[Beowulf] Mosix

[Robert G. Brown]

On Tue, 18 Jan 2005, Rajesh Bhairampally wrote:

> Hi,
> 
> I am newbee to cluster computing; so if my question sounds stupid, please
> excuse me.
> 
> i am wondering when we have something like mosix (distributed OS available
> at www.mosix.org ), why we should still develop parallel programs and
> strugle with PVM/MPI etc. Tough i never used either mosix or PVM/MPI, I am
> genunely puzzled about it. Can someone kindly educate me?

The best education is in the list archives, as this has be discussed and
described many times before.

In a nutshell -- MOSIX takes jobs submitted on any host in a LAN and
distributes them on other LAN hosts to keep global load balanced across
the LAN (which might or might not be a classical "cluster").  For
relatively simple jobs that have open files and sockets, it creates a
virtual network layer for forwarding their I/O back to the originating
host so if you open a file on your primary host and then the job
migrates, it doesn't crash.  It is one of several tools that is suitable
for running lots of embarrassingly parallel jobs at once on a pool of
systems (others being batch managers like e.g. SGE and/or policy tools
like condor, or any of a variety of fairly new gridware).

PVM or MPI are parallel communications (message passing) libraries.
They facilitate writing truly parallel jobs that are intended a priori
to run on a cluster, managing the passing of messages between what
amount to task threads running on different nodes.  The jobs thus
created may well NOT be embarrassingly parallel -- in fact generally
they won't be, as PVM is overkill for EP tasks although I've used it for
such in the past.

That is, MOSIX doesn't parallelize a serial job, it just runs lots of
serial jobs (independently) in parallel.  PVM or MPI run truly parallel
jobs with lots of NON-independent subtasks advancing a computation, with
nontrivial communications between the subtasks.

Clear?

   rgb

> 
> thanks,
> rajesh
> 
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-- 
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