OTOY Forums

I received a response from EVGA that seems at odds with info in Otoy’s FAQs:

EVGA:
Is the mother board on that Mac SLI compatible? For 2 cards in SLI the second card only helps in processing and does not use any of the VRAM so 2 2GB cards will only use 2GB. I would recommend just using 1 680 in the Mac Pro as I am not sure if Macs are capable of SLI. You would need to check the motherboard model to see if it has SLI compatibility.

However Otoy says that SLI is not needed and that using multiple cards is the way to scale performance:

Yes! Octane Render completely relies on the GPU for rendering performance and scales extremely well. If your motherboard can accept more than one video card, adding additional video cards will greatly improve Octane's rendering speed because Octane's performance scales perfectly with the number of GPUs (e.g. rendering with four GTX Titans will be 4x faster than using only 1 GTX Titan), without the need for SLI.
Does Octane Render take advantage of SLI?
No, but it can use multiple video cards for rendering (see above). It is also recommended to disable the SLI option in your Nvidia control panel to maximize Octane's rendering performance

Can anyone clarify if two GTX 680's for Mac) in a MacPro 5,1 with a single 27" display is a workable configuration?
(One card connected to nothing)?
Wouldn't two GTX 680s be a better choice for Octane than a GTX 5000 for Mac?

Hi Ristau,

Provided your mac has the PCIe slots and power supply sufficient for two 680's, this should work for octane.
SLI is used for gaming and it requires:
1) The motherboard to be capable of it
2) A special connector cable between the cards
3) Both cards to be identical (or at least only a little different, ie: different VRAM amount and/or clock speeds but same main GPU eg 580/680).

Octane sees GPU's as cuda GPGPU compute devices, so they are all separate devices that can be used. Hence you can mix and match card types and SLI is not required. They just need to be in the machine (motherboard doesn't need to be SLI capable or the connector attached).

Thanks
Chris.

Thanks Chris
Looks like there is a power problem with two 680's. So, how about a lower end NVIDIA card to drive the 27" LED display and a single 680? The 680 would be used for CUDA processing but connected to no display?

ristau wrote:Thanks Chris
Looks like there is a power problem with two 680's. So, how about a lower end NVIDIA card to drive the 27" LED display and a single 680? The 680 would be used for CUDA processing but connected to no display?

Yup, that should work. What do you have in the mac already?
What you need for display will depend on the complexity of the scenes you are working with.

The OctaneRender Team pointed me a posting on a Two EVGA GTX 680 (3,072 Cuda Cores) + 450 watt PSU rendering system in a MacPro:

http://forums.macrumors.com/showthread.php?t=1520765

Exactly what I've been looking for. (I think?)

i have 4 x EVGA GTX 780 ti and haven't noticed any increase in speed.
i've switch on/off different cards in the options and still get the same render times.

i'll do more testing, but i haven't noticed a different going from 1 to 4 over the period of time of acquiring the cards.

switching off 'use all gpus', then testing the render times by switching cards on/off did show speed increase, but it seems to be inversely exponential gains not linear,
i.e.
(2000x1000@1000samples with Pathtracing)
1 Card = 30 secs
2 Cards = 20 secs
3 Cards = 15 secs
4 cards = 13 secs

I'd take a look at the temperature of those cards while working all together, as if there's not sufficient cooling and temperatures go up, they may start throttling, hence loosing some performance.

If they are in the same PC without any space between them, they can throttle down a lot because of heat.
But you can see that with GPU-Z.
https://www.techpowerup.com/gpuz/