Re: Power-Mgt (Was: Re: cvs commit: src/sys/i386/cpufreq est.c )
- From: "Poul-Henning Kamp" <phk@xxxxxxxxxxxxxx>
- Date: Wed, 19 Mar 2008 12:19:29 +0000
In message <20080319064433.GA44676@xxxxxxxxxxxxxxxxxxxxxxx>, 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@xxxxxxxxxxx | 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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- Re: Power-Mgt (Was: Re: cvs commit: src/sys/i386/cpufreq est.c )
- From: Peter Jeremy
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