1 NVIDIA 3090 or 2 NVIDIA 3080TI on DaVinci Studio 17?

since DaviciResolve is hungry of RAM on the GPU side and since the Studio version allows the use of multiple GPUs in parallel a question arises: purely theoretically, the RAM of 1 Invidia 3090 (24GB) is equivalent to that of 2 INVIDIA 3080TI ( 12X2GB) as seen and used by NLE. the question is not accidental since on the calculation side it is probably better to use two GPUs.
Having planned the upgrade of my PC in this direction, ask the Forum friends who are more experienced than me for a clarification.

No, RAM doesn't add up. 5x12GB are still 12GB.

No, RAM doesn't add up. 5x12GB are still 12GB.

do you mean that DAVINCI sees only 12GB of Ram even if I install 2 or 3 GPUs each with 12 GB?

Resolve needs to be able to upload the same data to every gpu, because every device works independently, and must be interchangeable with others (frames get served randomly). So if data and kernels needed for rendering a frame exceed 12GB in size, having 1, 2 or 5 gpus does not make a difference, it won't fit into any of them. So in essence, yes, it only sees the minimum vram available over all enabled gpus.

OK but why to mount 2 or 3 GPUs? I have seen reviews positive in this sense from respectable Puget Systems

Resolve needs to be able to upload the same data to every gpu, because every device works independently, and must be interchangeable with others (frames get served randomly). So if data and kernels needed for rendering a frame exceed 12GB in size, having 1, 2 or 5 gpus does not make a difference, it won't fit into any of them. So in essence, yes, it only sees the minimum vram available over all enabled gpus.

For dimishing returns if a single GPU is not powerful enough. Adding GPUs neither adds up vRAM nor scales anything near linear. Expect +50% for a second GPU only.

OK but why to mount 2 or 3 GPUs? I have seen reviews positive in this sense from respectable Puget Systems

Because with device level parallelism it renders multiple frames at once, each gpu crunches on one frame at a time. But due to different overheads and io bounds it doesn’t scale linearly as Peter wrote.

[Peter Chamberlain]

While GPU ram is not additive, more GPUs is faster in many situations, and a lot more than 50% better for addition a second GPU for demanding grades.

That said, if you can get one fast and higher ram GPU that suits your needs, thats a simpler/better option.

OK but why to mount 2 or 3 GPUs? I have seen reviews positive in this sense from respectable Puget Systems

Resolve needs to be able to upload the same data to every gpu, because every device works independently, and must be interchangeable with others (frames get served randomly). So if data and kernels needed for rendering a frame exceed 12GB in size, having 1, 2 or 5 gpus does not make a difference, it won't fit into any of them. So in essence, yes, it only sees the minimum vram available over all enabled gpus.

let's say teh you have 25FPS (Europe/Italy), you have 1/25th of a second (40milliseconds) to execute the render of the frame otherwise you lose realtime. having two GPU identical, it means that you can afford to send even frames to GPU 1 and odd frames to GPU 2 and having 1/125th of a second to do the math (80 millisecond), so you can overcome difficult grades.

It will also speed up the render process because of that...

My personal take is: one powerful GPU, but one now if you can find it, then later on you get a SECOND one identical...

when the 1080ti came out, i got one, then when the 20xx series come out, i got a second card and double the rig with less money.

also one card will use less resources/bus than two for the time been....

Also consider PCIe lanes. With Ryzen it'll be 2x8 for your GPUs instead 1x16. Now 8 lanes PCI Gen4 is still plenty of bandwidth and might not be an issue. Still I'd probably move on to Threadripper for multi GPU.

Another thing is cooling. There is just no way I could add a second Strix 3090 despite having the slot as it would block air intake of the first card almost completely.

While GPU ram is not additive, more GPUs is faster in many situations, and a lot more than 50% better for addition a second GPU for demanding grades.

That said, if you can get one fast and higher ram GPU that suits your needs, thats a simpler/better option.

Great Peter!!
Actaully I own an AMD 5700X in my TRX40 platform with 3970X CPU and with render in 4k of my long footages it is noy enought. But following your skilled opinion I'd buy a single 3090 for NVIDIA and live happy... IF I ONLY COULD REACH ONE AT THE RIGHT PRICE!!!!!