Re: Status of X86_P4_CLOCKMOD?

From: Matt Mackall <mpm@selenic.com>
To: Dave Jones <davej@redhat.com>, Adrian Bunk <bunk@stusta.de>,
	Dmitry Torokhov <dtor_core@ameritech.net>,
	davej@codemonkey.org.uk,
	Zwane Mwaikambo <zwane@commfireservices.com>,
	Samuel Masham <samuel.masham@gmail.com>,
	Jan Engelhardt <jengelh@linux01.gwdg.de>,
	linux-kernel@vger.kernel.org, Andrew Morton <akpm@osdl.org>,
	cpufreq@lists.linux.org.uk, ak@suse.de
Subject: Re: Status of X86_P4_CLOCKMOD?
Date: Tue, 28 Feb 2006 14:47:20 -0600	[thread overview]
Message-ID: <20060228204720.GD13116@waste.org> (raw)
In-Reply-To: <20060228200916.GA326@redhat.com>

On Tue, Feb 28, 2006 at 03:09:16PM -0500, Dave Jones wrote:
> On Tue, Feb 28, 2006 at 01:46:29PM -0600, Matt Mackall wrote:
>  > On Thu, Feb 23, 2006 at 03:41:10PM -0500, Dave Jones wrote:
>  > > On Thu, Feb 23, 2006 at 08:59:37PM +0100, Adrian Bunk wrote:
>  > > 
>  > >  > > > >  config X86_P4_CLOCKMOD
>  > >  > > > > 	depends on EMBEDDED
>  > >  > > > 
>  > >  > > > This one is an x86_64 only issue, and yes, it's wrong.
>  > >  > > 
>  > >  > > That's for P4, not X86_64... And since P4 clock modulation does not provide
>  > >  > > almost any energy savings it was "hidden" under embedded.
>  > >  > 
>  > >  > But the EMBEDDED dependency is only on x86_64:
>  > >  > 
>  > >  > arch/i386/kernel/cpu/cpufreq/Kconfig:
>  > >  > config X86_P4_CLOCKMOD
>  > >  >         tristate "Intel Pentium 4 clock modulation"
>  > >  >         select CPU_FREQ_TABLE
>  > >  >         help
>  > >  > 
>  > >  > arch/x86_64/kernel/cpufreq/Kconfig:
>  > >  > config X86_P4_CLOCKMOD
>  > >  >         tristate "Intel Pentium 4 clock modulation"
>  > >  >         depends on EMBEDDED
>  > >  >         help
>  > >  > 
>  > >  > And if the option is mostly useless, what is it good for?
>  > > 
>  > > It's sometimes useful in cases where the target CPU doesn't have any better
>  > > option (Speedstep/Powernow).  The big misconception is that it
>  > > somehow saves power & increases battery life. Not so.
>  > > All it does is 'not do work so often'.  The upside of this is
>  > > that in some situations, we generate less heat this way.
>  > 
>  > This is perplexing. Less heat equals less power usage according to the
>  > laws of thermodynamics.
> 
> you end up taking longer to do the same amount of work, so you
> end up using the same overall power.

Doesn't make sense.

Power is energy consumption per unit time. Heat is energy dissipated
per unit time (both are measured in watts). So if you're saying "we
use the same amount of power", then conservation of energy implies "we
generate the same amount of heat." If you're instead saying "we use
the same amount of energy over a longer span of time", that means "we
draw less power from the battery" which means "battery lasts longer".

In short, power usage and heat production are _the same thing_.

-- 
Mathematics is the supreme nostalgia of our time.