GPU Scaling has long been a thorny issue.
The main problem is one of diminishing returns. Each additional graphics card adds system overheads and so the benefit you have isn't linear at all. Quadfire most certainly does not give you 4 times the performance of a single GPU, rather you end up at about 266%. One extra card helps massively, but after that you really need to want to flex your virtual masculinity and empty your wallet to bother going Triple or Quad.
To some degree this can be overcome with driver improvements, but generally even with a seriously powerful system that last card is just for show as opposed to go. With the fearsome performance of the HD6990 will another card help us achieve hitherto unheard-of heights?
Following on from the single reference review we had to see how Quadfire performed with the new HD6990 and thanks to PowerColor we are able to do so on launch day. This isn't the first time PowerColor have been there for us and once again we need to thank them for going the extra mile at a time when review-sample demands are at their highest.
As this is a reference card to all intents and purposes there isn't anything particularly different to mention so rather than regurgitate what you've only just read, here is the specification table once again. Of course if you have leapt straight in to this Crossfire review then you've only got yourselves to blame. Although you're probably reading the 3D Mark results and not the introduction.
Let's get cracking shall we.
Sword Of Justice
As this is a reference design the main features are the same. We have two 8-pin PCIe power inputs to keep both cards happily fed with all the juice they could require.
Whether it's the upcoming release of Dragon Age 2, or whether we're just nerds at heart, but the hardcore sword-wielding cover boy really looks great. Especially on that ice-blue background.
Four GPUs of loveliness ladies and gentlemen? Don't mind if we do.
One of the best anti-tampering solutions we've seen. You can steam it off all you like, but the word "VOID" stamped across the back is a bit of a giveaway.
At the exhaust end of the card we have the same DVI + 4 mini DisplayPort arrangement as on the reference. No great surprise there. Apparently all 6990s will ship with an Active DisplayPort adaptor, which will be amazing if true. Sadly this is a OEM version so we can't test the Eyefinity capabilities without one.
Second verse, same as the first. Only this time we have ... two cards to play with *cue Toccata and Fugue*
PowerColor HD6990 x2
Intel Core i7-950 @ 4GHz
ASUS Rampage III Extreme
Muskin Joule 1200w
6GB Mushkin Redline
Windows 7 Ultimate x64
The PowerColor managed to overclock 25MHz better than we saw from the reference AMD model. Naturally there is an element of the silicon lottery involved, but 25 MHz is just that.
Who are we to thumb our noses at any performance increase. We're Overclock3D after all.
As this is still a reference design we're still plagued by the heat and noise issues we were on the AMD, but there are no fault at all of PowerColor who can only work with what they're given.
3D Mark Vantage
It's not often we come across a situation in which our 4GHz i7-950 becomes the limiting factor in our testing but with the lack of PhysX on the Radeon chip, and the bias towards PhysX capable GPUs in Vantage, our poor i7 is having to run 4 PhysX threads as well as pump the data to the cards.
There definitely is phenomenal power available in this Quadfire setup. Nearly 32000 X Marks? Insanity.
3D Mark 11
Thankfully for us 3D Mark 11 has no such qualms about cards without PhysX capabilities and the score reflects that. Relative to a single card we're just past 300% efficiency which is highly impressive. We're starting to see thermal issues in the overclock though as the score is actually lower than that of stock.
If GPUs came with a sub-title I think the PowerColor's would be "Redefining Boundaries". 200FPS maximum in Unigine Heaven 2.1 is madness and yet the Crossfire HD6990s just put their shoulder to the wheel and strolled towards 400FPS.
It's not all top end either, with 150+ average making mincemeat of an otherwise tough benchmark.
Fancy 8x AA? Does the slider go higher? We've only lost 17FPS going from 0 Anti-Aliasing into 8x. Remember when just adding 2x AA would render anything unplayable?
This, boys and girls, is an entirely new type of power. If a GTX580 SLI setup can be left breathless then there isn't much we can add.
Alien vs Predator
Of course not everything is designed to take advantage of as many cards as you can fit into your tower, and AvP is one. All of the results are much of a muchness showing that once you hit two GPUs the engine isn't interested in anything else you have to hand.
Furthering the proof that bigger isn't always necessarily better because developers don't always create 3D engines that are scalable, Crysis Warhead remains its indefinable self. How it places such demands upon the system that the extra card hinders performance to this level is disappointing to say the least.
Far Cry 2
Ignoring the overclock result for a moment we can see how scaling is greatly affected by the quality of the underlying game engine. The Dunia engine is one that your humble scribe has long felt didn't get the use it deserved and at least it takes advantage of the four GPUs we have available with our PowerColor setup today.
Is there no end to the talents of the 4A Development team and their 4A Engine? As badly as it performs with a single GPU at the helm, it just eats extra hardware for breakfast, making 100% use of it, then sits there like the omnom from Cut the Rope just waiting for the next bit.
If the HD6990 was impressive, two are stunning. Sure it's a grands worth of kit, but to run out of eye-candy settings on Metro 2033 and still be well into triple digit frame-rates is almost worthy of a round of applause.
Phew. What a hell of a bunch of results. It's rare that we come across a product that leaves us breathless. Frustrated, but breathless.
Let me take you for a little stroll into an apt comparison for a moment. Racing Drivers are separated into the quick and the slow. That's all that really matters. Sure it's important that they don't wreck every race, but in general speed trumps reliability when analysing raw talent. You can teach a super-fast guy to not crash as much, but it's infinitely harder to teach a slow guy to be quick. You've either got it or you haven't. Rough edges can always be trimmed.
Welcome back. That's the HD6990 in a nutshell. It's like a hyper-talented Rookie. Sure it's brutally hot and loud. In a war between this and the reference GTX480 we're not sure which would be the louder and hotter of the two. But... but...
Like that hypothetical Rookie driver, its speed is innate. Rip the reference cooler off and hand the PCB to the tuners, and we guarantee you'll spend the next week picking carpet fluff off your chin from where your jaw keeps hitting the floor.
That flaw is nothing to do with PowerColor though. Anyone who has even a tiny bit of experience with them knows that their PCS+ and PCS++ variants always deliver and we're positive that in this case it'll be no different.
Strap two of these beasts together and you'll never be found wanting. With a multi-monitor setup you will definitely be in gaming heaven. If you haven't got multiple monitors then it's very debatable if you'll make the most out of the Quadfire HD6990s. Although of course if you've a thousand smackers to lay down on a GPU setup it's very likely you've already got at least an Eyefinity setup, if not the 5 monitor setup capable with the PowerColor HD6990.
There is no doubt that if you want performance capable of stopping every passer by within 10 miles, the PowerColor HD6990 is the card for you. But personally, we'll advice holding out for the non-reference models with an alternative cooler equipped.
Thanks to PowerColor for providing the HD6990s for our Crossfire review. Discuss in our forums.