[Beowulf] New HPCC results and the Myri viewpoint

Stuart Midgley sdm900 at gmail.com
Wed Jul 20 21:59:55 PDT 2005

Actually, I tend to disagree with your comment here.  The curve tells  
you one of the characteristics of the network, which is VERY useful  
in evaluating a network before you expend time/effort testing your  
code on it (assuming you know your code well).  On its own (without  
lots of other micro benchmarks) I agree that it is useless.

In my own experience, I tend to find that most codes are not latency  
sensitive (that is, QsNetII, Infinipath, Myricom etc are effectively  
the same, on a latency sense, to most codes)... until they try and  
scale to the 1000's of cpu's.  All of a sudden simple things like  
barriers and synchronisation etc can become expensive on networks  
with higher latencies.  Things that the software writer wasn't  
expensive start to dominate their code.  Hence, the ping-pong  
latencies and ring latencies are useful in giving you an idea of how  
well the larger codes will scale.

> An example for your curious and open mind: many interconnect people  
> advertize the streamed bandwidth curve, where the sender just keeps  
> sending messages as fast as possible. How often does this  
> communication pattern happens in my reality ? Never. I have never  
> seen an application sending enough messages back to back to fill up  
> the pipeline. So why optimizing for this case ? because the curve  
> looks good and people likes to think they have a bigger pipe than  
> their friends.

Dr Stuart Midgley
sdm900 at gmail.com

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