Re: dynamic-hz

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Mon, Dec 13, 2004 at 08:30:51AM -0700, Zwane Mwaikambo wrote:
> Hi Russell,
> 
> On Mon, 13 Dec 2004, Russell King wrote:
> 
> > This is an easy thing to grab hold of, but rather pointless in the
> > overall scheme of things.  Those of us who have done power usage
> > measurements know this already.
> > 
> > The only case where this really makes sense is where the CPU power
> > usage outweighs the power consumption of all other peripherals by
> > at least an order of magnitude such that the rest of the system is
> > insignificant compared to the CPU power.
> > 
> > Note: the above CPU power consumption figures were taken from
> > the Intel PXA255 processor electrical specifications, and the
> > "rest of the system" current consumption taken from a real life
> > device.  The timer interrupt taking 2us is probably an over-
> > estimation.  Only the battery lifetime of 24 hours is ficticious.
> 
> While i do not disagree with your research and resultant conclusions for 
> the PXA255 processor i think it may not be as representative of some of 
> the target systems we're interested in, that is, x86 (cringe, cringe). A 
> number of i386 systems enter model defined partial suspend states when 
> execution of the hlt instruction takes place, resuming from these suspend 
> states draws more power for a short period of time thus doing this every 
> millisecond is going to be detrimental to total power consumption over 
> time. But this isn't only an i386 trait as other desktop/workstation 
> processors are similar.

I think you missed the emphasis of my mail - one on measurement to
validate if this technology actually buys you anything.

The second thing you missed is that drawing a lot of power for a short
time may result in a rather negligable reduction in the overall scheme
of things.  Until you measure it and do the calculation, you'll never
know.

I can make the same comments as you above about the PXA255 processor.
"The PXA255 has a special idle mode which reduces power consumption
via the use of a special instruction.  Resuming from this state to
service the timer interrupt results in more power being drawed for
a short period of time, thus doing this every millisecond is going
to be determental to the total power consumption over time."

See?  Measurements in reality give the true story.  Words are just
that - words - which may not reflect reality.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
                 2.6 Serial core