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AMD's Radeon HD 7990 graphics card reviewed

How much does adding a second GPU really help?

Well, this is quite the turn of events. We recently started using some new GPU testing tools from Nvidia that measure precisely when frames are being delivered to the display, and in the process, we found that Radeon-based multi-GPU solutions have some troubling problems. As I said in that article, our next task is to address the issue of multi-GPU microstuttering in more depth. Just as we published that article, we learned of AMD's plans to introduce a killer new multi-GPU graphics product: the Radeon HD 7990.

So I guess our work is cut out for us.

By most measures, the introduction of a card like the Radeon HD 7990 should be simple, because it is unreservedly the most powerful graphics card the world has ever seen. The formula is straightforward enough: two Tahiti GPUs, like those driving the Radeon HD 7970, working together in CrossFire on one graphics card. That's like... twin Clydesdales pulling your wagon, a pair of Ferrari V12s driving all four wheels, like various other poor analogies involving large-scale parallelism and testosterone. Point is, the 7990's hardware is world-class, second-to-none stuff, capable of crunching more flops, bits, and texels than anything else you can plug into a PCIe slot.

Reviewing this thing should be, you know, fun.

But we have some difficult questions to ask about the 7990's true performance along the way. I think that makes our task interesting, at least. Although we have loads of data collected by multiple tools, our goal is to take a very practical approach that should yield some definitive answers to the questions at hand. Let's have a closer look at the 7990's formidable hardware, and then we'll dive into the performance results.

From Tahiti to New Zealand to Malta
The Radeon HD 7990's introduction is more interesting than usual because it's either really late, already over, or nearly didn't happen. I'm not sure which, entirely. You see, back when AMD unveiled the Radeon HD 7970 at the end of 2011, the firm let slip a code name, New Zealand, for an upcoming dual-GPU graphics card and said it was "coming soon." Since we in the media are given to fits of speculation, we pretty much expected to see a dual-Tahiti graphics card from AMD at some point in early 2012. That product didn't arrive as anticipated, and we nearly gave up hope that it ever would.

Eventually, several board makers, including Asus and PowerColor, slapped two Tahiti chips onto a single card, but those products didn't ship until late last year, in extremely limited volumes. We tried to get our hands on one of the water-cooled Asus ARES II cards for review but were told the cards were completely sold out practically as soon as the product was introduced.

We figured that was it for the 7990, but then the news broke that AMD would be extending the tenure of the Radeon HD 7000 series until the end of 2013. At that time, the company told us it had more 7000-series products on the way. Then came GDC last month, when we got our first peek at the 7990. Now here we are, well over a year since the Radeon HD 7970 was introduced, looking at an official Radeon HD 7990 reference card.

This thing even has its own code name, "Malta." AMD tells us New Zealand is an umbrella code name that refers to all dual-Tahiti products from itself and its partners, including those in the FirePro lineup, while Malta refers specifically to this reference design, proving once and for all that codenames are almost infinitely malleable. The fact Malta exists as a reference design from AMD matters, though. AMD tells us this card will be widely available through all of its partners, a true mass-market product. Also, the level of refinement evident in this card and cooler goes well beyond what we'd expect out of a science project from a board maker.

The biggest revelation about this reference design comes courtesy of those three fans spread atop a massive, board-long array of heatpipes and fins. This card is much, much quieter than its predecessor, the Radeon HD 6990, which set some records on the Damage Labs decibel meter. Heck, the 7990 seems like a faint whisper next to the reference 7970's cooler. AMD has put some work into searching out a quieter cooling solution. They claim air turbulence, not fan noise, creates noise in most coolers; this cooling setup reduces turbulence by pushing air down directly through the heatsink fins. The payoff will be obvious to your ears.

Pretty much anything else you can say about the 7990 requires big numbers. With dual Tahiti GPUs, it has a total of 4096 shader ALUs providing 8.2 teraflops of compute power. The board packs 6GB of GDDR5 memory. The true memory capacity is half that, with 3GB allocated to each GPU, but the memory interface is effectively 768 bits wide, with all the bandwidth that entails.

