Date: Monday , October 24, 2016
Last week when we published our AMD & NVIDIA GPU VR Performance: Serious Sam VR:TLH review, we discussed that Croteam did mention that the new SSVR Beta (that we did use for testing), was using some AMD LiquidVR specific technologies. If you are not familiar with AMD LiquidVR, it is akin to NVIDIA VRWorks. LiquidVR and VRWorks are techniques that are used to make VR titles run faster. I know that NVIDIA has been working hard to get its VR Works code paths into both the Unity and Unreal Engine VR engines.
One of the first titles that we have reviewed here that we saw VRWorks being used in is Pool Nation VR. While we have not covered it, Everest VR also uses several VRWorks advanced rendering techniques. Pool Nation VR uses MRS, or Multi-Res Shading, that renders different parts of the screen at different levels that basically save resources for better rendering at the middle of your view in the HMD.
While we have been hearing a lot about VRWorks from NVIDIA and developers using it, we have heard nothing about LiquidVR from AMD, or any game developers...until last week when Croteam mentioned it was using LiquidVR in it newest SSVR Beta.
Improved performance with LiquidVR on AMD driver 16.10.
Given that we have not been afforded much exposure to LiquidVR, I honestly did not know what exactly that meant, and Croteam was being a bit cryptic about it as well. After publishing our SSVR Review, Alen Ladavac, CTO at Croteam, was kind enough to reach out to us and give us some insight as to what exactly what going on with LiquidVR and SSVR.
I read your excellent article on testing GPUs with SSVR. You got some great data there, thanks for laying down all the details!
Just wanted to clarify the LiquidVR mention from our update... At the moment, the game only supports LiquidVR in mGPU mode. So if you have 2x 480 it will render much faster, as each GPU renders one eye. The support for single-GPU stereo is coming in the next few days, hopefully.
Note that this thing is very early and hasn't been extensively tested, so we didn't trumpet it very widely yet. If you get a chance to try it, I would be thankful if you can provide feedback.
First and foremost, it is simply awesome to get this kind of feedback and support from Croteam, and I want to extend a very sincere, "Thank you so much for reaching out!" The VR developer community overall has been very open and accessible, much more-so than I ever saw in desktop gaming.
Given that Croteam informed us that mGPU was working in the SSVR Beta, we immediately tested it, and did find that it was very much working with the AMD RX 480. When I purchased our RX 480 card, I went ahead and bought two simply because we were hoping to get to see mGPU in action. Multi-GPU, is akin to "CrossFire," but it is a bit different. Here is what AMD says about mGPU in its LiquidVR online documentation (which is a bit sparse in detail).
Affinity Multi-GPU: faster multi-GPU performance - Affinity Multi-GPU delivers significantly increased stereo rendering performance when using two or more GPUs. By assigning one or more GPUs to each eye and intelligently partitioning workload, Affinity Multi-GPU can cut rendering time nearly in half while doubling bandwidth and computational resources.
So the simple explanation is that when we use two-GPU mGPU in a VR rendered game, each card is responsible for the image in each eye. I am not sure how it works with more than two GPUs, and AMD does allude to that being a possibility in its quote above.
We immediately did some mGPU testing with two AMD RX 480 cards and SSVR and this is what we found.
Two card RX 480 mGPU is very much working in the SSVR Beta, and working well. We still are having some BSOD issues with the RX 480 and SteamVR, but this issue is not related to SSVR or mGPU, and I mention this so you know there are still issues that AMD needs to overcome with SteamVR and your experience might not be perfect. However, when it does work, it is working very well inside Serious Sam VR. I had zero issues with RX 480 mGPU in the game itself.
Using mGPU RX 480 in SSVR brought our average GPU Render Time down from 13.71ms to 8.755ms, far below our 11.1ms "cutoff" threshold. The visual quality difference is extremely noticeable and simply a much better VR experience as this bought SSVR gaming out of Reprojection, eliminating frame judder, and providing a much higher IQ level. Keep in mind that we are using "High/High/High" Performance IQ levels for our testing which I do consider GPU-intensive.
So where does this leave us in terms of comparing mGPU RX 480 to its NVIDIA competition? We use RX 480 8GB cards that sell for around $240 according to PC Hound. The NVIDIA GTX 1060 6GB sells for around $240 as well. The much more powerful GTX 1070 sells for around $400.
The mGPU RX 480 blows away the single-GPU GTX 1060 in performance, as you might expect since mGPU RX 480 is double the cost. Not much of a surprise there.... It is worth mentioning that while the GTX 1060 does not support SLI, I believe there is no reason that mGPU inside VR should not work with the GTX 1060, but we will be watching for this in the future and reporting on that.
The mGPU RX 480 8GB configuration, while $100 more than the single GTX 1070, comes close to catching the GTX 1070 in terms of performance data. The mGPU RX 480's GPU Render Time is 8.755ms while the GTX 1070 logs a 8.377ms average GPU Render Time. That said, keep in mind that the actual in-game performance is identical inside the HTC Vive HMD.
While we do not have RX 480 4GB cards to test, it is feasible that two of those for under $200 each could compete with the single GTX 1070, but if I was buying new video cards for a VR system, I would not skimp on VRAM personally.
This is the not the first time we have seen mGPU working in a VR title. NVIDIA's own VR Fun House supports mGPU as well as Trials on Tatooine with NVIDIA cards (testing data in the review), so we know mGPU works in VR. But Serious Sam VR is the first commercial VR gaming title that we have seen mGPU working in. It is exciting and certainly promising to see mGPU working with AMD's LiquidVR technologies. Alen Ladavac at Croteam also mentioned that they hoped to have "single-GPU stereo" working soon as well, which I assume is similar to NVIDIA's VRWorks Single-Pass Stereo, which basically cuts the geometry workload in half, which of course frees us GPU resources to do other things.
We are very happy to see Croteam leading the way with its SSVR title and these new rendering technologies inside LiquidVR that will make our VR gaming experiences that much better. I expect we will see Croteam leverage NVDIA's VRWorks as well in SSVR.