I am having a system built for use with cycles, and I see contradictory answers to the question of multiple GPU and PCIe lane usage.
The answer I see most often in this forum and others is that the lane usage for cycles is very low- so with 3 GPU you can expect to render almost 150% faster than 2 GPU, and almost 3 times faster than one. This claim is supported by the benchmark spread sheet here: https://docs.google.com/spreadsheet/ccc?key=0As2oZAgjSqDCdElkM3l6VTdRQjhTRWhpVS1hZmV3OGc#gid=0
Specifically rows 15, 16, and 17: 1 gtx 670 renders a scene in 77 seconds, 2 gtx 670 render the scene in 39 seconds, and 3 gtx 670 do it in 27 seconds… 77/27 =2.85 faster for 3 cards compared to 1 card…
The other answer I see is that by adding more cards you cripple the performance of your PCIe bandwidth and that rendering speed will increase far less than linearly- THIS argument is supprted by benchmarks like this: https://www.youtube.com/watch?v=j8wK6DOXzAA&feature=youtu.be
Where he goes up to 4 titans:
Time: 0 min 28 seconds (GPU - titan black) (256x256 tiles)
Time: 0 min 13 seconds (GPU - 4x titan black) (256x256 tiles)
4 cards BARELY twice as fast as 1! (but since its only 30 seconds it could be misleading since part of that may be build time?)
The fellow doing my computer build INSISTS that 3 cards will perform very poorly due to PCIe bandwidth and it would be a waste of money to go for 3 cards… Does anyone have a definitive answer with the technical explanation as to why this is or is not the case?
If you have a 3 or 4 gpu card system benchmarks for 1,2,3, and 4 cards would be the best data I could have!