Cheap PCs from Wal-Mart

Jim Lux James.P.Lux at jpl.nasa.gov
Tue Jun 3 10:04:29 EDT 2003


> > The C3 at 800 MHz dissipates 6.65W average and peaks at 12W
> > The Nehemiah at 1 GHz dissipates 11.25/15 W ave/pk
>
> I suspect VIA's "peak" is not equivalent (lower) than Intel's TDP.

Hard to know until someone sticks a current probe on the devices...



>
> > This is really a first order approximation. You'd have to look at
> > peripheral power, memory power, and power supply conversion and power
> > distribution efficiency.  Peripherals and memory probably scale with
memory
> > speed fairly linearly.
>
> huh?  do you mean "the P4 drives ram much faster and so the ram
> will also dissipate more power"?

Precisely.. CMOS, to a first order, has power dissipation proportional to
clock frequency. Cycle the bus at 200 MHz and it draws twice as much power
as cycling the bus at 100 MHz.  I don't know if the P4 or C3 have the same
bus width, too? Wider buses draw more power (for the line
drivers/receivers).


>
> > The higher powered device will have more I^2R
> > losses in the power supply and distribution. (Supplying 50 or so Amps at
> > 2-3V is no easy feat)
>
> well, the PS supplies 3.3V; the Vcc for the CPU only has to travel
> a couple of inches, and that's probably in a plane.  I don't really
> thing that's an issue.

It's a huge issue... The efficiency of the PS is lower at 3.3V than at 5V or
12V, for instance.  As far as the CPU core voltage regulator, the same
applies... If you push power through anything, you're going to have more IR
losses at 1.8V than at 2.5V.  There's only so much copper available on the
board to carry the current, and the pin or ball is only so big.  On chip,
there's the issue of the power carrying conductors in the metalization.  The
saving grace is that, for CMOS, the power dissipation also scales with
voltage, so as the feature size goes down (increasing IR losses), the power
consumed goes down too (reducing current).

However, consider.. a contact/trace resistance of 5 milliohms, carrying a
current of 20 amps, disspates 2 Watts... 5 mOhm is pretty darn low..
especially for a pin only 25 mils on a side.



>
> my whole point was that people must not even start to think that
> these low-power CPUs are close in performance.  and even if they were,
> I don't think people really have an intuitive sense for what "scales well"
> means - I doubt *anyone* has a task that is as undemanding of the system
> as dhrystone is, for instance.
>
> > computing, I'd like to see more attention paid to computing per joule or
> > computing rate/watt.  There are different tradeoffs whether you are
>
> sure, but please, let's use specFPrate per watt.  even that's a bit
> dated, since it's so cache-friendly.




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