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

[Greg Lindahl]

On Mon, Feb 14, 2005 at 05:42:42PM +0000, Ashley Pittman wrote:

> If you had a bunch of sends to do to N remote processes then I'd expect
> you to post them in order (non-blocking) and wait for them all at the
> end, the time taken to do this should be (base_latency + ( (N-1) * M ))
> where M is the recpipiocal of the "issue rate".  You can clearly see
> here that even for small number of batched sends (even a 2d/3d nearest
> neighbour matrix) the issue rate (that is how little CPU the send call
> consumes) is at least as important that the raw latency.

Unless I completely misunderstand your formula, M is not only the CPU
the send call consumes. It's easy to find situations (fast cpu, slow
network) where the cpu consumed isn't a part of M at all. Even for a
modern 1 GByte/sec network, cpu consumed might not be a part of M.

Reducing CPU consumed can't hurt. But reasoning about it seems to be
less useful than testing actual applications.

-- greg