Power-Mgt (Was: Re: cvs commit: src/sys/i386/cpufreq est.c )

[Poul-Henning Kamp]

In message <20080319064433.GA44676 at 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 at 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.