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SLI and CrossFire setups getting memory stacking soon

3 min read


Having run a dual card for the past three years, I can honestly say that SLI/Crossfire configurations should be reserved for PC gamers with way too much money to spend on their rigs. There are obvious performance gains, but the price of an additional card comes with many handicaps. The inability to stack memory is the most glaring one, made even more disappointing given requirements of modern games. But it’s going to change very soon.

Right now, two cards configured in SLI/Crossfire don’t stack memory, meaning two 4GB cards effectively still produce 4GB of memory instead of 8GB. It’s something that most seasoned PC gamers have come to accept, resting on the fact that the performance gain is enough of a boost to justify the cost. But AMD isn’t satisfied with that answer, and rightly so considering their massive push into dual-card territory recently. The issue surrounding the limitation, however, has already been addressed, with Mantle offering options to let the stacking begin.

Robert Hallock, head of global technical marketing at AMD, has explained via Twitter that their low level API can already bridge the gap to allow memory stacking. With “to-the-metal” access to GPUs, developers can allow for multiple cards to add onto the memory stack, with the cards not needing their own copy of the game running on them individually. It’s all down to optimisation, which will become easier with future APIs like DirectX 12.

Mantle is the first graphics API to transcend this behaviour and allow that much-needed explicit control. For example, you could do split-frame rendering with each GPU ad its respective framebuffer handling 1/2 of the screen. In this way, the GPUs have extremely minimal information, allowing both GPUs to effectively behave as a single large/faster GPU with a correspondingly large pool of memory.

That doesn’t mean it’s in effect yet, especially since this is a feature developers themselves need to add. However, the idea of multiple cards being hindered by memory could be a distant memory in the near future, which opens up PC gaming in a significant way.

Ultimately the point is that gamers believe that two 4GB cards can’t possibly give you the 8GB of useful memory. That may have been true for the last 25 years of PC gaming, but that’s not true with Mantle and its not true with the low overhead APIs that follow in Mantle’s footsteps.

With DirectX 12 creeping its way into titles soon, it could be just a matter of months before memory stacking for multiple graphics cards becomes a reality. That makes offerings like the GTX 970 and R2 290X even more enticing, especially since cards exceeding 4GB of memory are quite expensive still. But it would be a game changer in the mid-range market. Imagine being able to stack the memory you need for full texture settings without breaking the bank? That’s the future right there.

Last Updated: February 3, 2015

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