> Date: Tue, 6 Mar 2012 09:16:24 -0500
> From: bogs...@pobox.com
> To: discuss@blu.org
> Subject: Re: [Discuss] AMD FX-8120 update
>
> On Tue, Mar 6, 2012 at 8:44 AM, Chuck Anderson <c...@wpi.edu> wrote:
> > On Tue, Mar 06, 2012 at 04:27:07AM -0500, Shirley Márquez Dúlcey wrote:
> >> The fact that the correct strategy for maximum performance may be
> >> different from the best power management strategy is likely to require
> >> new OS capabilities (changing the scheduling strategy based on power
> >> management settings). If I have four things running on a desktop system
> >> I'd likely prefer to spread them among all four modules for maximum
> >> speed, but on a laptop I might prefer to put them on only two modules
> >> and take the performance hit to get the power reduction of shutting off
> >> the other two modules.
> >
> > I'm not so sure you would gain anything by shutting off modules. It
> > was found with CPU frequency governors that slowing down a processor
> > actually used MORE power, because it took longer to complete running
> > tasks. It is better to have the CPU always run as fast as it can
> > while there are running tasks, and then halt the processor when it is
> > truley idle.
>
> Depends on the kind of task. Some tasks have time limits (viewing a
> video) rather then fixed computational goals.
>
That is not a good example. Video or audio is processed by a separate dedicated
engine. It has a fixed compute and power profile, does not use any significant
CPU (core) resources and the OS typically cannot do much else to reduce the
power consumption. In fact, that is why both Apple and Microsoft (for Win8)
have explicit power specs for audio and video that hardware vendors have to
meet. These have nothing to do with the main CPU's power/performance profile or
capabilities. In general, Chuck's assertion is correct: it's better to execute
fast and go to sleep quicker than to slog along slowly -- the latter takes much
more power. Why? Because the CPU's power consumption is, these days, only a
small fraction of the overall system power. Things like system buses, the IO
buffers on the chips, voltage regulators, PLLs, DLLs, etc. consume a lot of
static power and is independent of how fast the CPU is running. So the quicker
you can shut these down, the better off you are in terms of saving power.
-Nilanjan
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