(no subject)

Robert G. Brown rgb at phy.duke.edu
Thu Apr 11 15:58:18 PDT 2002

On Thu, 11 Apr 2002, Erik Paulson wrote:

> > >If you've got N nodes, submit N copies of SETI at home to your queuing system,
> > >and your cluster will get an N times speedup over a single node. I don't
> > see
> > >how you can hope to do better than that.
> > 
> > I was aware of this possibility, but do not have the skills to implement it.
> Yes you do.  Download Condor, or PBS, or Sun Grid Engine, or buy Platform LSF,
> and:
> A. Install it on N nodes
> B. Submit N copies
> or, install Scyld or MOSIX. Type:
> my_program &
> N times.

And not even for SETI will you get an Nx speedup on N nodes.  There is
ALWAYS a serial fraction even for embarrassingly parallel applications,
and the time required to send the jobs out to the nodes (relative to
just looping N times on the node) is part of it.  In Amdahl's Law N-fold
speedup is the upper bound, not the general, practical limit.

This is the basis of Eric's observation about embarassingly parallel
jobs being ideal for clusters -- they're the ones that often get very
close to N-fold speedup on N nodes for nearly arbitrary N.  "Real"
parallel jobs (ones with nontrivial communications built on MPI or PVM
or raw sockets or even shared memory or some sort of specialized
communications channel) almost never do this well, and more often than
not will only speedup at all up to some maximum number of nodes and then
actually run more slowly if further partitioned.

It's also interesting that master-slave jobs were cited as being "real"
parallel applications as in many cases the master is nothing more than
an intelligent front end for an embarassingly parallel application core.
What's the difference between using a script or Mosix or even a bunch of
rsh's as the "master" that distributes the jobs and collects the results
and using PVM to do exactly the same thing?  Not much, really, but
perhaps a small edge in network efficiency for that part of things.
This may matter -- if the jobs run a short time and communicate with the
master a long time it will matter -- but in cases where this paradigm
makes sense at all (where the ratio of run to communication is the other
way around -- lots of computation, a little communication) it won't
matter much.

Most of this is in any decent book on parallel computing, including at
least one that is freely available on the web.  Then there is my online
book (which I make no claim for being "decent", but it is free:-).  Lots
of these resources are on or linked to various cluster sites, including:



Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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