AMD Radeon HD 6990 4 GB Review: Antilles Makes (Too Much) Noise

Several months late and supposedly only a couple of weeks ahead of Nvidia's own dual-GPU flagship launch, AMD's Radeon HD 6990 has no trouble establishing performance superiority. But does speed at any cost sacrifice too much of the user experience?

In drag racing, they say ‘a chase is a race.’ In other words, if you floor it and the guy next to you follows suit, that’s a race, and you’d better be prepared to pay up at the finish if it’s a money contest.

Both AMD have ridiculous dual-GPU hot rods they’ve been tweaking and tuning for months. Understandably, they want to stay secretive about their respective power plants. But neither one seems willing to mash the pedal and risk an embarrassing second-place finish. It’s a good thing that these two companies don’t live their lives a quarter-mile at a time. I can just see Vin, shaking his head in disappointment.

But come on already, guys! The AMD Radeon HD 6990 was supposed to be a 2010 model, and here we are in March wondering if AMD overpromised during its press briefing last October. We even heard rumours that the 6990 was cancelled.

Au contraire, Pierre. It looks like AMD is making the first move with its blown Charger, daring Nvidia to throw-down with a twin sequential turbo-charged Supra...you probably know it as the rumoured GeForce GTX 590. We received a single Radeon HD 6990 4 GB one week ago, beta drivers a couple of days later, and updated Catalyst Application Profiles a couple of days after that. Needless to say, the benchmarking marathon that went on in our Bakersfield, CA lab made the 24 Hours of Le Mans look like kart racing at an amusement park.

Meet Radeon HD 6990 4 GB

It just sounds majestic, doesn’t it? 6990. 4 GB. Unlike anything we’ve ever seen from AMD on the desktop. But don’t let naming trickery disarm you like the beautiful rosso corsa of Ferrari’s racecars.

The Radeon HD 6990 follows in the pedigree of Radeon HD 4870 X2 and Radeon HD 5970. It’s a dual-GPU card with graphics processors running, by default, at slightly reduced clock speeds compared to the company’s fastest single-chip board. Its 4 GB of memory are divided between both ASICs. So, you’re essentially looking at two 2 GB configurations on a single PCB, running in CrossFire.

Although it was previously referred to by the code name Antilles, Radeon HD 6990 centres on two of the Cayman-based GPUs found in Radeon HD 6970 and 6950 graphics cards. If you remember from Radeon HD 6970 And 6950 Review: Is Cayman A Gator Or A Crock?, Cayman employs a slightly modified architecture, designed to extract more performance per square millimetre of die space. There are situations where this VLIW4 architecture could underperform AMD's older VLIW5 design, but the company says those situations are rare.

Bottom line: the highest-end Cayman configuration offers fewer ALUs than the most complex Cypress processor (found in the Radeon HD 5800-series cards). However, Cayman’s ALUs are more capable. For a deeper background on Cayman’s architecture, check the second page of our launch coverage.

Each Cayman GPU serves up 1536 ALUs spread across 24 SIMDs. SIMDs are tied to four texture units, totalling 96. Radeon HD 6990 utilizes Cayman in its uncut form, so you get 3072 ALUs and 192 texture units between the pair of GPUs. As mentioned, the 4 GB frame buffer is divided up, 2 GB of GDDR5 per processor, connected via a 256-bit bus.

AMD unifies the two Cayman GPUs using the exact same 48-lane PCI Express 2.0 switch from PLX found on the Radeon HD 5970. Sixteen of those lanes serve the slot interface, 16 go to GPU 1, and 16 go to GPU 2.


Radeon HD 6990
Radeon HD 6970
Radeon HD 6950
GeForce GTX 580
Manufacturing Process
40 nm TSMC40 nm TSMC
40 nm TSMC
40 nm TSMC
Die Size
2 x 389 mm²389 mm²389 mm²520 mm²
Transistors
2 x 2.64 billion2.64 billion
2.64 billion
3 billion
Engine Clock
830 MHz880 MHz
800 MHz
772 MHz
Stream Processors / CUDA Cores
3072
1536
1408
512
Compute Performance
5.1 TFLOPS
2.7 TFLOPS
2.25 TFLOPS
1.58 TFLOPS
Texture Units
192
96
88
64
Texture Fillrate
159.4 Gtex/s
84.5 Gtex/s
70.4 Gtex/s
49.4 Gtex/s
ROPs
64
32
32
48
Pixel Fillrate
53.1 Gpix/s
28.2 Gpix/s
25.6 Gpix/s
37.1 Gpix/s
Frame Buffer
4 GB GDDR5
2 GB GDDR5
2 GB GDDR5
1.5 GB GDDR5
Memory Clock
1250 MHz
1375 MHz
1250 MHz
1002 MHz
Memory Bandwidth
2 x 160 GB/s (256-bit)176 GB/s (256-bit)
160 GB/s (256-bit)
192 GB/s (384-bit)
Maximum Board Power
375 W
250 W
200 W
244 W

Of course, we’re ecstatic that AMD is using fully functional 40 nm Cayman GPUs—the kind you’d find on a Radeon HD 6970. But that product is already rated for up to 250 W maximum board power. Keeping the 6990’s thermal output manageable meant turning down the clocks from 880 MHz (Radeon HD 6970) to 830 MHz (Radeon HD 6990). AMD also uses a lower memory clock (1250 MHz rather than 1375 MHz). The resulting compute power adds up to 5.1 TFLOPS of single-precision math or 1.27 TFLOPS double-precision.

But AMD also arms this card with a couple of surprises that "break the rules" in the name of more muscle.