[Beowulf] Which is better; 4 Mountains or 5 Hills

[Robert G. Brown]

On Sat, 31 Mar 2007, amjad ali wrote:

> Hi All,
>
> Which one of the following two choices are better (assuming both
> clusters have nearly same cost):
>
> 1) A 4-node cluster having 8 AMD Opteron of 2.6GHz each and 2GB RAM/node
>
> 1) A 5-node cluster having 10 AMD Opteron of 2.2GHz each and 2GB RAM/node

What is the meaning of this word "better"?  Better for what?

A)
   4x2.6 = 10.4 aggregate GHz (whatever that means)
   Can do a distributed calculation that fits into 8 GB RAM
   Fewer faster nodes means less Amdahlian penalty for parallel code.
   Probably uses less power, has slightly lower aggregate maintenace
     costs

B)
   5x2.2 = 11 aggregate GHz
   Can do a distributed calculation that fits into 10 GB RAM
   More slower nodes means a bit more flexibility and less penalty if a
     node is down
   Probably uses more power, has higher costs.

Which is "better"?  B) has more aggregate GHz (and presumably FLOPS by
at least some measures) so it is better.  It can also hold a larger
partitioned problem, which for some people makes it MUCH better, and for
others just wastes money.  A) might well run faster for some kinds of
parallel code, and it probably costs less in the long run, once
(expensive) human time and infrastructure costs are accounted for.

> Consider GiGE (max 1000mbps) as the interconnect.
> Is there any significant performance difference between the two?

Sure, maybe, for some code.  For example, try running a 9 GB partitioned
computation on A and oooo, slow (as it swaps to run at all).  Try
running a computation with lots of little messages, but there are just
plain fewer of them between four nodes than for five, and A might shine.

> Does the compasison depends on the MPI communication among nodes, for
> a given application?

It depends on EVERYTHING.  The task mix, how the task(s) are executed,
whether, how, and how much they intercommunicate during execution, how
big they are (or might be) and whether four boxes plugged into your only
available circuit doesn't blow the breaker where five does.

    rgb

>
> Thanking all in anticipation.
> Regards.
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-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu