RSH scaling problems...

[Trent Piepho]

On Tue, 17 Dec 2002, Greg Lindahl wrote:
> >    Our users who have tested ssh have told us that the wall clock
> > time is much larger than if they use rsh (I'm sorry I don't have any
> > numbers :) This are MPI jobs running over about 12-24 hours.
> > Have you seen this?
> 
> I haven't seen it. If your jobs run that long, just about any method
> of starting will be efficient enough to not increase the runtime.

I know with LAM-MPI, any console I/O from MPI processes is sent back over the
rsh or ssh connection.  If you use ssh instead of rsh, that means all your
output to stdout and stderr, and any input to stdin, will get encrypted and
decrypted.  If you have a lot of i/o this way, that could add up to a few
cycles.  

I wish there was some kind of ssh option to not encrypt content for ssh and
especially scp.  It's one thing to encrypt interactive sessions where you
might type in passwords, but it's slow and unnecessary to when copying
terabytes of atmospheric data from one machine to another.