The Design Experiment: Could Intel Build a GPU?

Larrabee is fundamentally built out of existing Intel x86 core technology, which not only means that the chip design isn't foreign to Intel, but also has serious implications for the future of desktop microprocessors. Larrabee isn't however built on Intel's current bread and butter, the Core architecture, instead Intel turned to a much older architecture as the basis for Larrabee: the original Pentium.

The original Pentium was manufactured on a 0.80µm process, later shrinking to 0.60µm. The question Intel posed was this: could an updated version of the Pentium core, built on a modern day process and equipped with a very wide vector unit, make a solid foundation for a high-end GPU?

To first test the theory Intel took a standard Core 2 Duo, with a 4MB L2 cache at an undisclosed clock speed (somewhere in the 1.8 - 2.9GHz range I'd guess). Then, on the same manufacturing process, roughly the same die area and power consumption, Intel sought to find out how many of these modified Pentium cores it could fit. The number was 10.

So in the space of a dual-core Core 2 Duo, Intel could construct this hypothetical 10-core chip. Let's look at the stats:

  Intel Core 2 Duo Hypothetical Larrabee
# of CPU Cores 2 out of order 10 in-order
Instructions per Issue 4 per clock 2 per clock
VPU Lanes per Core 4-wide SSE 16-wide
L2 Cache Size 4MB 4MB
Single-Stream Throughput 4 per clock 2 per clock
Vector Throughput 8 per clock 160 per clock


Note that what we're comparing here are operation throughputs, not how fast it can actually execute anything, just how many operations it can retire per clock.

Running a single instruction stream (e.g. single threaded application), the Core 2 can process as many as four operations per clock, since it can issue 4-instructions per clock and it isn't execution unit constrained. The 10-core design however can only issue two instructions per clock and thus the peak execution rate for a single instruction stream is two operations per clock, half the throughput of the Core 2. That's fine however since you'll actually want to be running vector operations on this core and leave your single threaded tasks to your Core 2 CPU anyways, and here's where the proposed architecture spreads its wings.

With two cores, each with their ability to execute 4 concurrent SSE operations per clock, you've got a throughput of 8 ops per clock on Core 2. On the 10-core design? 160 ops per clock, an increase of 20x in roughly the same die area and power budget.

On paper this could actually work. If you had enough of these cores, you could get the vector throughput necessary to actually build a reasonable GPU. Of course there are issues like adapting the x86 instruction set for use in a GPU, getting all of the cores to communicate with one another and actually keeping all of these execution resources busy - but this design experiment showed that it was possible.

Thus Larrabee was born.

Index Not Quite a Pentium, Not Quite an Atom: The Larrabee Core


View All Comments

  • TonyB - Monday, August 04, 2008 - link

    but can it play crysis?! Reply
  • phaxmohdem - Monday, August 04, 2008 - link

    Can your mom play Crysis? *burn* Reply
  • JonnyDough - Monday, August 04, 2008 - link

    I suppose she could but I don't think she would want to. Why do you care anyway? Have some sort of weird fetish with moms playing video games or are you just looking for another woman to relate to?

    Ooooh, burn!
  • Griswold - Monday, August 04, 2008 - link

    He is looking for the one playing his mom, I think. Reply
  • bigboxes - Monday, August 04, 2008 - link

    Yup. He worded it incorrectly. It should have read, "but can it play your mom?" :p Reply
  • Tilmitt - Monday, August 04, 2008 - link

    I'm really disappointed that Intel isn't building a regular GPU. I doubt that bolting a load of unoptimised x86 cores together is going to be able to perform anywhere near as well as a GPU built from the ground up to accelerate graphics, given equal die sizes. Reply
  • JKflipflop98 - Monday, August 04, 2008 - link

    WTF? Did you read the article? Reply
  • Zoomer - Sunday, August 10, 2008 - link

    He had a point. More programmable == more transistors. Can't escape from that fact.

    Given equal number of transistors, running the same program, a more programmable solution will always be crushed by fixed function processors.
  • JonnyDough - Monday, August 04, 2008 - link

    I was wondering that too. This is obviously a push towards a smaller Centrino type package. Imagine a powerful CPU that can push graphics too. At some point this will save a lot of battery juice in a notebook computer, along with space. It may not be able to play games, but I'm pretty sure it will make for some great basic laptops someday that can run video. Not all college kids and overseas marines want to play video games. Some just want to watch clips of their family back home. Reply
  • rudolphna - Monday, August 04, 2008 - link

    as interesting and cool as this sounds, this is even more bad news for AMD, who was finally making up for lost ground. granted, its still probably 2 years away, and hopefully AMD will be back to its old self (Athlon64 era) They are finally getting products that can actually compete. Another challenger, especially from its biggest rival-Intel- cannot be good for them. Reply

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