Safe to use two different GPUs ?

[OlaHaldor]

Hello!
I've just purchased my license, and I'm super thrilled. If I only had known about this earlier.. I bet you've all had (and possibly still have) the same excitement!

I have a GTX Titan (the first generation) and I have some space to spare in my case and on the motherboard. So I started looking for GPUs on the second hand market. I found a couple of Titans, but there's like a flood of GTX 780 and GTX 780Ti.

I'm just wondering if there's any drawback to install a GTX 780Ti together with my Titan, or if I should try to get my hands on one of the very rare Titans.


I'm especially curious about this because the GTX 780 have half the RAM that I have on my Titan, and I'm afraid that will kind of pull down the performance of the Titan. Am I nuts to think like this? Or is Octane just happy to see another GPU loaded with CUDA cores and RAM?

[whersmy]

Octane Render welcomes any CUDA enabled gpu (from gtx 480 and on...I thought)

780`s are pretty fast. Be sure to install the right drivers from the nvidia site.

Though Octane can only use the max. RAM of the smallest RAM available. So only the 780`s amount of RAM will be available.

[OlaHaldor]

Thanks for the quick reply!
So in other words, it will utilize 3GB from each GPU, a total of 6GB (unless I can find a GTX 780 6GB here)?

How much does RAM count during render?

[whersmy]

Nope, another thing: you can only use an amount of 3GB that comes with every gpu, in your case. For example: my gtx590s have 3GB total onboard, but is actually 1,5GB times two. (there are 2gps on one pcb)
In games it`s the same principe I thought.

[OlaHaldor]

Ok. So, my Titan isn't actually using all the 6GB alone either then?
I'm really tempted to grab one of those second hand GPUs and go nuts (with the limited budget I got).

But if you had the choice, would you go for a GTX Titan 6GB, GTX 780 6GB or a GTX 780 3GB in combo with my Titan, regardless of price.

[whersmy]

RAM doesn`t affect speed
In my case, I don`t do a lot of interiors/exteriors or use a lot of (high-res) textures. Textures and HDRs are used by the RAM.
I`m not sure what card (titan or 780) is the fastest, but for me the fastest card would be the best. I almost never max out my ram.

http://render.otoy.com/forum/viewtopic.php?f=9&t=7144
In this topic you can measure the speed of the two, there are also a lot of comparisons in that thread.

I`m a big fan of second hand gpus, that`s a good starting point for the budget.

[voon]

The "smallest amount of RAM" thing is because every GPU involved in rendering must have a complete copy of the entire scene. That means the smallest card is dictating the limit of ram/scenesize. You could not have a 6 GB scene, because your smaller 3GB card can not hold a complete copy of it. They do not add up, share or distribute etc ... they all have a complete copy. It also doesn't matter how many cards you have. If you had 500 cards with 3 GB, your biggest scene is 3 GB.

[OlaHaldor]

Thanks a billion guys for fast replies and good info!
Looks like I either go for a second hand Titan (before they are sold!) or a 780 6GB, just to stay on par with RAM.