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GPU Renderers Multiple Cards and Vram

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I'm new to GPU rendering and researching graphics cards for a new build.


I understand you can stack cards, but have read that renderers do not combine vram from multiple cards.


Is this true across the board for all GPU renderers?


Also, if you have a card that has 8 GB of memory, and add a card that has 12 GB, how much memory will be available to the renderer?

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That is true, for now, that the memory cannot be combined with multiple GPUs. The reason for this is the way they work.  The renderer needs to load the entire scene into the card (including the materials) so the card can process the scene.  If you have multiple GPUs, then each GPU needs to have the complete scene loaded.  Having said that, I believe that Redshift uses the memory a little differently, but I'm not sure.  


Your second question is a good question that a lot of people ask.  Basically, you will be limited by 8Gb in your example.  Like I said, the entire scene needs to be loaded in memory for the GPU to process the scene.  So in your example, if you load 10Gb, you will need to exclude the 8Gb card in your render settings for that scene.


The amount of memory may not be an issue.  8Gb is a lot, and that may be more than what is needed.  I have 11Gb in my GPU and have not even come close to using half of that.  Most scenes that I have rendered stay around 3Gb to about 4Gb.  I'm not sure what kind of work you do, but 8Gb is a massive amount of data.


Finally, I would check the renderer that you choose and make sure that it will support multiple GPUs.  For example, I do not believe that Maxwell supports more than one GPU at this time.



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Yeah it's sad, but specifically for Octane, I just read in their faq that if you use multiple cards, you "will be limited to the size of that card with the least amount of VRAM present in your machine or in the network where all GPUs used for GPU rendering resides".  I'm just getting over VRAM not adding on multiple cards =8-Q.  I have so much to learn.


I measured as many of my previous scenes as I could before I got terminally bored...lol, and came up with 4.5GB max (though I'm sure I missed something big as I thought I was real texture hog).  However, last I checked, Win 10 was still reserving a portion of VRAM on even cards that were no connected to monitors.  So I'm still going with a 1080Ti for my new build (and hoping to add another 1 or 2 next year) and maybe a pair of 1070Ti's to turn my old 980X box into a GPU renderer... hopefully the prices will drop a bit after Xmas.

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