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

[Peter Jeremy]

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 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.  In any case,
repeatability and monotonicity are more of an issue than absolute
accuracy: As long as we can meaningfully do relational comparisons
then we can make progress.  I suspect the results are going to vary
significantly between systems anyway.

>affect the results), but few off them allow you to measure power
>(ie: Watts) without hooking up GPIB and accumulating a lot of
>current measurements by hand.

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.  If you want to monitor energy then, yes you probably need to
hook it up to an external logger.  You can buy a multimeter with a USB
interface for AUD140 (<EUR100).

Monitoring mains is more of an issue because not only is the power
factor unlikely to be close to 1 for cheap PSUs but it's also likely
to vary with load.

Note that Silicon Chip published a mains power/energy meter in July
and August 2004: http://www.siliconchip.com.au/cms/A_102045/article.html
ISTR it uses a normal low-voltage transformer to measure current, rather
than a true current transformer so I'm not sure of the absolute accuracy.

-- 
Peter Jeremy
Please excuse any delays as the result of my ISP's inability to implement
an MTA that is either RFC2821-compliant or matches their claimed behaviour.
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