Cache and Memory Hierarchy: Architected for Low Latency Operation

Intel has had a lot of experience building very high performance caches. Intel's caches are more dense than what AMD has been able to produce on the x86 microprocessor front, and as we saw in our Nehalem preview - Intel is also able to deliver significantly lower latency caches than the competition as well. Thus it should come as no surprise to anyone that Larrabee's strengths come from being built on fully programmable x86 cores, and from having very large, very fast coherent caches.

Each Larrabee core features 4x the L1 caches of the original Pentium. The Pentium had an 8KB L1 data cache and an 8KB L1 instruction cache, each Larrabee core has a 32KB/32KB L1 D/I cache. The reasoning is that each Larrabee core can work on 4x the threads of the original Pentium and thus with a 4x as large L1 the architecture remains balanced. The original Pentium didn't have an integrated L2 cache, but each Larrabee core has access to its own L2 cache partition - 256KB in size.

Larrabee's L2 pool increases with each core. An 8-core Larrabee would have 2MB of total L2 cache (256KB per core x 8 cores), a 32-core Larrabee would have an 8MB L2 cache. Each core only has access to its L2 cache partition, it can read/write to its 256KB portion of the pool and that's it. Communication with other Larrabee cores happens over the ring bus; a single core will look for data in its L2 cache, if it doesn't find it there it will place the request on the ring bus and will eventualy find the data in its L2.

Intel doesn't attempt to hide latency nearly as much as NVIDIA does, instead relying on its high speed, low latency caches. The ratio of compute resources to cache size is much lower with Larrabee than either AMD or NVIDIA's architectures.

  AMD RV770 NVIDIA GT200 Intel Larrabee
Scalar ops per L1 Cache 80 24 16
L1 Cache Size 16KB unknown 32KB
Scalar ops per L2 Cache 100 30 16
L2 Cache Size unknown unknown 256KB

 

While both AMD and NVIDIA are very shy on giving out cache sizes, we do know that RV670 had a 256KB L2 for the entire chip cache and can expect that RV770 to have something larger, but not large enough to come close to what Intel has with Larrabee. NVIDIA is much closer in the compute-to-cache ratio than AMD, which makes sense given its approach to designing much larger GPUs, but we have no reason to believe that NVIDIA has larger caches on the GT200 die than Intel with Larrabee.

The caches are fully coherent, just like they are on a multi-core desktop CPU. The fully coherent caches makes for some interesting cases when looking at multi-GPU configurations. While Intel wouldn't get specific with multi-GPU Larrabee plans, it did state that with a multi-GPU Larrabee setup Intel doesn't "expect to have quite as much pain as they [AMD/NVIDIA] do".

We asked whether there was any limitation to maintaining cache coherence across multiple chips and the anwswer was that it could be possible with enough bandwidth between the two chips. While NVIDIA and AMD are still adding bits and pieces to refine multi-GPU rendering, Intel could have a very robust solution right out of the gate if desired (think shared framebuffer and much more efficient work load division for a single frame).

How Many Cores in a Larrabee? Programming for Larrabee
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  • del - Friday, August 15, 2008 - link

    Don't be a hater. :P Intel has got it goin' on right now. Believe in the POWAH of Larrabee... unless it proves to be a failure upon release.

    :)
    Reply
  • atlmann10 - Saturday, August 09, 2008 - link

    Think about this ok AMD originally was a private IBM cpu manufacturer. Then bought out and run as a side unit of INTEL, that was dropped after they were done with them. So in a way the were partners and I'm sure there was some friendliness. As it's always been said keep your friends close but your enemies closer. There have been some things especially in these past two years that struck me kind of odd. Such as AMD's graphics chips running fine on a x38/48 chipset and the physics collaboration things as well as a few other rumors. Then Nvidia starts spouting off about how they could kick INTELS A77 etc. Now AMD has a definite GPU coprocessor in ATI and they wanna break into the market of GPU's etc. They know that there will be graphics competition with Nvidia being there largest competitior because there dedicated to GPU's solely and have a reputation. However now AMD has some chips that compete straight on weakening Nvidia to a point. Then AMD is getting more and more out of there cpu's gpu's and chipsets so INTEl jumps in the CPU GPU market just like AMD. Either way it turns out more are going to go with INTEL cpu's and many other products where AMD is kind of a fringe player. Who would you rather compete against full on 2 major GPU manufacturers or attempt to kind of co-align yourself with there competetitor while the somewhat down. Then throw out a whole new way to do graphics that performs well Nvidia is already loosing market share. So more people try it and the same number of people go with ATI. That leaves a much lower market for Nvidia plus there paying back what some 200 million dollars in bad GPU's right now as well and a few other problems they been having. Now this is not anything I know but knowing INTEL loves to stick it to competitors when there weak think about it. Reply
  • benkantor - Wednesday, August 06, 2008 - link

