PostHeaderIcon Fast Forward: Hybrid CrossFire for Fusion

Ever since multicore processors appeared a few years ago, programmers have been complaining about them. Distributing a software workload among multiple CPUs isn’t as easy as running a single-threaded program on a single CPU. Now AMD is doing something even more difficult—but it’s the future of computer science.


Image courtesy CNET

As you’ve heard, AMD’s new Fusion processors combine multiple CPUs and a GPU on one chip, like Intel’s “Pineview” Atom processors. In some cases, programmers can distribute workloads across all the CPUs and the GPU, and those workloads needn’t be graphics. This general technique is called asymmetric multiprocessing on a heterogeneous multicore processor.

No doubt you’re familiar with AMD’s CrossFire technology, which boosts performance by linking multiple graphics cards together. Usually, this is symmetric multiprocessing on homogeneous processors (identical GPUs). But AMD’s Hybrid CrossFire works with different graphics cards or even with a graphics card and an integrated-graphics chipset. Before, adding a graphics card usually bypassed the weaker integrated graphics.

AMD derives Fusion graphics from ATI Radeon discrete graphics, so the latest integrated GPUs aren’t the weaklings they used to be. Why waste the Fusion GPU if the user upgrades to a full-fledged graphics card? So AMD will allow Hybrid CrossFire configurations using Fusion graphics and discrete graphics.


Image courtesy Legit Reviews

It’s logical but nontrivial. The graphics driver must balance the workload across an integrated GPU and a discrete GPU that are related like cousins but not identical like twins. The GPUs use different memory and different I/O buses. In addition, some programs will try to use the GPUs and the multiple CPUs!

If you just heard a scream, it was probably a programmer. Buy the poor soul a Jolt cola. Asymmetric multiprocessing on heterogeneous multicore processors is a tough challenge, but it’s the future of computing. I’m confident the programmers are ready.

Tom Halfhill was formerly a senior editor for Byte magazine and is now an analyst for Microprocessor Report.

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