Meet Larrabee, Intel's answer to a GPU

WE FIRST TOLD you about Intel's GPU plans last spring, and the name, Larrabee last summer. That brings up the question of just what the heck it is, other than the utter death of Nvidia.

VR-Zone stumbled across some details about Larrabee recently (nice catch guys)*, so I guess that makes it open season on info. VRZ got it almost dead on, the target is 16 cores in the early 2009 time frame, but that is not a fixed number. Due to the architecture, that can go down in an ATI x900/x600/x300 fashion, maybe 16/8/4 cores respectively, but technically speaking it can also go up by quite a bit.

What are those cores? They are not GPUs, they are x86 'mini-cores', basically small dumb in order cores with a staggeringly short pipeline. They also have four threads per core, so a total of 64 threads per "CGPU". To make this work as a GPU, you need instructions, vector instructions, so there is a hugely wide vector unit strapped on to it. The instruction set, an x86 extension for those paying attention, will have a lot of the functionality of a GPU.

What you end up with is a ton of threads running a super-wide vector unit with the controls in x86. You use the same tools to program the GPU as you do the CPU, using the same mnemonics, and the same everything. It also makes things a snap to use the GPU as an extension to the main CPU.

Rather than making the traditional 3D pipeline of putting points in space, connecting them, painting the resultant triangles, and then twiddling them simply faster, Intel is throwing that out the window. Instead you get the tools to do things any way you want, if you can build a better mousetrap, you are more than welcome to do so. Intel will support you there.

Those are the cores, but how are they connected? That one is easy, a hugely wide bi-directional ring bus. Think four not three digits of bit width and Tbps not Gbps of bandwidth. It should be 'enough' for the average user, if you need more, well now is the time to contact your friendly Intel exec and ask.

As you can see, the architecture is stupidly scalable, if you want more CPUs, just plop them on. If you want less, delete nodes, not a big deal. That is why we said 16 but it could change on more or less on a whim. The biggest problem is bandwidth usage as a limiter to scalability. 20 and 24 core variants seem quite doable.

The current chip is 65nm and was set for first silicon in late 07 last we heard, but this was undoubtedly delayed when the project was moved from late 08 to 09. This info is for a test chip, if you see a production part, it will almost assuredly be on 45 nanometres. The one that is being worked on now is a test chip, but if it works out spectacularly, it could be made into a production piece. What would have been a hot and slow single threaded CPU is an average GPU nowadays.

Why bring up CPUs? When we first heard about Larrabee, it was undecided where the thing would slot in, CPU or GPU. It could have gone the way of Keifer/Kevet, or been promoted to full CPU status. There was a lot of risk in putting out an insanely fast CPU that can't do a single thread at speed to save its life.

The solution would be to plop a Merom or two in the middle, but seeing as the chip was already too hot and big, that isn't going to happen, so instead a GPU was born. I would think that the whole GPU notion is going away soon as the whole concept gets pulled on die, or more likely adapted as tiles on a "Fusion like" marchitecture.

In any case, the whole idea of a GPU as a separate chip is a thing of the past. The first step is a GPU on a CPU like AMD's Fusion, but this is transitional. Both sides will pull the functionality into the core itself, and GPUs will cease to be. Now do you see why Nvidia is dead?

So, in two years, the first steps to GPUs going away will hit the market. From there, it is a matter of shrinking and adding features, but there is no turning back. Welcome the CGPU. Now do you understand why AMD had to buy ATI to survive? µ

* Update
We originally stated that Intel had briefed VRZone, but it transpires that wasn't the case. Apologies.