Update, May 6, 2014: AMD maintains that the only supported way to hook its Radeon R9 295X2 up to a 4K monitor is through DisplayPort, and that no enthusiast will use the dual-HDMI method required for us to benchmark with our FCAT toolset. With that last point, I agree completely. DisplayPort is the only practical way to go in the real world.
But that also means leaning on Fraps exclusively for performance evaluation. We’ve already opened Pandora’s Box on that, and we know why Fraps isn’t the best solution for testing multi-GPU configurations. There’s no going back, and I just wouldn’t feel comfortable presenting Fraps data and asking you to make an exception and trust it. So, in addition to re-running all of the FCAT-generated numbers with lots of extra airflow and in two different platforms (I swapped the cards out of iBuyPower's box and into my own as a sanity check), we also ran Fraps numbers hooked up through DisplayPort. All of the results for quad-CrossFire are mentioned on the benchmark pages.
In two cases, a comparison between the results suggested an issue that went undetected in the first piece. Battlefield 4 and Grid 2 should have been much faster with four Hawaii GPUs compared to two, but weren’t. I have some guesses as to what happened, and I’ve presented them, along with all of my testing notes, to AMD’s driver team. We now report closer-to-expected frame rates in those titles.
Assassin’s Creed, Metro, Thief, and Tomb Raider come fairly close to what we saw a couple of weeks ago, and today’s re-worked charts make the same point over again. Arma 3 speeds up today versus our first evaluation of quad-CrossFire, but again, all of our testing is best-case, with maximum airflow through an open chassis.
Quantitatively, we end up with a couple of games in which AMD serves up impressive scaling given the historically difficult move from two to four GPUs. A few others demonstrate more modest scaling. And the last couple don’t scale at all, really. As a matter of principle, just based on the numbers, I stand by our original take on quad-CrossFire using two Radeon R9 295X2s: they’re a work in progress for gaming at 3840x2160. Idiosyncrasies abound, though AMD tells us that the experience from an integrator like Maingear would be significantly different.
But because some of the performance metrics suggest more attractive performance in games that we weren’t seeing before, it’s also important to consider the qualitative experience of gaming on four GPUs. In Battlefield 4, for example, the idea of 84 FPS from two 295X2s is sexy compared to one board’s 48 FPS. The stuttering you run into, however, makes the benchmark result irrelevant—you’d unplug the second card before ever trying to play that way. Assassin’s Creed IV is similarly intolerable, though AMD says that one's problematic because of Nvidia and its GameWorks library. Arma and Tomb Raider don’t escape this “Quality Index” unscathed, either. Only Grid 2 and Metro looked smooth enough to enjoy, and in the latter title, quad-CrossFire doesn’t affect performance at all.
What on earth could be to blame? First, consider the complexity of what AMD is trying to do. You have two dual-GPU boards communicating over the PCI Express bus. Each has its own PLX switch to facilitate communication on-card. However, the boards have to reach across the bus to synchronize with each other. At 3840x2160, each frame is about 33.2 MB. AMD has mechanisms in its driver to evaluate available bandwidth and cope with shortcomings by switching to software compositing. Still, ignoring one of its best practices (using a link smaller than 16 lanes or a motherboard with another PLX switch) could cause issues at 4K. But we tried isolating those variables and continued to see the same issues.
Obviously, this information is relevant to a privileged few able to consider three-grand worth of graphics hardware. But 4K is the future for a great many enthusiasts, and driving that many pixels isn’t easy. Gamers buying today are almost certainly looking at multi-GPU configurations of some sort, making twin Radeon R9 295X2s an attractive option (particularly given their outward-venting closed-loop liquid coolers). And although I continue saying good things about the 295X2, and wouldn’t have any problem using one in my personal system, there are still issues to work out with a pair operating in tandem.