ConclusionAfter running through these tests, it’s impossible to feel wholeheartedly convinced by the quad GPU solutions. Indeed the reality is a lot more complex than the few almost perfect scaling case studies that appear in the AMD and NVIDIA documentation show.
What with games which give very good results but aren’t much use in practice given the fact that the available bi-GPU solutions already provide more than sufficient performance, games which saturate the memory of the multi-GPU systems, games limited by the CPU and games which refuse to recognise the fourth GPU, such extreme graphics systems only come into their own with a few titles, such as Crysis, S.T.A.L.K.E.R. Call of Pripyat and Metro 2033 at 2560x1600 with antialiasing, as well as Battleforge and at least in part, Bulletstorm and F1 2010 in surround gaming. Of course, this is no longer the case if your screen is limited to full HD.
Although none of the solutions tested is completely convincing, there is one we can exclude right away: the quad CrossFire X based on two Radeon HD 6990s. In our view, this is the least effective solutions because of the fact that the drivers are not optimised to allow you to benefit from the fourth GPU. This solution also makes a great deal of noise, indeed we have never tested as noisy a system as this and remember this was with the “low energy consumption” bios… Unless you go for watercooling, we can only advise you to avoid this system.
Using a Radeon HD 6970 with a Radeon HD 6990 looks a better bet as things stand, especially as this combination costs a good deal less. Unfortunately it’ll still make a good deal of noise as the Radeon HD 6990 is far from quiet.
Among the NVIDIA solutions, the GeForce GTX 580 in 3-way SLI and the GeForce GTX 590s in SLI are comparable, with a small advantage going to the GeForce GTX 580s as the tri GPU yield is, as you’d expect, higher than the quad GPU. What makes the biggest difference here are the formats used. The GeForce GTX 580s take up 6 slots and require an appropriate motherboard but expel hot air out of the casing while the GeForce GTX 590s take up just 4 slots on any SLI compatible motherboard but only expel part of the hot air. A small advantage with the GTX 590s is that attributing one of the GPUs to GPU PhysX processing only monopolises 25% of the processing power compared to 33% for the GeForce GTX 580s.
While our preference would be for one of these NVIDIA solutions, they do suffer from the fact that their memory is limited to 1.5 GB. Although this is a relatively generous amount of memory, it’s not enough in extreme situations, which is to say in those very situations which have most need of such graphics power. None of these extreme graphics solutions really covers all the bases therefore. There are merely several imperfect systems to choose from depending on what you want to use it for and you’ll just have to accept that you can’t be sure how your choice will handle future games…
To end up with, we want to raise the issue of specific profiles for each video game, which are unfortunately still required. While there are few developers really interested in multi-GPU solutions, AMD and NVIDIA still haven't managed to implement robust generic solutions to support multi-GPU systems. How much of a gain multi-GPU systems bring therefore depends largely on how quickly the manufacturers get these profiles out. While NVIDIA has done pretty well, it has to be said that AMD still has an awful lot to do and could even do with overhauling its profiles policy entirely. Even if a developer hasn’t been particularly cooperative upstream, the fact that it took so long to deliver a fully functioning profile for a game such as Crysis 2 is totally unacceptable.