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Multi GPU uses

Question asked by jl5086 on Nov 24, 2019
Latest reply on Dec 11, 2019 by jl5086

I'm wondering about the actual usefulness of being able to use more than one GPU in Blender for rendering.  I have an RX5700 XT and an RX580 8GB in my system, my question is what benefit do I get from having the option to use both GPUs?  What I have found is that, when using ProRender, any time I render using anything other than just one GPU, the render speed drops dramatically.  Just like selecting the option to use the CPU + GPU: it renders at about 1/2 the speed of just the GPU.  If I select just both GPUs: a specific render goes from 18 seconds up to 31 seconds.   Also when I use Cycles and I choose either the 580 or 5700, individually, the render times are what I would expect, the 5700 being a bit faster than the 580.  If I choose both in cycles it does impact the speed, but not as drastically as in ProRender.   It's harder to compare because in ProRender, if I select just the 580 as the sole render device, it still uses only the 5700 (which has my monitor plugged into it), so it only actually uses the second GPU if I have BOTH of them set to be used, and of course then it's about 30% slower than just the 5700.  If I add the CPU into the mix, then it's about half as fast as that!  So if that is normal, which it may be, then what exactly is the benefit of being able to use any of them together, and to not be able to use a secondary GPU on it's own?  I assume it's a bit of a bottleneck communicating between the 2 GPUs and the CPU, so that may be slowing it down.  I can't see the speed being any benefit, maybe being able to use the extra memory in the additional GPUs?  Even then you have the out of core memory option to use also.  It just seems pretty useless to have your render be a third slower and also have both GPUs using 150w of power each, unless it's in some way to aid in multitasking render layer situations.