SLI And Virtual Reality Applications
A word of warning is in order if you're using an Oculus Development Kit 2, or are planning to purchase the Oculus Rift once it is released (it's rumored to be coming in late 2015). Virtual reality headsets, by nature, rely on the lowest possible level of latency between head movements and display updates. Anything else can result in uncomfortable motion sickness-type reactions.
Nvidia recently introduced a "Virtual Reality pre-rendered frames" setting in its Control Panel (with a default setting of "1") to help reduce latency to its lowest level possible. Unfortunately, this setting does not apply to SLI; it only affects single GPUs. Because of the way SLI works, the CPU needs to pre-render at least two frames at any given point in time for a measurable performance benefit.
The Oculus DK2 supports refresh rates no higher than 75Hz. That means the minimum displayable frame time with v-sync is 13.3 milliseconds. Pre-rendering an extra frame from the CPU results in an additional latency of the same amount, which sounds small but is actually quite significant to VR.
Note: Without delving into a rather complex discussion of why v-sync is essentially necessary to VR, or debunking the common "v-sync is evil" attitude in the desktop gaming space, trust us on this one. You'll want v-sync on in your VR applications.
In situations where your game cannot be rendered at a constant 75 FPS, having SLI enabled will help. A frame pre-rendering delay is always desirable over the stuttering caused by having v-sync enabled and "missing" a frame refresh, thus showing the previous frame again.
Furthermore, if the latest Crescent Bay prototype from Oculus is any indication of where the consumer version is headed, maintaining a 90Hz (90 FPS) frame rate at 1280x1440 per eye with v-sync enabled may in fact require two cards in SLI. In that case, the minimum displayable frame time with v-sync is reduced to 11.1 milliseconds, which is certainly an improvement.
In short, SLI is neither required nor necessarily desirable for the current DK2 version of the Rift. But it might actually be helpful for an ideal experience once the final consumer version is introduced, particularly if you want to play with the eye candy turned up in current-generation games. If you're shooting for the absolute lowest possible latency, however, you'll want instead to drop your detail settings and go with a single GPU.
Do you think we'd see 1080p monitors with 200hz+ in the future? Would it even make a difference to the human eye?
I also believe that alternating frames is utter crap. The fact that this has become the go to standard is a travesty. I dont care for fake fps, at the expense of consistent frames, or increased latency. If one card produces 60fps in a game. I would much rather have 2 cards produce 90fps and both of them work on the same frame at the same time, then for 2 cards to produce 120 fps alternating frames.
The only time 2 gpus should not be working on the same frame, is 3d or vr, where you need 2 angles of the same scene generated each frame. Then ya, have the cards work seperatly on their own perspective of the scene.
However, If i need to buy 2 980s to run a VR set or a 4K display Ill just wait till the prices are more mainstream.
I mean, in order to have a good SLI 980 rig you need a lot of spare cash, not to mention buying a 4K display (those that are actually any good cost a fortune), a CPU that wont bottleneck the GPUs, etc...
Too rich for my blood, Id rather stay on 1080p, untill those technologies are not only proven to be the next standard, but content is widely available.
For me, the right moment to upgrade my Q6600 will be after DX12 comes out, so I can see real performance tests on new platforms.
I had my eye on the two Acer monitors, the curved 34" 21:9 75Hz IPS, and the 27" 144HZ IPS, either one really for a future build but this piece of info tells me my i5 will be a problem.
Could it be that Intel CPUs are stagnated in performance compared to GPUs, due to lack of competition?
Is there a way around this bottleneck at 1440P? Overclocking or upgrading to Haswell-E or waiting for Sky-lake?
While true from a certain perspective, it should be clarified that you need 2 of the same number designation. As in two 980's or two 970's. I fear that new system builders will hold off from going SLI because they can't find the same *brand* of card or think they can't mix an OC 970 with a stock 970 (you can, but they will perform at the lower card's level).
PS. I run two 670's just fine (one stock EVGA and one OC Zotac)
What you say -was- true with 6xx class cards. With 9xx class cards, requirements for the cards to be identical have become much more stringent!