[Beowulf] What is a "proper" machine count for a cluster

Mark Hahn hahn at mcmaster.ca
Thu Mar 15 09:22:12 PDT 2007

> Now comes the pitch.  The "sweet spot" in cluster nodes at the moment seems 
> to be a dual socket, dual core Opteron or Intel machine with 2 gigs of RAM 
> per core, so each box is a 4-way SMP
> (I will be flayed alive by the list for such cavalier numbers).

merely scourged ;)

I agree with the config you mention for many purposes.  I would almost
certainly consider using 4-core chips for a serial cluster, though I don't
know offhand whether dual-socket would be more cost-effective than single.

it's also worth pointing out that the first fork in the decision-tree is 
really whether to use desktop or server components.  with servers, the 
sweet-spot does tend towards 2-socket.  there are many attractive things 
about this approach, such as frequent builtin dual-gigabit, more robust 
components such as ECC, and important managability features such as IPMI.

OTOH, there is some attraction to going cheap (arguably more beowulfy!)
by using desktop-grade single-socket boards/chips.  if you take this
approach, you're guided towards some other config details - you probably
won't get ECC, for instance (saving some price, but should be considered).
to be cost-effective, you also need to choose a cheap chassis - desktop
boxes piled on wire racks are reasonable, though there are cheap 1U chassis,
as well.  such an approach is cost-sensitive, so you want to configure with
no more ram than necessary and quite possibly no local disk.

> Now, such a box is likely to have oodles (scientific term) more processing 
> power than four pentiums.

just some blue-sky numbers: a 5-year old machine will have individual 
performance figures which are about 1/4 of todays hardware.  that includes
interconnect (gigabit was uncommon then), memory bandwidth, peak flops,
onchip cache, etc.  any real app will probably not pin _all_ the components,
but the fractions are multiplicative, so a delivered performance is some 
weighted _product_ of the factors.  I'd expect somewhere between 5:1 and 20:1.

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