Messages in this thread | | | Date | Wed, 18 Mar 2020 09:24:52 +0100 | From | Morten Rasmussen <> | Subject | Re: [PATCH V2] sched: fair: Use the earliest break even |
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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.
Morten
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