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

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Message-ID: <alpine.LFD.2.11.1405271140050.17310@knanqh.ubzr>
Date:	Tue, 27 May 2014 12:02:06 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Daniel Lezcano <daniel.lezcano@...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 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.

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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