Messages in this thread | | | Date | Wed, 18 Mar 2020 15:38:23 +0100 | From | Morten Rasmussen <> | Subject | Re: [PATCH V2] sched: fair: Use the earliest break even |
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On Wed, Mar 18, 2020 at 11:17:49AM +0100, Daniel Lezcano wrote: > On 18/03/2020 09:24, Morten Rasmussen wrote: > > On Tue, Mar 17, 2020 at 06:07:43PM +0100, Daniel Lezcano wrote: > >> On 17/03/2020 15:30, Morten Rasmussen wrote: > >>> On Tue, Mar 17, 2020 at 02:48:51PM +0100, Daniel Lezcano wrote: > >>>> On 17/03/2020 08:56, Morten Rasmussen wrote: > >>>>> On Thu, Mar 12, 2020 at 11:04:19AM +0100, Daniel Lezcano wrote: > >>>>>>>> In order to be more energy efficient but without impacting the > >>>>>>>> performances, let's use another criteria: the break even deadline. > >>>>>>>> > >>>>>>>> At idle time, when we store the idle state the CPU is entering in, we > >>>>>>>> compute the next deadline where the CPU could be woken up without > >>>>>>>> spending more energy to sleep. > >>>>> > >>>>> I don't follow the argument that sleeping longer should improve energy > >>>>> consumption. > >>>> > >>>> May be it is not explained correctly. > >>>> > >>>> The patch is about selecting a CPU with the smallest break even deadline > >>>> value. In a group of idle CPUs in the same idle state, we will pick the > >>>> one with the smallest break even dead line which is the one with the > >>>> highest probability it already reached its target residency. > >>>> > >>>> It is best effort. > >>> > >>> Indeed. I get what the patch does, I just don't see how the patch > >>> improves energy efficiency. > >> > >> If the CPU is woken up before it reached the break even, the idle state > >> cost in energy is greater than the energy it saved. > >> > >> Am I misunderstanding your point? > > > > Considering just the waking then yes, it reaches energy break-even. > > However, considering all the CPUs in the system, it just moves the idle > > entry/exit energy cost to a different CPU, it doesn't go away. > > > > Whether you have: > > > > |-BE-| > > ____ ____ > > CPU0: ___/ \__/ \___ > > > > CPU1: ____________________ > > > > Or: > > |-BE-| > > ____ > > CPU0: ___/ \___________ > > ____ > > CPU1: ___________/ \___ > > > > _ > > = CPU busy = P_{busy} > > _ = CPU idle = P_{idle} > > / = CPU idle exit = P_{exit} > > \ = CPU idle entry = P_{entry} > > > > The sum of areas under the curves is the same, i.e. the total energy is > > unchanged. > > It is a counter-intuitive comment, now I get it, thanks for the > clarification. It is a good point.
No problem.
> Taking into consideration the dynamic, in the case #1, the break even is > not reached, the idle duration is smaller and that leads the governor to > choose shallower idle states after and consequently CPU0 will be used in > priority. We end up with CPU0 in a shallow state and CPU1 in a deep state.
Indeed. I was speculating earlier if the opposite could happen too. If we extended the second case to form a repeating pattern, could we prevent somehow prevent CPU1 from reaching a deeper state? Could we have pattern that would keep both CPUs in shallow state where it would have been more efficient to consolidate the wake-ups on CPU0 and let CPU1 enter deeper states?
> > With the case #2, we can have the CPUs in both deep state and the > governor should be keeping choosing the same idle state.
Ideally yes. However it depends on the break-even times of the deeper states and when the next wake-ups happen.
> I don't know what is more energy/perf efficient. IMO this is very > workload dependent. The only way to check is to test. Hopefully I can > find a platform for that.
Moving the wake-up shouldn't impact energy directly, although it have a positive latency impact as you are more likely to avoid waking up CPUs that haven't finished the idle entry sequence. However, changing the wake-up pattern could have an indirect energy impact, positive or negative. It isn't clear to me either what outcome to expect.
Morten
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