is only 50% busy in this case. And we will go at max OPP > > when reaching ~492 (~64% of 768). > > > > That said, I guess this might work as a first solution, but we will > > probably need something better in the future. I understand Rafael's > > concerns regardin margins, but it seems to me that some kind of > > additional parameter will be probably needed anyway to fix this. > > Just to say again how we handle this in schedfreq, with a -20% margin > > applied to the lowest OPP we will get to the next one when utilization > > reaches ~410 (80% busy at curr OPP), and so on for the subsequent ones, > > which is less aggressive and might be better IMHO. > > Well, Peter says that my idea is incorrect, so I'll go for > > next_freq = C * current_freq * util_raw / max > > where C > 1 (and likely C < 1.5) instead. > > That means C has to be determined somehow or guessed. The 80% tipping > point condition seems reasonable to me, though, which leads to C = > 1.25. > Right. So, when using freq. invariant util we have: next_freq = C * curr_freq * util / curr_cap as util_raw = util * max / curr_cap What Vincent is saying makes sense, though. If we use arch_scale_freq_capacity() as denominator instead of max, we can use a single formula for both cases. Best, - Juri -- 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
