Blender 2.79 - One graphics card no longer works

Hi Blenderers,

I recently upgraded to Blender 2.79 and since then my dual graphics cards have not been working.

I do a render with both graphics cards turned on and the render takes double the time of only having one turned on.
It’s baffling me. Any advice on how to debug this problem? I miss my renders only taking 10 minutes, rather than 20!

This happens for both full renders and viewport renders.

Based on what?
Right now we don’t even know your OS, model and brand of your graphics cards, driver versions…

And if using both cards is actually slower than using only one, I don’t think the issue is one card not working correctly. Because in that case the render time wouldn’t change at all - as still only the one card is rendering, right?

Hmmm, you raise a good point. 1 card vs 2 cards (but one not working) should theoretically render in the same time. I find that even more confusing!

Okay, I’ve done a little more testing, and this problem only seems to arise when I use “Branched Path Tracing”.

Here’s the results using “Path Tracing”
Single GPU: 1.38 mins
Dual GPU: 55 seconds

And here using “Branched Path Tracing”
Single GPU: 31 seconds
Dual GPU: 1min 51 seconds

And here’s my Computer Specs:
OS: Windows 8
Graphics card: NVIDEO GeForce GTX670 (Dual)
Driver Version: (updated 18/07/2017)
Blender Version: 2.97