When trying to render my scene with my GTX 970 and cycles I get a cuda error saying that I was out of memory. However according to gpu-z I only got up to using 3228MB of VRAM, and according to blender my scene was using a little over 2GB, somewhere in the 2200MB range. Is there a work around, or a know reason for this?
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Unless you have multiple graphics cards you are unlikely to be able to max out the memory use. Some of the memory will be used by other processes and windows. I've found it is always worth closing all other windows and any other unnecessary processes when I'm close to the limit. They all use RAM which could affect your render but they all will likely have staked a claim on some VRAM too. I don't know whether minimising the Blender program windows makes any difference but I do it anyway. If you are on Windows I've also found that switching themes can help. Current Windows versions and their UI effects are not a particularly efficient users of VRAM so dropping back to the Classic theme will probably help. I've also read that changing font size to normal and switching off clear type and anti-aliasing can help although I've no evidence to support that.
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$\begingroup$ I've forced every app using my discrete GPU to use the iGPU, including Windows system apps. My GPU now shows 25% VRAM is free (before there was only ~3% free), yet Blender still refuses to use it. I even shut down the Blender UI and rendered from the command line with all UI apps shut down, same thing: 25% unused VRAM. Blender will even swap out whatever it is using to system RAM and pull in more to the VRAM, killing my render time (12 hours vs 18 minutes). The whole time there is 20%-25% VRAM sitting idle according to Task Manager, Open Hardware Monitor, and AIDA64. No way around it. $\endgroup$ Jun 19, 2022 at 16:49
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$\begingroup$ I think Windows may keep some in reserve, because if I open an app that can use the extra VRAM, the usage will spike to 95-97% while the app is open. Maybe Linux can allow full VRAM usage for a single app... $\endgroup$ Jun 19, 2022 at 16:53