a few more RT gpu questions..

[arobbert]

@3dvizual

thanks for the reply, yes its best not to get too technical with me.. i have 0 programming knowledge.

wrt the use of cpu and gpu together, i appreciate that a cpu is a fraction of a gpu in performance, but its still significant; especially if you have several dual 10 core xeons.

quoting the gpu benchmark thread.. here is a comparison between an amd card, and amd card plus modern cpu:

W9100 - 2 minutes 55 seconds using 3ds Max 2014 and V-Ray 3.00.07

W9100 + i74930K using APP - 1 minute 53 seconds using 3ds Max 2014 and V-Ray 3.00.07

and quoting vlado’s comment on this result:

"CPUs are not too shabby; in fact a K6000 card (which has similar performance to a Titan) is just slightly faster for pure raytracing than a high-end dual Xeon machine. The reason GPU renderers are so much faster is because they don’t have all the baggage of years of existing code, plus their code is naturally simpler because of all the restrictions that are there. This is demonstrated by the fact that some of the newer CPU renderers are quite fast too. Of course, a big advantage of GPUs is that you can just put more cards in the system and it gets proportionally faster. Multi-CPU machines on the other hand are very expensive and the performance increase is not there, although one can simply buy more machines. It might also be cheaper to buy one or two graphics cards for an older machine, than it would be to buy one or more high-end CPU render machines.

Best regards, Vlado"

- so of course its infinitely preferable to buy more gpus, but most of us are heavily invested in cpu hardware. to have to let all that essentiall sit there doing nothing just cos we bought a couple of gpus, is a bit of a shame. i want to buy a gpu, even one, and see my rendertimes improve drastically. as it is even if i buy a titan x, ill likely only see a doubling in speed (not insignificant i know) since my 3x 4.6ghz 6 cores will be sitting sleeping. if i had invested in a 20-node cpu renderfarm this would be an even more thorny issue. if i could simply get a gpu and *add* it to my setup instead of replacing it, it would definitely make moving to RT easier.

I recently built a machine with an OC 5960X and a GTX 970. In my tests I can usually get a faster CLEAN result with my CPU compared to RT GPU. I think this is largely because of the adaptive sampler that can’t be used with RT GPU. I would have to add another card to get a speed increase.

It’s nice to know that you can take an older render node or workstation and add a new GPU or two and potentially make it as fast as a machine with a newer processor. I also see advantages for building a render node with a lower end cpu and lots of GPU’s. Potentially that machine could be much faster than an expensive high end CPU based option. It will also require fewer licenses. Of course the big limitation is GPU memory, but that will change soon.