NVlink enabling and multiple GPus

[Tristan V]

Hello,

I couldn't find the answer on the internet, so I'm trying here.

I use Vray GPU 5 for rendering, with two RTX 2080ti bridged with NVlink. I often have issues when rendering because I'm lacking some memory, and I guess, optimizations apart, it is because one of my GPU has two monitors connected to it and some of the Vram is allocated to them.

I have ordered a new workstation now, and have 3 slots on my motherboard. I would like to plug a third GPU which is an old GTX 1080. My questions are : can I use this last GPU for monitoring only, while using the others 2080ti with NVlink as a supercalculator ? Would Vray recognize the NVlink bridge when rendering, even if there would be no display attached to the GPUs it is linking ?

Thanks for your quick answer.

 

[Alex Glawion]

Yes, this should be possible. You'll only be able to render on the two connected GPUs, though. The third will have to sit idle. Or you can use a render manager to render a second task on the third GPU only.

 

[Tristan V]

Yes, this should be possible. You'll only be able to render on the two connected GPUs, though. The third will have to sit idle. Or you can use a render manager to render a second task on the third GPU only.

Thank you for your answer.
The guy who made my new workstation told me that he couldn't activate the NVlink because it requested that at least one display should be attached to one of the bridged GPU, and therefore I would be forced either to plug at least one display on my GTX 1080, and the other one on one of the bridged 2080ti (which was the opposite of what I intended to do (I wanted the two displays to be connected to the GTX)), or just run without the GTX, only with the 2 RTX.
Should I use a dummy dongle to work my way around this issue ? Or should I attach one of my display on a 2080ti, activate NVlink through the Nvidia control panel, and then unplug it afterwards to re-plug it, along the other display, to the GTX ?

I'm sorry, maybe these are dumb questions, but I am a bit lost.

Regards.

 

[Alex Glawion]

What is requesting this? The nvidia control panel? Or the v-ray renderer?

 

[Tristan V]

I guess it was the Nvidia control panel, because Vray is not installed on this computer.
As I asked Chaos group (Vray developer) the same question, I received this answer :

"In general, it is recommended to use a dedicated GPU for the monitors. Yes when you use one of the GPUs for monitors, there will be some GPU memory dedicated for them.

NVlink for Geforce cards requires all the cards to be in WDDM mode. This mode requires one of the GPUs to be connected to a monitor as far as we know.
For Quadros for example, they can use the TCC mode with NVlink which does not require a monitor to be attached.
There is more info on the article about NVlink you already checked.

If this is still true you will need a monitor for the 2x2080ti so they can work, but things with GPUs change frequently so we will double-check with our team and post an update.
In case you are able to contact Nvidia, they will know more about this."

It's not a definitive answer. I asked Nvidia too. Waiting for their reply.