linux-kernel - Re: [PATCH v2 0/6] sched: expel confusing usage of the term "power"

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Message-ID: <5384B9EE.10203@linaro.org>
Date:	Tue, 27 May 2014 18:14:38 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH v2 0/6] sched: expel confusing usage of the term "power"

On 05/27/2014 06:02 PM, Nicolas Pitre wrote:
> On Tue, 27 May 2014, Daniel Lezcano wrote:
>
>> On 05/27/2014 12:19 AM, Nicolas Pitre wrote:
>>> "Power" is a very bad term in the scheduler context.  There are so many
>>> meanings that can be attached to it.  And with the upcoming "power
>>> aware" scheduler work, confusion is sure to happen.
>>>
>>> The definition of "power" is typically the rate at which work is performed,
>>> energy is converted or electric energy is transferred.  The notion of
>>> "compute capacity" is rather at odds with "power" to the point many
>>> comments in the code have to make it explicit that "capacity" is the
>>> actual intended meaning.
>>>
>>> So let's make it clear what we man by using "capacity" in place of "power"
>>> directly in the code.  That will make the introduction of actual "power
>>> consumption" concepts much clearer later on.
>>>
>>> This is based on the latest tip tree to apply correctly on top of existing
>>> scheduler changes already queued there.
>>>
>>> Changes from v1:
>>>
>>> - capa_factor and SCHED_CAPA_* changed to be spelled "capacity" in full
>>>     to save peterz some Chupacabra nightmares
>>>
>>> - some minor corrections in commit logs
>>>
>>> - rebased on latest tip tree
>>>
>>>
>>>    arch/arm/kernel/topology.c |  54 +++----
>>>    include/linux/sched.h      |   8 +-
>>>    kernel/sched/core.c        |  87 ++++++-----
>>>    kernel/sched/fair.c        | 323 ++++++++++++++++++++-------------------
>>>    kernel/sched/sched.h       |  18 +--
>>>    5 files changed, 246 insertions(+), 244 deletions(-)
>>
>> Hi Nico,
>>
>> it is a good initiative to replace the 'power' word by another to prevent
>> confusion for future code. Personally I have a preference to 'strength'
>> instead of 'capacity', in case that matter.
>
> Proper usage does matter:
>
> Strength could mean many things.  Among them:
>
> Physical ability
>
>       * Physical strength, as in people or animals
>
> As an abstract or psychological trait
>
>       * Virtue and moral uprightness
>       * Courage or fortitude in the face of moral or social pressure
>       * Persuasiveness of an argument
>       * The exercise of willpower
>
> Physics
>
>       * Strength of materials, ability to withstand an applied stress
>         without failure
>            + Compressive strength, capacity to withstand axially directed
>              pushing forces
>            + Tensile strength, maximum stress while being stretched or
>              pulled before necking
>            + Shear strength, the ability to withstand shearing
>       * Strength (explosive), the ability of an explosive to move
>         surrounding material
>       * Field strength, the magnitude of a field's vector
>       * Signal strength, the magnitude of an electric field at a
>         reference point
>
> I have difficulty referring to "CPU strength" without still be confused
> about what exactly this would mean.  None of the above definitions would
> provide a sufficiently close analogy to be applied without ambiguity.

Ok, fair enough.



> On the other hand, the definition for capacity is much narrower:
>
> 1.
> a. The ability to receive, hold, or absorb.
> b. Abbr. c. A measure of this ability; volume.
>
> 2. The maximum amount that can be contained: a trunk filled to capacity.
>
> 3.
> a. Ability to perform or produce; capability.
> b. The maximum or optimum amount that can be produced: factories
> operating below capacity.
>
> Etc.
>
> Here the analogy with "CPU capacity" or "compute capacity" is clear and
> natural for what we are applying this term to.
>
>
> Nicolas
>


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