Re: [PATCH 0/9] GPU-bound energy efficiency improvements for the intel_pstate driver.

From: Francisco Jerez <currojerez@riseup.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: intel-gfx@lists.freedesktop.org, linux-pm@vger.kernel.org,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Eero Tamminen <eero.t.tamminen@intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Subject: Re: [PATCH 0/9] GPU-bound energy efficiency improvements for the intel_pstate driver.
Date: Thu, 12 Apr 2018 11:34:54 -0700	[thread overview]
Message-ID: <87r2nk46k1.fsf@riseup.net> (raw)
In-Reply-To: <20180412085824.GX4082@hirez.programming.kicks-ass.net>

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Peter Zijlstra <peterz@infradead.org> writes:

> On Wed, Apr 11, 2018 at 09:26:11AM -0700, Francisco Jerez wrote:
>> "just like" here is possibly somewhat unfair to the schedutil governor,
>> admittedly its progressive IOWAIT boosting behavior seems somewhat less
>> wasteful than the intel_pstate non-HWP governor's IOWAIT boosting
>> behavior, but it's still largely unhelpful on IO-bound conditions.
>
> So you understand why we need the iowait boosting right?
>

Yeah, sort of.  The latency-minimizing state of this governor provides a
comparable effect, but it's based on a pessimistic estimate of the
frequency required for the workload to achieve maximum throughput
(rather than a plain or exponential boost up to the max frequency which
can substantially deviate from that frequency, see the explanation in
PATCH 6 for more details).  It's enabled under conditions partially
overlapping but not identical to iowait boosting: The optimization is
not applied under IO-bound conditions (in order to avoid impacting
energy efficiency negatively for zero or negative payoff), OTOH the
optimization is applied in some cases where the current governor
wouldn't, like RT-priority threads (that's the main difference with v2
I'm planning to send out next week).

> It is just that when we get back to runnable, we want to process the
> next data packet ASAP. See also here:
>
>   https://lkml.kernel.org/r/20170522082154.f57cqovterd2qajv@hirez.programming.kicks-ass.net
>
> What I don't really understand is why it is costing so much power; after
> all, when we're in iowait the CPU is mostly idle and can power-gate.

The reason for the energy efficiency problem of iowait boosting is
precisely the greater oscillation between turbo and idle.  Say that
iowait boost increases the frequency by a factor alpha relative to the
optimal frequency f0 (in terms of energy efficiency) required to execute
some IO-bound workload.  This will cause the CPU to be busy for a
fraction of the time it was busy originally, approximately t1 = t0 /
alpha, which indeed divides the overall energy usage by a factor alpha,
but at the same time multiplies the instantaneous power consumption
while busy by a factor potentially much greater than alpha, since the
CPU's power curve is largely non-linear, and in fact approximately
convex within the frequency range allowed by the policy, so you get an
average energy usage possibly much greater than the optimal.

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