Hi. I was just curious if SLI or the new NVlink could be incorporated into the pimax software or into a future crossover box? I read somewhere that the yet to be priced Starvr has this feature. According to the reviews the 5k/8k seem to eat gpu’s
No. SLI of any type needs game support, not pimax support. Even most games that support SLI dont support VR SLI aswell as its a different thing (basically needs split frame rendering instead of alternate frame that most games use).
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Alternate frame rendering should be possible as the bandwidth with nvlink is more than enough to see exceptional speedup and is a game agnostic feature. Unfortunately rtx 2080 and to have nvlink disabled on them and still use the old sli. If you want to use the nvlink you need to use the Quattro which have the feature unlocked.
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on RTX 2080 you don’t have bidirectional P2P communication, but bandwidth would be really bigger and faster than old SLI, and that’s what counts in VR gaming.
it was latency, and not the power, the main problem of the old SLI
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Don’t know if you’ve seen this video?
2080 TI NV Link Vs 1080 TI SLI Vs 2080 Benchmarks…
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The problem with AFR is that each GPU is working on a frame at the same time, so from the start of rendering to when the 2nd frame is displayed is actually a bigger lag with SLI. They do have frame pacing to try to even this out but its still not low latency enough for VR.
90 fps gives a GPU 11ms to generate a frame, but with AFR SLI each GPU is being given 22ms to render each frame - 90 frames are being rendered but half of those frames are 11ms “late”.
VR sli uses one GPU per eye, which is split frame rendering.
from the related article: NVLink RTX 2080 Ti Benchmark: x16/x16 vs. x8 & GTX 1080 Ti SLI | GamersNexus
most interesting graph for us are those:
so, nVLink is able to perform consistently under 11ms, that’s what we need, if I’m not wrong
From all that I have gathered to date, nvlink addresses the issues with bandwidth and latency, no?
What is the frame timing for nvlink though?
Nvlink enables higher bandwidth between the 2 cards for higher resolutions, it doesn’t solve the issue that each card is buffering frames to be displayed next interval, the latency in sli issue isn’t a communications latency issue.
The aggregate of each card performing under 11ms in AFR doesn’t always help when both cards start rendering at the same time so the 2nd card’s frame gets displayed at 22ms when it was rendered at 11ms. You can see from the graph that single card is taking around 16ms to render each frame which at 90fps woukd be buffered and fed out at the 22ms point, the cards are overlapping each other and the end result for the end user would be equivalent of 45fps but without any reprojection being done.
I don’t know quite how else to explain it, AFR is bad for VR, it doesn’t work. Nvlink can’t solve the buffering issue because it isn’t a comms issue, its just inherent in the way AFR works.
but in VR-SLI AFR works in a different way I think, each card renders only one eye, so half load, am I wrong?
VR SLI is SFR not AFR (split frame instead of alternate frame), you’re correct that its one gpu per eye, thats why its split instead of alternate
so, with VR-SLI on 2080Ti do you think nVLink can give us our beloved 11ms? (12.5 on Pimax 8K )
It depends what game and what settings just like every other game/headset/gpu combination.
Not many games support VR SLI so its not a GPU setup I’d ever recommend.
I meant, theoretically. From what I understand VR-SLI on Pascal was useless even though the game supported it, because the latency was always too high. That’s why actually only few games support it.
In flat, on the other hand, you often get the desired performance with a single card, so even here there’s little interest in supporting SLI
But if VR-SLI on Turing’s nVlink works, the VR and the new high-resolution headsets could be an interesting target audience.
I wish VR SLI was a simpler thing to implement for developers. I have zero idea how hard it is to implement right now… but whatever backend Nvidia / AMD got going for that functionality, they ought to simplify it as much as possible for devs, so that it wouldn’t be a question of “do we have enough developer resources to implement this, when less than 1% have an SLI/Xfire setup?” - instead it should be a simple flick of a switch for devs. But I’m speaking way outside my area of expertise here, so no idea if it’s even feasible. 
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Ideally, it needs to be ZERO work for the developers. That’s the only way it would get widely supported.
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VR SLI isn’t really related to us. This is a capability for VR that has been included in Nvidia’s VR Works and AMD’s Liquid VR and must be implemented by software developers. To date only 3 full titles are known to have any support for SLI (The Lab, Serious Sam VR and VR Funhouse). There are probably more but support is tiny and the vast majority of the time SLI systems actually have a lower framerate than without.
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