The twin GPUs are joined by a bridge chip from PLX with 48 lanes of PCIe bandwidth, 16 lanes to each GPU and 16 to the PCIe slot. AMD claims the board has "96 GB/s" of "inter-GPU bandwidth," but do the math with me. Each PCIe x16 link can transfer 16 GB/s in one direction or 32 GB/s bidirectionally. That means GPU 1 can transfer, say, a big texture to GPU 2 at a peak rate of 16 GB/s. That's, you know, considerably less than the claimed 96 GB/s. It should be more than sufficient, regardless.

The card can drive up to five displays simultaneously, four via DisplayPort and one via DVI, though plug adapters of various types are available. As you can see above, the board has a CrossFire connector, so you can double up on 7990s if you're feeling a little unsure whether just two Tahiti chips will suffice. AMD says a quad CrossFire config would be ideal for driving 4K display resolutions. (Note to self: test this claim ASAP.)

Poking up out of the top of the 7990 are two eight-pin auxiliary power plugs, to the surprise of no one. The board requires 375W of power, which is 25% less than the power requirements for two separate Radeon HD 7970 GHz Edition cards. Some of the power savings likely come from clock frequencies that are a smidgen lower. The 7970 GHz Edition has a 1GHz base clock and a 1050MHz "boost" frequency. The 7990 clocks in at 950MHz base and 1000MHz boost.

At 12", the 7990 is a full inch longer than its most direct competitor, the GeForce GTX 690, and at this point, the jokes just write themselves. The Radeon's additional endowment may prove to be inconvenient, though, if you're trying to install the board into any sort of mid-sized PC enclosure. You'll want to check to see whether your case has enough room before ordering up a 7990.

Speaking of which, perhaps the largest number of all associated with this card is its price: $999.99. Gulp.

Yep. Nvidia started it with the GTX 690 and Titan, and AMD is following suit by pricing its latest premium graphics card at one shiny penny shy of a grand. I was initially surprised by this move, since the 7990 doesn't have the distinctive industrial design touches that the GTX 690 and Titan do, such as magnesium-and-aluminum cooling shrouds, LED-lit logos, and blowers with Crisco-and-gold-dust bearing lube. The 7990 is handsome—and I've already told you the cooler is quiet—but it mostly just looks like another Radeon covered in shiny plastic. Asking this much is also a bit of a risk because you can buy a Radeon HD 7970 for 400 bucks at Newegg right now, so two of them presumably would be 800 bucks, which I understand is less than a grand.

However, AMD has a couple of awfully decent justifications for charging as much as it does. First of all, it's taking this whole Never Settle game bundle concept to its terrifyingly wondrous logical conclusion. The 7990 will come with a coupon—right there in the box, to prevent redemption hassles—for the following games: BioShock Infinite, Tomb Raider, Crysis 3, Far Cry 3, Far Cry 3 Blood Dragon, Hitman: Absolution, Sleeping Dogs, and Deus Ex: Human Revolution. That's an eleventy billion dollar value, purchased separately. But the 7990 comes with all of 'em.

The other reason AMD can get away with asking a grand for this card is simply that the specs justify it. Have a look at the 7990 versus the competition:

Peak pixel
fill rate
GeForce GTX 680 34 135 135 3.3 4.2 192
GeForce GTX Titan 42 196 196 4.7 4.4 288
GeForce GTX 690 65 261 261 6.5 8.2 385
Radeon HD 7970 GHz 34 134 67 4.3 2.1 288
Radeon HD 7990 64 256 128 8.2 4.0 576

As I've said, the GeForce GTX 690 is the 7990's nearest competitor, and the 7990 has substantially higher peak rates in the two most critical categories for modern GPU hardware: shader flops and memory bandwidth. Granted, the GK104 chips driving the GTX 690 have proven to be formidably efficient performers, but Tahiti's larger shader array and 384-bit memory interface cannot be denied. One can see why AMD would price the 7990 directly opposite the GTX 690 and its theoretically less powerful single-GPU sibling, the GTX Titan. Add in the value of that stupendously stuffed game bundle, and the 7990 practically looks like a bargain by comparison. Also, I think there's some funky psychology at work at the ultra high end of the market: lower prices communicate inferiority, and any signal that carries that message is directly at odds with the 7990's whole mission.