[Beowulf] many cores and ib

Gilad Shainer Shainer at mellanox.com
Tue May 6 21:20:15 EDT 2008


Patrick Geoffray wrote: 
> > It is the same benchmark that QLogic were and are using for MPI 
> > message rate, and I guess you know that better then me, 
> don't you?....  
> > I want to make sure when one do a comparison he/she will be 
> using the 
> > same benchmark/output to compare.
> 
> It is not the benchmark, it's the MPI implementation. 


My apologizes. I meant the MPI includes an option to collect several MPI
messages into one network message. For applications cases, sometimes it
helps with performance and sometimes it does not. OSU have shown both
cases, and every user can decide what works best for him.


> The benchmark in itself is stupid, because it sends a gazillion 
> messages to a single node. The MPI implementation is 
> dishonest, because it says "eh, you are trying to send a 
> gazillion messages to a single node, let me pack them into a 
> single message on the wire for you", completely changing what 
> the benchmark is trying to measure.


As long as you use the same code and the same benchmark (and the same
platform), you can use any benchmark to compare between different
network devices. You can claim that if you want to count the network
messages and not the MPI messages, using this way to do it will give you
the wrong number, but if you want to compare between 2 interconnects,
this is valid. 


> You are a marketing guy, 


I see it as a compliment, so thanks, much appreciated. 


> you just repeat the numbers without 
> understanding what they mean. 


This becomes personal now... :-) you don't need to be angry all the
time. Bad for your health. I could have replay back mentioning all the
FUD and misleading information you provide wherever you go, and we saw
couple of examples just few weeks ago, but I wont.   


> I can imagine the next optimization from here: if you happen 
> to send messages full of zeros in your ping-pong, MVAPICH 
> will "compress" them for you. And somewhere, someone will 
> claim a gazillion bytes per second...


Old joke. I am trying to use jokes only once. At the end of the day, the
best way is to benchmark your applications and see what gets you better
performance. If someone want to benchmark IB he can use the benchmark
center at Mellanox. 

By the way, since ISC08 conf is just around the corner, you are welcome
to visit our booth and see several IB QDR demonstrations. 

Gilad.

> 
> Patrick
> 

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