[Radiance-general] radiance + openmosix = :)

[Carsten Bauer]

Hi Francesco,

I can partly confirm the observations from your parallel rendering 
experiments. I didn't use openmosix so far, but some time ago I wrote a 
parallel Radiance using the PVM-Library ('Parallel Virtual Machine').

This PVM provides a collection of routines for interprocess 
communication on different sorts of machines. Instead of using rpiece I 
changed the Radiance code to produce a set of master and slave modules, 
one for the distribution of the work, the others for doing the rendering 
and  collecting the results to produce the final picture. By this, I 
also could avoid the Linux lock manager problem for the ambient files, 
as I shared ambient values among the processes with help of the PVM 
routines.

Although PVM offers to connect all sorts of machines and OS-es together, 
it is of course advisable to use a symmetric cluster built from machines 
with comparable speed. (BTW, by this you also can use the same data 
format on all machines and don't need to codec data into a separate 
exchange buffer).

I started with 4 equal 400MHz processors, when I later added faster 
machines, I quickly observed that, e.g  the two 1.2 GHz machines already 
finished almost all the blocks of the divided image when the four old 
ones hardly managed to trace one block each. So it really doesn't pay 
off to add slow machines to your cluster.

Additionally, the overhead from parallelization was considerable. I 
didn't really care about benchmarking, but I remember to have had an 
overhead of rougly about 20% (with my PVM version), meaning when using 
e.g. 4 machines, rendering was not 4 times but only ~3.2 times faster.( 
-nb: PVM does no migrating, at least not the version from several years 
ago). Its important to make the blocks small, because you almost always 
will end up with the following: all machines finished long time ago but 
only one still working on it's last block :-)

Another totally different problem I observed was that sometimes the 
block boundaries have been visible, i.e. the image showed a checkered 
pattern, different blocks appeared with different brightnesses. I came 
so far to attribute this effect to the direct calculation, but haven't 
completely found out where exactly the cause lies.

Nevertheless that migrating of openmosix sounds interesting, if I find 
the time I'd like to have a look at it..

so long & see you in Fribourg

-Carsten