Originally Posted by Xerosnake90
Repped. Thanks a bunch. Read up on some performance results and all that, now reading about how it actually works.
Now, as asked earlier. How does CPU performance factor into a crossfire setup? If the 7870 is a decently powerful GPU by itself, would a crossfire setup require just about the strongest CPU out there? Or would the 8120 be powerful enough to run a dual setup already? I'm asking so I can get an idea of how powerful a cpu would be needed.
There is a lot of info to take in on the subject, that is for sure. Like the guy above said, the most common way CF/SLI works these days is called AFR (alternate frame rendering) where each card renders a frame in sequence. This is the easiest to implement but also the most prone to stuttering issues due to the load not being equal between frames, ex. frame 1 is a wall which card 1 renders in 10ms while frame 2 is a complex scene which card 2 renders in 30ms, you can start to see the little hitches in smoothness if you pay attention. Nvidia uses some frame metering techniques to smooth this while AMD does not, so some people who are sensitive to rate changes like SLI better than crossfire. The problem is also lessened when more than two cards are involved, 3-way setups are typically the smoothest although they may not have the best scaling. The other main type is SFR (split-frame rendering) which hasn't been used in years, it can be smoother than AFR but is less commonly implement in games and drivers. Naturally developers follow the path of least resistance because the SLI/CF crowd is pretty small compared to the full player base of any given game, so AFR is the preferred method although it is not the best quality.
Scaling is the next important issue. Depending on the drivers, system itself, and the game being played you will almost never see two identical cards put together being twice as fast as a single card of the same type. Anything over 80% scaling is decent, while some things like synthetic benchmarks -3Dmark11 for example- can see 95%+ scaling. The more cards you add, the worse the scaling is especially as you add more GPU power than you actually need for the resolution of your display setup. 4-way setups are usually plagued with problems because they are simply very rare and the drivers from both major companies tend to only give cursory support to that type of setup.
Another thing to remember is that your Vram is not doubled, a 2GB card combined with another 2GB card still only offers you 2GB of usable graphics memory, because the same data must be mirrored between the two. That is why dual GPU cards claiming the full memory on them is simply a marketing tactic to make it sound better, for example the GTX 690 may have 4GB of physical memory on it, but since both GPU's fill their Vram with the same data, there is only effectively 2GB to use and if you happened to have something that used more than 2GB worth of textures you would run into the Vram limit, not at 4GB like they put on the front of the box in big bold letters.
CPU performance, ah theres the question. It depends to be honest, mainly on the game, graphical settings, and resolution. This is the simplest I can put it and still be somewhat accurate: the difficulty lies with how fast the CPU can provide information to the GPU's, the faster your GPU setup gets the faster the CPU has to work. The solution to a CPU bottleneck is to increase resolution or graphical detail, if you cause the frame rate to slow down because the cards render frames slower, the CPU doesn't have to work as hard to keep the graphics cards running at full steam. In the end, as long as the game plays how you want it to it really isn't a big concern. Some people stress over not getting the extra 10fps a faster CPU would give them, others do not. I tend to not worry as long as it plays how I want it to at the settings I like.
As far as crossfiring your 7870 in the future? Nobody can say except you, when that time comes look at benchmarks of the games you want to play and determine if its worth it or better to just buy a single more powerful card.Edited by Scorpion49 - 9/16/12 at 11:14am