[Radiance-general] why the rtrace "-n" option doesn't reduce simulation time ?

[Ji Zhang]

I did the same Radiance simulation, one without specifying the "-n" option
for rtrace, the other with "-n 8" specified for rtrace. The former took 9
min to finish, whereas the latter used 13 min ...


I was quite happy to see all the 8 cores of the CPU are fully utilized in
the beginning, but I was quite upset when the results turned out to be
worse than not specifying anything ...


Why the -n option didn't reduce the simulation time but increases it ? ...


Did I miss something obvious here regarding Radiance parallel rendering ?


The two bash scripts are something like:

oconv ./Sky_m3_d21_h13.rad ./materials.mat ./Context.rad >
./Octree_files_m3_d21_h13.oct

cat ./sensor.txt | rtrace -I -h -w -oov -u- -aa 0.1 -ab 6 -ad 1024 -ar 256
-as 512 ./Octree_files_.oct > ./Results_m3_d21_h13_ir_RGB.txt


and:


oconv ./Sky_m3_d21_h13.rad ./materials.mat ./Context.rad >
./Octree_files_m3_d21_h13.oct

cat ./sensor.txt | rtrace -I -h -w -oov -u- -aa 0.1 -ab 6 -ad 1024 -ar 256
-as 512 -n 8 ./Octree_files_.oct > ./Results_m3_d21_h13_ir_RGB.txt


The test was done on a Macbook Pro, OS X 10.8.2, 2.3 GHz Intel Core i7 (8
cores), with 8GB REM.


The images showing the CPU core usage for the two simulations are put at:

http://episode-hopezh.blogspot.sg/2012/10/rtrace-parallel-rendering-myth.html


Advices are appreciated! and thanks in advance!

- Ji
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