I currently use a 2 GPU system and am thinking about upgrading to a 4 GPU setup.
The wiki states that using 2 GPUs will increase tracking speed by a factor of 1.6. Can you expect the tracking speed to increase by a factor of 1.6 for each additional gpu or are there diminishing returns?

In other words, if I use 4 GPUs instead of 2, can I expect the tracking speed to roughly increase by a factor of 2.56 or will it be less?

We have not actually tested any multi-GPU configurations besides 2 GPUs.
I suppose that the performance gain is likely 60% on each doubling of GPU count rather than on adding each new GPU.
So, I expect the speed of 4 GPUs will be about 2.5 times faster than that of 1 GPU. This is just a speculation, though.

Thank you, if I am able to test with a 4 GPU machine I will post the results.

I'm late to this thread, but iPi Mocap Studio brings up the multi-gpu setup and it only shows the first 2 GPU's, even if you have more than 2.

Also, I noticed no performance increase from a Quadro 4000 to dual GTX 780's for "refine" track. Both are a little over 1 second per frame. That's the only thing I've had a chance to test with my new setup (which has 3 GTX 780's, btw).

Hello,
we currently do not support odd GPU numbers. So if you have 3 GPU you can use only 2 of them. 4 GPU should work and be faster than 2 GPUs. As to you tracking speed, measured speed look strange. Expected tracking speeds for single GPU can be found here
http://ipisoft.com/pr/GPU_speed_v_14_04.pdf
These are speeds with head tracking turned off. Turning it on lead to around twice slower speeds. Refine speed approximately equal to Track Forward speed.
Quadro GPUs are MUCH slower for iPi Mocap, so you should not get the same speed as with gtx 780. Maybe you measured tracking speeds with different options / number of cameras.
Also, bottleneck is not necessarily GPU. The speed may be limited by CPU.

_________________
iPi Soft

I just wanted to update you after I've done a little more testing...

I've now taken one of my GTX 780 cards and installed it in a different machine and that machine, which is a 1st generation i7 is now refining at less than .5 seconds per frame. That means it's about three times as fast as a 3rd generation i7 with two GTX 780 cards.

Why is this?

Definitely on your 1st PC there were some configuration or hardware problems. Maybe on your 1st PC SLI was turned on. How to turn of SLI:
http://wiki.ipisoft.com/Multiple_GPUs

Another good test it to try only one GTX 780 on both PCs and compare the results (speed should be the same).

_________________
iPi Soft