[Beowulf] The Walmart Compute Node?

[Robert G. Brown]

On Thu, 8 Nov 2007, Peter St. John wrote:

> Recently, probably you noticed, Walmart began selling a $200 linux PC.
> (Apparently the OS is just Ubuntu 7.10 with a small xindow manager
> instead of Gnome or KDE). Now Slashdot points to
> http://www.linuxdevices.com/news/NS5305482907.html, the MB being sold
> separately for $60 ("development board"). It has 1.5GHz CPU,
> unpopulated memory (slots for 2GB), one 10/100 connection. Does this
> look to y'all like fair FLOPS/$ for a kitchen project? I'm thinking 6
> of them as compute nodes per 8 port router, with a bigger head node
> for fileserving. (actually I'll use a spare room but you know what I
> mean). An arrangement like this might be faster RAM access per core,
> compared to multicore, since each core has no competition for is't own
> memory, right?

Well by now you surely have heard the YMMV litany enough times not to
hear it again from me, but YMMV quite a bit here so let me indicate a
few potential difficulties.

  a) For this money, I'm guessing the CPU is a 32 bit Celery, which has a
very small L2.  For some code this won't matter, but if you're worrying
about multiple cores and a memory bottleneck, let me assure you the L2
bottleneck on a single 32-bit channel will likely be much worse.

  b) Amdahl's law rewards higher clock and fewer CPUs over lower clock
and more CPUs almost (but not quite) without exception.  I doubt that
you are an exception.

  c) A 64-bit CPU has some superlinear speedup compared to a 32-bit CPU
at constant clock, for memory bound code especially.  64-bit CPUs have
much larger caches as well.  This CAN work against you for very cache
unfriendly code, but again in 99% of all applications it will work for
you -- it is what a cache "does".

  d) A perfectly fair question is to what extent the memory bus is
oversubscribed on a 64-bit dual core, say, a very cheap AMD-64 at
roughly twice the clock, with more than twice the total memory
bandwidth, and with two cores.  This is the question that depends in
detail on YOUR APPLICATION.  Many applications are de facto CPU bound
and you get clock speed scaling within a CPU family all the way down to
small cache Celerons.  Others are vehemently not.  "YMMV", so you have
to analyze YOUR application to figure out which it is, where the easiest
way by far to find out is to just try it.

Sounds like it will cost you somewhere between $100 and $200 to set up a
minimal system -- cheap case/power, motherboard, memory, a borrowed
video card.  You can probably beg, borrow, or buy a dual core AMD at
some middling low (but much higher!) clock for no more than $400.  Run
your presumably EP application on the one, and on the other two at a
time.  Buy lots of the winner, use the loser as a desktop or head node
(even the Celery should be fine for that, especially on a 100 Mbps
network).

Now, I'm a gambling man (as you may not know) and I will bet you one
bottle, can, or glass of ice-cold or cellar cool clean and refreshing or
thick and chewy beer as the winner prefers, to be delivered at a
mutually convenient time (such as both of us sitting side by side at in
a venue that purveys said beverages), that the medium-low end AMD-64
kicks the ass of the maximally cheap Celery in price-performance on your
application (where I have an unfair advantage in that I know something
about your application, but I'd make the same bet if I didn't).

To go into detail, I expect that at contant cost you'll end up with
somewhere in the ballpark of 2-3x aggregate bogomips/$ from the AMD,
that memory bottlenecks will eat up no more than a small part of it (I
actually expect the AMDs to win here TOO because of the probably at
least doubled total memory bandwidth and larger cache), that when you
factor in a roughly 4x increase in required system volume and 3x
increase in total power consumption required to run the same number of
Celeries that will match the AMD, at a marginal cost of roughly
$200/year in increased power costs and some increased investment of your
"free" time to install and mange the extra systems... well, let's just
say that I think that the Celeries will look ugly.  And I'd expect
similar savings from the lowball dual core Xeons, honesly -- system
price around $350-500 stripped to match where you vary in this range to
find the sweet spot in terms of total memory, processor clock, and other
configuration details.

Before you turn me down, note that this is a win-win bet for both of us,
since the winner gets to buy the next round...;-)

    rgb

> Thanks,
> Peter
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-- 
Robert G. Brown
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone(cell): 1-919-280-8443
Web: http://www.phy.duke.edu/~rgb
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