Cheap PCs from Wal-Mart

Mark Hahn hahn at physics.mcmaster.ca
Mon Jun 2 19:36:29 EDT 2003


> I went to the linked page at Toms Hardware and found reasonably close results
> Athlon XP 3200+, and P4 3GHz...

right, as I said "high-end".

> Now, what's the power consumption of those two?
> I think the P4 dissipates a max of 85W (poking around on the same Tom's page)

that would be max TDP.

> 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.

> So, looking at the scaling and comparing P4 against Via C3 Nehemiah
> Speed 6809/1591  4.3:1
> Power   85/15     5.7:1
> 
> So the Via looks like it does more computes/Joule...

well, for dhrystone at least.  which isn't really surprising if you
think about the fact that dhry is entirely on-chip, and much of 
the power dissipated by a high-end CPU is actually the external 
interface.

if you consider a *real* workload which has some external load,
you'd see much higher throughput on the thick system.

> 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"?

> 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.

> Here's the upshot... if you're in an environment where ultimate performance 
> is important, than you're probably best going with as few big fast 
> processors as you can get (the original Cray paradigm). On the other hand, 
> if you're in an environment where maximizing computation for a fixed amount 
> of power/heat dissipation resources (i.e. spaceflight or portable 
> operation), then things like the Via C3 start to look attractive, assuming 
> your task scales well.

almost tautological, it's so unobjectionable ;)

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.

but the real question is: can you afford to use a cluster node which 
has, say, 10-20% of the performance?  you can stretch Amdahl's law a bit
and see that the further you push wimpy nodes, the smaller a problem
domain you can address (requires ever looser coupled programs, longer
latency of individual work-units, etc).

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