From: Peter Zijlstra <peterz@infradead.org>
To: Douglas Raillard <douglas.raillard@arm.com>
Cc: linux-kernel@vger.kernel.org, rjw@rjwysocki.net,
viresh.kumar@linaro.org, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
qperret@google.com, linux-pm@vger.kernel.org
Subject: Re: [RFC PATCH v4 0/6] sched/cpufreq: Make schedutil energy aware
Date: Fri, 14 Feb 2020 13:21:19 +0100 [thread overview]
Message-ID: <20200214122119.GK14879@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <4a664419-f5a6-882f-83ee-5bbf20ff33d3@arm.com>
On Thu, Feb 13, 2020 at 05:49:48PM +0000, Douglas Raillard wrote:
> > So even when:
> >
> > boost = util_avg - util_est
> >
> > is small, despite util_avg being huge (~1024), due to large util_est,
> > we'll still get an effective boost to max_cost ASSUMING cs[].cost and
> > cost_margin have the same curve.
>
> I'm not sure to follow, cs[].cost can be plotted against cs[].freq, but
> cost_margin is a time-based signal (the boost value), so it would be
> plotted against time.
Suppose we have the normalized energy vs frequency curve: x^3
( P ~ V^2 * f, due to lack of better: V ~ f -> P ~ f^3 )
1 +--------------------------------------------------------------------+
| + + + + *|
| x**3 ******* |
| ** |
0.8 |-+ ** +-|
| ** |
| * |
| ** |
0.6 |-+ ** +-|
| ** |
| ** |
| *** |
0.4 |-+ *** +-|
| ** |
| *** |
| *** |
0.2 |-+ **** +-|
| **** |
| ****** |
| + ********** + + |
0 +--------------------------------------------------------------------+
0 0.2 0.4 0.6 0.8 1
where x is our normalized frequency and y is the normalized energy.
Further, remember that schedutil does (per construction; for lack of
better):
f ~ u
So at u=0.6, we're at f=0.6 and P=0.2
+ boost = util_avg - util_est_enqueued;
So for util_est = 0.6, we're limited to: boost = 0.4.
+ max_cost = pd->table[pd->nr_cap_states - 1].cost;
+ cost_margin = (cost_margin * max_cost) / EM_COST_MARGIN_SCALE;
Which then gives:
cost_margin = boost = 0.4
And we find that:
P' = P + cost_margin = 0.2 + 0.4 = 0.6 < 1
So even though set out to allow a 100% boost in energy usage, we were in
fact incapable of achieving this, because our cost_margin is linear in u
while the energy (or cost) curve is cubic in u.
That was my argument; but I think that now that I've expanded on it, I
see a flaw, because when we do have boost = 0.4, this means util_avg =
1, and we would've selected f = 1, and boosting would've been pointless.
So let me try again:
f = util_avg, P = f^3, boost = util_avg - util_est
P' = util_avg ^ 3 + util_avg - util_est
And I'm then failing to make further sense of that; it of course means
that P'(u) is larger than P(2u) for some u, but I don't think we set
that as a goal either.
Let me ponder this a little more while I go read the rest of your email.
next prev parent reply other threads:[~2020-02-14 12:21 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-22 17:35 [RFC PATCH v4 0/6] sched/cpufreq: Make schedutil energy aware Douglas RAILLARD
2020-01-22 17:35 ` [RFC PATCH v4 1/6] PM: Introduce em_pd_get_higher_freq() Douglas RAILLARD
2020-01-22 17:35 ` [RFC PATCH v4 2/6] sched/cpufreq: Attach perf domain to sugov policy Douglas RAILLARD
2020-01-22 17:35 ` [RFC PATCH v4 3/6] sched/cpufreq: Hook em_pd_get_higher_power() into get_next_freq() Douglas RAILLARD
2020-01-23 16:16 ` Quentin Perret
2020-01-23 17:52 ` Douglas Raillard
2020-01-24 14:37 ` Quentin Perret
2020-01-24 14:58 ` Quentin Perret
2020-02-27 15:51 ` Douglas Raillard
2020-01-22 17:35 ` [RFC PATCH v4 4/6] sched/cpufreq: Introduce sugov_cpu_ramp_boost Douglas RAILLARD
2020-01-23 15:55 ` Rafael J. Wysocki
2020-01-23 17:21 ` Douglas Raillard
2020-01-23 21:02 ` Rafael J. Wysocki
2020-01-28 15:38 ` Douglas Raillard
2020-02-10 13:08 ` Peter Zijlstra
2020-02-13 10:49 ` Douglas Raillard
2020-01-22 17:35 ` [RFC PATCH v4 5/6] sched/cpufreq: Boost schedutil frequency ramp up Douglas RAILLARD
2020-01-22 17:35 ` [RFC PATCH v4 6/6] sched/cpufreq: Add schedutil_em_tp tracepoint Douglas RAILLARD
2020-01-22 18:14 ` [RFC PATCH v4 0/6] sched/cpufreq: Make schedutil energy aware Douglas Raillard
2020-02-10 13:21 ` Peter Zijlstra
2020-02-13 17:49 ` Douglas Raillard
2020-02-14 12:21 ` Peter Zijlstra [this message]
2020-02-14 12:52 ` Peter Zijlstra
2020-03-11 12:25 ` Douglas Raillard
2020-02-14 13:37 ` Peter Zijlstra
2020-03-11 12:40 ` Douglas Raillard
2020-01-23 15:43 ` Rafael J. Wysocki
2020-01-23 17:16 ` Douglas Raillard
2020-02-10 13:30 ` Peter Zijlstra
2020-02-13 11:55 ` Douglas Raillard
2020-02-13 13:20 ` Peter Zijlstra
2020-02-27 15:50 ` Douglas Raillard
2020-01-27 17:16 ` Vincent Guittot
2020-02-10 11:37 ` Douglas Raillard
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