Date: Wed, 19 Mar 2008 12:19:29 +0000 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Peter Jeremy <peterjeremy@optushome.com.au> Cc: arch@freebsd.org Subject: Re: Power-Mgt (Was: Re: cvs commit: src/sys/i386/cpufreq est.c ) Message-ID: <9664.1205929169@critter.freebsd.dk> In-Reply-To: Your message of "Wed, 19 Mar 2008 17:44:33 %2B1100." <20080319064433.GA44676@server.vk2pj.dyndns.org>
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In message <20080319064433.GA44676@server.vk2pj.dyndns.org>, Peter Jeremy write s: >On Tue, Mar 18, 2008 at 07:52:02PM +0000, Poul-Henning Kamp wrote: >>When we talk about macroscopic efforts, turning of hardware we don't >>use, spinning down disks, common sense says that power is saved and >>we can leave it at that. > >Except that it takes more power to spin up a disk than keep is >spinning. Even neglecting the disk life issue, powering a disk down >for a short period and then powering it back up may use more energy >than keeping it running. I was talking in the context of having a facility for spinning disks down vs. always letting them run. You're talking about when we spin them down, which is a matter of tuning. Yes, obviously our defaults should be sensible, as always. >>I have not tried to find out how exact the power measurements ACPI >>offers on laptops are, I know some of the chips used but have >>never double-checked the result. > >I don't believe ACPI lets you get at the numbers with sufficient >resolution to manage anything particularly meaningful. I'm not so sure, the chips have pretty good resolution and high accumulation rate, it's ACPI which only ask the chip every 30 seconds. >Any decent bench supply should be stiff enough to treat the voltage as >a constant so just monitoring the current is adequate to calculate >power. The problem with this approach, is that you need to accumulate current measurements at least 500 times per second, to get a realistic picture of the power content of the spikes. You can of course do a lot to smooth this out, but then it turns into (even more) of an electronics task. The only PSU's I know that can do this themselves are the HP/Agilent "extra 3" supplies like the 66311 and similar. If you want to measure on the high-voltage side, the best and cheapest strategy is to get a utility-class powermeter (like the DIN unit i linked to in the other mail) -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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