Multi GPU support Mac Pro Cycles Metal

I have a 2019 Mac Pro with a single Pro Vega II.

I have an option to buy a second card for a fair price. One of my main motivations would be Cycles Metal GPU acceleration.

I’m having a hard time finding benchmarks for single vs multi GPU setups for Mac with the latest 3.1 and 3.2a Blender.

Anyone else out there with a Mac Pro and dual (or more) cards willing to share their experience? Thanks!

I have been following the development of Blender and Metal over the past few months and was excited to see that with Mac OS release of 12.3 that Metal would be supported with older Intel Macs with AMD graphics cards. Before I installed 12.3 on. my 2017 5K iMac I ran some benchmarks on a simple Blender scene and some rather strange results.

On OS 12.2.2
Blender 3.0. 45.55 seconds
Blender 3.1b 49.18 seconds
Blender 3.1 today release 49.88 seconds

OS 12.3
Blender 3.1 today release without GPU support 52.23
Blender 3.1 today release with RX580 GPU support 2.15.45

The strange thing is not only is it significantly slower using the Metal but when a start a render I just a spinning lollypop and under force quit it says blender is not responding. If I wait long enough the window will open and the render will complete and reflect to actual total time even including window not open time.

Bottom line here Apple and Blender still have some work to do to implement Metal in Blender.

It appears that a weird bug in 12.3 is causing a lot AMD GPU’s to run at half speed. It would appear like a driver issue.

I’m using a W6800X Duo and it screams in CyclesX. Renders three times faster than my 24-core processor. If Blender/Apple can squeeze even more out if it I’ll be very happy!

I will also say, GPU rendering on cards with only 8GB has proven really flaky for me in the past. I used Octane with C4D and it was a coin toss whether it would crash. As soon as I updated to the 16GB GPU, it rendered much more smoothly and was pretty much crash-free.

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Nice. Does it show up as two cards that you select individually in Blender’s settings?

Have you tried deselecting one of them to see if time doubles, or stays somewhere in between?

Yep, two separate cards (and also the 5700XT I have to drive the monitor). Turn one off and the render times are doubled. Weirdly, I get faster speeds if I don’t include the CPU in the render calculations!

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