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I'm rendering a fairly dense interior scene, and my mac is switching itself off when the render is around 90% complete. It doesn't happen every time, maybe every third render. I suspect my GTX 1080ti is getting too hot, is that the likely cause?

Running 12 core Mac Pro 5.1 3.46GHz, with 1 x 1080ti and 1 x 980ti. High Sierra and Octane 2020.1 RC3

Yes, overheating could be one of the causes, especially if it happens after some time of rendering.
Have you tried to keep the lateral panel opened?
Maybe you can use a vent to push the hot air out of the case.
ciao Beppe

I do have the side panel off, as the 1080ti is powered externally. I've got a 120mm USB powered fan coming today that I'm going to use to blast air across the GPUs.

Is there a way to tweak the settings in Octane though, so it renders for longer but not as hot?

Not sure what's causing these complete restarts...

Halved the number of samples, so render time is about 50% quicker, and got a powerful desk fan and blasted air into the side of the mac.

The mac still switched itself off towards the end of the render.

I'm using denoising - could that be the cause, as it happens post render?

We've got a similar Franken-Mac with two 1080 TIs in it, a second PSU, and a supplemental 120mm cooling fan. The extra fan is directly in line with the built-in PCI cooling fan, tucked in between the built-in fan and the back end of the GPUs. It's counter-intuitive, but in my experience, the case needs to be closed to get proper air flow over the cards.

Power draw spikes can also shut down your Mac. If you're not already doing so, definitely power the 1080 TI with the external PSU, not the 980. You can also tap that external PSU to power the fan, just to be sure.

I think for sure it's a power spike caused by the denoiser kicking in rather than a cooling issue. Always happens right at the end of the render. The 1080ti is powered externally and I've set the 1080ti to be the only card doing the denoising, and got another couple of renders out, but it's all pretty random whether it's going to crash or not.

Hi Mike,
probably is time to check or change the external PSU, with a more powerful one.
What are the specs?
ciao Beppe

It's a 650W, plenty for the 1080ti. I seem to have solved the problem by making sure the internally powered 980ti does not do any denoising.

If you haven't done so yet, try running with one card only for a while, then running only with the other. That might narrow the problem down a bit. The problem could be with one specific card, and not a product of the two together. It may very well still be a combined card problem, but at least you can eliminate a few possibilities this way.

I have the same problem with my GTX 1080 Ti connected via eGPU. I noticed that if I cool it, keeping it open and with air, it doesn't crash...
I have to keep the graphics card in the fridge??