[Beowulf] scalability

Gus Correa gus at ldeo.columbia.edu
Wed Dec 9 18:11:44 PST 2009

Hi Amjad

There is relatively inexpensive Infiniband SDR:
Not the latest greatest, but faster than Gigabit Ethernet.
A better Gigabit Ethernet switch may help also,
but I wonder if the impact will be as big as expected.

However, are you sure the scalability problems you see are
due to poor network connection?
Could it be perhaps related to the code itself,
or maybe to the processors' memory bandwidth?

You could test if it is network running the program inside a node
(say on 4 cores) and across 4 nodes with
one core in use on each node, or other combinations
(2 cores on 2 nodes).

You could have an indication of the processors' scalability
by timing program runs inside a single node using 1,2,3,4 cores.

My experience with dual socket dual core Xeons vs.
dual socket dual core Opterons,
with the type of code we run here (ocean,atmosphere,climate models,
which are not totally far from your CFD) is that Opterons
scale close to linear, but Xeons get nearly stuck in terms of scaling
when there are more than 2 processes (3 or 4) running in a single node.

My two cents.
Gus Correa
Gustavo Correa
Lamont-Doherty Earth Observatory - Columbia University
Palisades, NY, 10964-8000 - USA

amjad ali wrote:
> Hi all,
> I have, with my group, a small cluster of about 16 nodes (each one with 
> single socket Xeon 3085 or 3110; And I face problem of poor scalability. 
> Its network is quite ordinary GiGE (perhaps DLink DGS-1024D 24-Port 
> 10/100/1000), store and forward switch, of price about $250 only.
> ftp://ftp10.dlink.com/pdfs/products/DGS-1024D/DGS-1024D_ds.pdf
> How should I work on that for better scalability?
> What could be better affordable options of fast switches? (Myrinet, 
> Infiniband are quite costly).
> When buying a switch what should we see in it? What latency?
> Thank you very much.
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