[Beowulf] Begginers question # 1

Douglas Eadline deadline at eadline.org
Tue Oct 5 05:40:34 PDT 2010

> If I build a cluster with 8 motherboards with 1 single core each would it
> be the same as using just one motherboard but with two quad core
> processors? I wanna build one of these but wanna save money and space and
> if what counts is the amount of cores to process info I think fewer
> motherboards with dual six-core processors is definitely cheaper just
> because I wont be needing that many mothers power supplies etc. thanks

First the short and easy answer: "It all depends"

Now the longer answer.

A single 8-way system has plenty of advantages and four 2-way or
eight 1-way systems certainly have a more overhead, cables, space etc.
If you want to play with parallel computing and MPI the 8-way
system will work just fine. (And yes, MPI works just fine on
SMP systems.) OpenMP is also an option in this case, but remember,
OpenMP will not "scale beyond the motherboard" (or at least
there are no guarantees) while MPI can. I recently did a whole
bunch of tests using both MPI and OpenMP a 12-way (dual 6-core)
SMP box I will be posting soon.

As I see it, one of the issues with the higher core counts is
memory contention. An 8-way parallel program that hits hard
on the memory may not scale as well as eight 1-way cores.
This is where "it all depends" comes into play because it
is very application dependent.

I have a small script that I run on multi-core systems
that uses the NAS parallel suite (single process) to
give a hint at memory performance. I call it "effective cores."
Check these two articles for some recent results:


(you have to register, rather painless)

<Shameless Plug>
Note, the Limulus Project is an attempt to lower
the overhead for small personal clusters. I'll have some
news "real soon" about an 18-core design (one 6-way and three 4-way)
that fits in one case and uses one power supply.
More here: http://limulus.basement-supercomputing.com/
(there are links to pics and video)
</Shameless Plug>


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