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.