[Beowulf] interconnect and compiler ?

Patrick Geoffray patrick at myri.com
Fri Jan 30 17:03:42 EST 2009


Greg Lindahl wrote:
> little computation. InfiniPath gets a speedup on lots of codes that
> you wouldn't predict given the raw latency and bandwidth; how else
> would you explain it?

There are a tons of variables. The one I keep thinking about is PIO 
sending for larger message size than usual. If the data is in cache 
(reasonable assumption for send side), it can remove a lot of load from 
the memory bus compared to DMA. If your code is memory bandwidth bounded 
(aren't they all on multi-core ?), then you have a speedup.

>> Ok, my turn to bite :-) What is a negative "g" ?
> 
> It means that the interconnect is ready to send a 2nd message before
> the 1st one is on the wire. Think pipelining. Or you could ask

That's a warping of the (old and getting older) logp model :-) g cannot 
be negative, the best it could be is null, which means the messages will 
be send on the wire with no bubble between them. You cannot use a 
negative g to express a NIC overhead lower than host, because a negative 
g would compensate o for a single core, and it's not true.

Patrick
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