Two graphics chips. One card. And a whole lotta rendering fun: that's the basic philosophy behind the ATI Radeon HD 4870 X2, AMD's latest dual-GPU 3D card.
Actually, that's the philosophy AMD has adopted for high-end graphics in general. No longer intending to duke it out with NVIDIA at the very top of the market, instead of producing one monstrous GPU, AMD's plan is to slap NVIDIA silly with two slightly more compact GPUs crammed onto a single card.
But there's a problem. In the past, two chips on one board has been a recipe for double trouble, not double your fun. Multi-GPU technology in multi-card form has always been marginal in terms of stability and reliability; a somewhat worrisome fact for a single card that relies on multi-GPU performance scaling for its very existence.
NVIDIA's progression
NVIDIA kicked off the modern day version of the twin-chip trend with GeForce 7950 GX2. That board worked fairly well at first. But then NVIDIA launched a whole new graphics architecture and, quite frankly, its driver team stopped caring about the 7950 GX2.
And drivers are absolutely crucial when it comes to any form of multi-GPU technology. Both NVIDIA's SLI platform and ATI's competing CrossFire technology use driver profiles to detect games and apply correct multi-GPU scaling methods.
If there's no driver profile for a given game or if the game is not detected correctly, you're in trouble. At best, you'll get single-GPU performance. At worst, the game won't run at all.
The task for the new Radeon HD 4870 X2 is therefore clear. To be taken seriously it must expunge all hint of multi-GPU unreliability and deliver the sort of stability and ease of use that users expect from a single-chip graphics card.
Enticing upgrades
On paper, there are reasons to be optimistic. For starters, AMD has upgraded its inter-GPU PCI Express bridge chip from 1.0 to 2.0 spec and added a 5GB bi-directional sideport to each GPU. Consequently, overall inter-GPU bandwidth has been boosted from 6.8Gbps to 21.8Gbps.
Likewise, AMD has made a really smart move in stuffi ng fully 2GB of graphics memory onto this card. At really high resolutions, 512MB per GPU may not be enough to store all the game data.
When that happens, a graphics card is forced to use the PCI Express bus to fetch data. And that means hideously slow frame rates. But with 1GB per GPU, the 4870 should suffer from no such shortcoming.
The 4870 X2 is also extremely user-friendly by multi-GPU standards. It shows up as single device for driver installation purposes and there's no need for users to even think about enabling multi-GPU scaling, it all happens automatically. Unlike most other multi-GPU boards, the 4870 X2 is also capable of full multi-display support.
But, what the 4870 X2 conspicuously does not do is to move the game forward in a technological sense. One day, a company will produce a multi-chip solution that behaves likes a single rendering device, thereby sidestepping current multi-GPU problems. But that day has not arrived. In essence, the 4870 X2 remains CrossFire on a card and is every bit as dependent on driver support as any other multi-GPU technology currently available.
The GPUs are essentially lifted from the single-chip 4870 board. It's the same 55nm GPU die running at an identical 750MHz. The 4870 X2 therefore packs 1,600 stream processor and a theoretical maximum compute performance of 2.4TFLOPs. One helluva lot, in other words.
Likewise, the 3.6Mbps data rate of the GDDR5 graphics memory is identical to the single-chip 4870. And like every other 4800 series card, the 4870 X2 offers DirectX 10.1 support. For what it's worth, NVIDIA's competing GPUs remain 10.0 bound.
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