[Beowulf] Re: Re: Home beowulf - NIC latencies

Joachim Worringen joachim at ccrl-nece.de
Fri Feb 11 13:49:48 EST 2005


Greg Lindahl wrote:
> On Thu, Feb 10, 2005 at 01:35:06PM -0700, Maurice Hilarius wrote:
>>If I have a fantastic device that uses infinitely small time (latency) 
>>and moves huge amounts of data (bandwidth) but in doing so it takes 80% 
>>of a CPU, we do not have a useful solution..
> 
> If large cpu usage is a problem, it will show up nicely in real
> application benchmarks.

True. I always wonder what the low-CPU-usage-advocates want the MPI 
process to do while i.e. an MPI_Send() is executed. For small messages 
(which are critical for many applications), it's somewhat like 
requesting that a local memory-write has to show low CPU usage.

Of course, I can think of scenarios in which data transfers w/o CPU 
usage do promise advantages, and I have implemented and evaluated such 
techniques myself. But in the end (for the application), it always 
boiled down to latency and bandwidth as most applications don't honor 
"true" asynchronous communication.

The latest unsuccessful case of uncoupling computation and MPI 
communication I read about was BG/L when using the second CPU as a 
message processor. Maybe Myrinet MX will behave differently by making 
the MPI itself more concurrent on hardware level (is this a correct 
description, Patrick?) - but it will need matching applications, too.

  Joachim

-- 
Joachim Worringen - NEC C&C research lab St.Augustin
fon +49-2241-9252.20 - fax .99 - http://www.ccrl-nece.de
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