[Radiance-general] genBSDF multiple processes

[Greg Ward]

Hi Stephen,

The rcontrib command underpinning rfluxmtx and therefore genBSDF can become i/o bound if the model is not particularly complicated and/or there are few non-specular rays being generated.  If your system is purely specular, this is the sort of behavior I might expect.  As the number of receiving bins increases from -t4 5 to -t4 6, you go from 1024 output directions per input direction to 4096.  If you leave your sampling (-c) parameter at 2000, then you're actually sending fewer samples per incident direction than you have output bins.  This might be OK, depending on your model, but it means that the number of bin results being sent from one process to another exceeds the number of rays being calculated, thus i/o becomes the bottleneck over CPU if you have enough processes.  In a sense, you could increase your -c setting by a factor of 2 or 3 and get better accuracy in the same calculation time.  Try it and see if it doesn't improve your CPU utilization.

Remember that if your number of processes exceeds the number of physical cores (not virtual ones), then your time-linearity will go down dramatically, even if your CPU utilization shows 100%.

Cheers,
-Greg

> From: Stephen Wasilewski <stephen at coolshadow.com>
> Date: October 4, 2016 6:14:24 PM PDT
> 
> I'm getting some curious (to me) behavior when running genBSDF to calculate a rank 4 tensor tree (It also happens with '-t3').  When the sampling resolution is 4 or 5 the program utilizes most of my processing (I notice it starts 9 rcontrrib processes an 1 rfluxmtx process with '-n 8', so the threads are each around 80%).  When I change the resolution to 6 (without changing anything else) the processors top out at 25% and if I increase -n it still uses no more than ~25% of my total cpu power.  There is plenty of unused memory.  
> 
> here is my command:
> genBSDF -n 8 -c 10240 -f +b -r '-ab 10 -ad 1 -ss 0 -st .02' -t4 5 -dim 0 2.673 0 2.706 -1.207 0 -geom millimeter test.rad > test.xml
> 
> Does anyone know what is going on?
> Is there something in the genBSDF program limiting this?
> Is it a hardware limitation?
> Is it some environmental variable (im running mac OSX 10.9.5)?
> 
> Thanks,
> 
> Stephen Wasilewski
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