[Beowulf] Mosix

[Michael Will]

On Tuesday 18 January 2005 11:31 am, Rajesh Bhairampally wrote:
> 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. 

Because Mosix does not work?

This of course is not really true, for some applications Mosix might be appropriate,
but what it really does is transparently move processes around in a cluster, not
have them become suddenly parallelized. 

Let's have an example:

Generally your application is solving a certain problem, like say taking an image and apply
a certain filter to it. You can write a program for it that is not parallel-aware, and does not use
MPI and just solves the problem of creating one filtered image from one original image.

This serial program might take one hour to run (assuming really large image and really 
complicated filter). 

Mosix can help you now run this on a cluster with 4 nodes, which is cool if you have 4
images and still want to wait 1 hour until you see the first result.

Now if you want to really filter only one image, but in about 15 minutes, you can program your
application differently so that it only works on a quarter of the image. Mosix could still help you
run your code with different input data in your cluster, but then you have to collect the four pieces
and stitch them together and would be unpleasently surprised because the borders of the filter
will show - there was information missing because you did not have the full image available but just
a quarter of it. Now when you adjust your code to exchange that border-information, you are actually
already on the path to become an MPI programmer, and might as well just run it on a beowulf cluster.

So the mpi aware solution to this would be a program that splits up the image into the four quadrants, 
forks into four pieces that will be placed on four available nodes, communicates the border-data between
the pieces and finally collects the result and writes it out as one final image, all in not much more than 
the 15 minutes expected.

Thats why you want to learn how to do real parallel programming instead of relying on some transparent
mechanism to guess how to solve your problem.

Michael



> Tough i never used either mosix or PVM/MPI, I am 
> genunely puzzled about it. Can someone kindly educate me?
> 
> thanks,
> rajesh
> 
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