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From: Morten Rasmussen <morten.rasmussen@arm.com>
To: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>,
linux-kernel <linux-kernel@vger.kernel.org>,
Qais Yousef <qais.yousef@arm.com>,
Valentin Schneider <valentin.schneider@arm.com>
Subject: Re: [PATCH V2] sched: fair: Use the earliest break even
Date: Wed, 18 Mar 2020 15:38:23 +0100 [thread overview]
Message-ID: <20200318143823.GC6103@e123083-lin> (raw)
In-Reply-To: <798e9bde-a207-3a0e-0f13-0e27d60fd286@linaro.org>
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
next prev parent reply other threads:[~2020-03-18 14:38 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-11 20:26 Daniel Lezcano
2020-03-12 8:36 ` Vincent Guittot
2020-03-12 10:04 ` Daniel Lezcano
2020-03-12 12:27 ` Vincent Guittot
2020-03-13 12:15 ` Daniel Lezcano
2020-03-13 13:15 ` Vincent Guittot
2020-03-13 13:17 ` Daniel Lezcano
2020-03-13 13:21 ` Vincent Guittot
2020-03-17 7:56 ` Morten Rasmussen
2020-03-17 13:48 ` Daniel Lezcano
2020-03-17 14:30 ` Morten Rasmussen
2020-03-17 17:07 ` Daniel Lezcano
2020-03-18 8:24 ` Morten Rasmussen
2020-03-18 10:17 ` Daniel Lezcano
2020-03-18 14:38 ` Morten Rasmussen [this message]
2020-03-18 7:51 ` Vincent Guittot
2020-03-18 8:33 ` Morten Rasmussen
2020-03-17 10:56 ` Valentin Schneider
2020-03-17 13:59 ` Daniel Lezcano
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