linux-kernel - Re: [11/11] system 1: Saving energy using DVFS

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140120171752.GC29971@arm.com>
Date:	Mon, 20 Jan 2014 17:17:52 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Pavel Machek <pavel@....cz>
Cc:	Morten Rasmussen <Morten.Rasmussen@....com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"markgross@...gnar.org" <markgross@...gnar.org>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [11/11] system 1: Saving energy using DVFS

On Mon, Jan 20, 2014 at 05:10:29PM +0000, Catalin Marinas wrote:
> On Mon, Jan 20, 2014 at 04:49:26PM +0000, Pavel Machek wrote:
> > > To save energy, the higher frequencies should be avoided and only used
> > > when the application performance requirements can not be satisfied
> > > otherwise (e.g. spread tasks across more cpus if possible).
> > 
> > I argue this is untrue for any task where user waits for its
> > completion with screen on. (And that's quite important subset).
> > 
> > Lets take Nokia n900 as an example. 
> > 
> > (source http://wiki.maemo.org/N900_Hardware_Power_Consumption)
> > 
> > Sleeping CPU: 2mA
> > Screen on: 230mA
> > CPU loaded: 250mA
> > 
> > Now, lets believe your numbers and pretend system can operate at 33%
> > of speed with 11% power consumption.
> > 
> > Lets take task that takes 10 seconds on max frequency:
> > 
> >       ~ 10s * 470mA     	     	    = 4700mAs
> > 
> > You suggest running at 33% speed, instead; that means 30 seconds on
> > low requency.
> > 
> > CPU on low: 25mA (assumed).
> > 
> >      ~ 30s * 255mA			    = 7650mAs
> > 
> > Hmm. So race to idle is good thing on Intel machines, and it is good
> > thing on ARM design I have access to.
> 
> Race to idle doesn't mean that the screen goes off as well. Let's say
> the screen stays on for 1 min and the CPU needs to be running for 10s
> over this minute, in the first case you have:
> 
> 	10s & 250mA + 60s * 230mA = 16300mAs
> 
> in the second case you have:
> 
> 	30s * 25mA + 60s * 230mA = 14550mAs
> 
> That's a 1750mAs difference. There are of course other parts drawing
> current but simple things like the above really make a difference in the
> mobile space, both in terms of battery and thermal budget.

BTW, the proper way to calculate this is to use the energy rather than
current x time. This would be J = Ohm * A^2 * s = V^2 / Ohm * s (so the
impact of the current is even bigger).

-- 
Catalin
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/