    if you could fit 10 Larrabees on 143 mm^2, you could fit 40 Larrabees on 286 mm^2, not 20... :P Reply
  • MamiyaOtaru - Saturday, August 09, 2008 - link

    For the love of education. We've already been through this. See the end of page 6 through page 7 in the comments section.

    143mm^2 doesn't mean 143*143. It means 143 square millimeters. 286 square millimeters is twice as many, allowing twice as many cores.
    http://img379.imageshack.us/my.php?image=squaremmh...">http://img379.imageshack.us/my.php?image=squaremmh...

    The article is right and you are so very wrong.
    Reply
  • Barack Obama - Wednesday, August 06, 2008 - link

    Derek and Anand deliver again! Reply
  • KGR - Wednesday, August 06, 2008 - link

    I am not a profeesional about software and hardware that is why maybe this question can sound nonsense .
    If larrabee will have a software renderer and programmed by C++ is it possible that it is not depended on windows? I mean if it doesnt need direct X can we run the games on Linux also??
    Reply
  • npoe1 - Tuesday, August 05, 2008 - link

    I enjoyed reading this so much. I think that this kind of articles is what Anandtech needs; I usually go to Arstechnica to read things like this one.

    Again, thanks!
    Reply
  • TrEmEnDo - Tuesday, August 05, 2008 - link

    I am definitely impressed with this new development and I expect that this technology will be disruptive down the road, however I feel that somehow they are about to commit another of their megalomaniac mistakes.
    Has anyone stopped for a sec and look where all gaming industry is heading into? Are PCs the future gaming platform? Maybe I am missing something but aren't the big guys already struggling to retain a 'decent' percentage of the multibillion gaming pie (PC gaming alliance anyone...)? I believe that whether us, tech enthusiast, hardcore pc gamers like it or not, it is the console arena where the big guns are going to be playing in a few years from now.
    Guys, we are seeing this happening everyday, we see tittles appearing and disappearing everyday b/c companies don't want to commit the resources to develop games for more than one or two platforms (normally doing a sloppy work BTW). Now that the grandpas of graphic hardware had manage to get DX/D3D derived engines into the last gen consoles (xenos, RSX) and a terribly inertial and rigid developer community avoids and whines about how difficult is to program for the few hardware 'jewels' that we have already in our hands (Cell/RV770/G200) do you think anyone except Intel is in the mood for yet another graphics industry spin?

    I have no doubt that this new development will have its own niche application or someone will definitely find something appropriate for it, but to say that Larrabee CAN do graphics and to say larrabee will kick ass so bad that in 3 years from now we all will be gaming from a Larrabee containing computer are two very different things.
    Congrats to Intel as the fathers of the creature, and congrats to us to see the tech world moving on....but just don't think this will change the world as we know it.
    Reply
  • hooflung - Tuesday, August 05, 2008 - link

    They are doing something very AMD like and taking it a step further and tossing in a few Power ideals in. I just wonder what the power profile will look like and who will partner up with Intel for it.

    I am sure they will have 4+ of these cores built into integrated chip sets for OEMs and laptops to really boost those areas. And people who buy laptops will see that they can get a desktop with 'bigger larrabee' and play their games faster than their budget/laptop computer.

    So it does make sense. However, it is an empire made on a lot of ifs. It will be fun to watch. Thanks anandtech for the informative article.
    Reply
  • christophergorge - Tuesday, August 05, 2008 - link

    is it just me or does it look like another transmeta crusoe in the making? Reply

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