On Tue, 25 Jul 2017 15:51:05 +0200 Peter Zijlstra <pet...@infradead.org> wrote:
> On Tue, Jul 25, 2017 at 09:03:08AM +0200, Luca Abeni wrote: > > > > I'm still confused.. > > > > > > So GRUB does: > > > > > > dq = Uact -dt > > > > > > right? > > > > Right. This is what the original (single processor) GRUB did. And > > this was used by the "GRUB-PA" algorithm: > > https://www.researchgate.net/profile/Giuseppe_Lipari/publication/220800940_Using_resource_reservation_techniques_for_power-aware_scheduling/links/09e41513639b2703fc000000.pdf > > > > (basically, GRUB-PA uses GRUB for reclaiming, and scales the CPU > > frequency based on Uact) > > > > > > > Now, you do DVFS using that same Uact. If we lower the clock, we > > > need more time, so would we then not end up with something like: > > > > > > dq = 1/Uact -dt > > > > Well, in the GRUB-PA algorithm GRUB reclaiming is the mechanism > > used to give more runtime to the task... Since Uact is < 1, doing > > dq = - Uact * dt > > means that we decrease the current runtime by a smaller amount of > > time. And so we end up giving more runtime to the task: instead of > > giving dl_runtime every dl_period, we give "dl_runtime / Uact" every > > dl_period... And since the CPU is slower (by a ratio Uact), this is > > equivalent to giving dl_runtime at the maximum CPU speed / frequency > > (at least, in theory :). > > > > > > > After all; our budget assignment is such that we're able to > > > complete our work at max freq. Therefore, when we lower the > > > frequency, we'll have to increase budget pro rata, otherwise > > > we'll not complete our work and badness happens. > > > > Right. But instead of increasing dl_runtime, GRUB-PA decreases the > > amount of time accounted to the current runtime. > > > > > > > Say we have a 1 Ghz part and Uact=0.5 we'd select 500 Mhz and need > > > double the time to complete. > > > > > > Now, if we fold these two together, you'd get: > > > > > > dq = Uact/Uact -dt = -dt > > > > Not sure why " / Uact"... According to the GRUB-PA algorithm, you > > just do > > dq = - Uact * dt = -0.5dt > > and you end up giving the CPU to the task for 2 * dl_runtime every > > dl_period (as expected) > > Yeah, I seem to have gone off the rails there... Bah I'm terminally > confused now. Let me try and get my brain the right way up. This stuff always confuses me too... :) The parts that gives me more headaches is how to combine GRUB-PA with non-reclaiming tasks, and how to cope with "real world issues" (such as an actual DVFS frequency different from the theoretical one, GRUB reclaiming less than 100% of the CPU time, etc...) Anyway, Claudio is running some experiments with this patchset, measuring power saving and missed deadlines for various sets of periodic real-time tasks... We hope to present the results at RTLWS. Luca