Intel's Larrabee Architecture Disclosure: A Calculated First Move
by Anand Lal Shimpi & Derek Wilson on August 4, 2008 12:00 AM EST- Posted in
- GPUs
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).
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iop3u2 - Monday, August 4, 2008 - link
First of all it's called d3d not directx.Secondly you seem to imply that direct3d/opengl will cease to exist at some point if larrabee succeeds. I thinks you don't quite get what they are. They are APIs. Larrabee won't make programming APIless. Are you serious anand or what?
The Preacher - Tuesday, August 5, 2008 - link
It could make programming D3D/OpenGL-less for programs/PCs that exploit Larrabee. And if the share of such programs/PCs increases, the share of competing solutions logically decreases and might eventually vanish (although not anytime soon).iop3u2 - Tuesday, August 5, 2008 - link
Just because you can for example write a c program without the c lib it doesn't mean that people follow that road. It's all about what programmers will choose to do.Also, even if they do vanish there will still be a need for an api. So there will either be a new api or they won't vanish. Both situations make no difference whatsoever to the fact that larrabee will always need api implementations.
ZootyGray - Tuesday, August 5, 2008 - link
right - and I will put hotels on boardwalk and park place :)I used to own an 815chipset - it was like version 14 or whatever so it didn't suk as bad as some of the earlier ones - but it did blow up - I think pixelated FarCry and Doom3 really killed it. But o sure, the software fixes and bubblegum patches made it good, for a while. I really do think I am going to wait for this just so I can watch the lineups of returns - or read the funny forums posts of sheep seeking help - baaaahaha :) The best part is that it doesn't exist - delay, postpone - kinda like the 64bit chip also. Maybe later, maybe. But the ads invade the livingroom.
Make sure you keep yer getouttajailfree card - receipt.
Ummm let's see: I think I will buy this one!
Reality is that 4870x2 is on deck. Not 'rumour and sigh'. I just know there will be a 16page article on that - not!
Pok3R - Monday, August 4, 2008 - link
Larrabee means good news for consumers, and definitely bad news for nvidia. Maybe the worst in decades...with AMD and Ati having enough human resources now to face it, and Nvidia having nothing but bad policies and falling stocks despite good $elling numbers...The future, today, is definitely Intel vs AMD/Ati.
initialised - Monday, August 4, 2008 - link
a miniature render farm (you know like they use to make films like Hulk and WALL-E) on a chip. Lets hope AMD and nVidia can keep up.ZootyGray - Monday, August 4, 2008 - link
Really? Guess again. There is NOT anything to keep up to.I do not accept that the grafx loser in the industry is going to simply become numero uno overnight.
You really think that nvidia and ati have been sleeping for decades?
Supporting the destruction of ntel's only competitors leaves us at the mercy of a group that's already been busted for monop and antitrst.
Well written article? Of course, but I think it's like you are all fished in on many fronts. Nothing is really known except spin. This is beachfront property in the desert.
There's nothing to watch except what we usually watch - released hardware benchmarks.
I tell you AMD is going to be the cpu of choice in a few months when the truth about the bias in the benchies is revealed. And try - try real hard - to imagine ati+amd creating the ultimate cpu+gpu powerhouse. ntel needs this hype because I am not the only one with vision here. they are rich and scared, for now.
but such talk seems to be frowned upon - so let's all cheer for the best grafx manufacturer - ntel = kkaakk! sorry to offend, so many of you just might be lost in the paid mob. so just watch and you will see for yourself- no need to believe me. I really know almost nothing - but I am free to see for myself. sorry to offend - I just can't cosign bs. but that's just me and a very few other posters here who have also been criticized. watch and see for yourself. watch...
Mr Roboto - Monday, August 4, 2008 - link
I'd have to agree with the skeptics here. While the article is well written and informative (What AnandTech articles aren't?) it's purely speculation that Intel can get all of the variables right. How does a company that hasn't made a competitive GPU since the days of the 486 suddenly jump to Nvidia and ATI GPU type levels on their first try, never mind surpassing them. It's absolutely absurd to think that these chips are going to replace GPU's in terms of performance. I believe Larrabee will kick the shit out of Intel's own IGP but then again that's not much of a feat.Again I have to agree with previous posters that Intel just isn't that innovative. Even as I speak their are many lawsuits pending against Intel, most of them having to do with accusations of stolen IP that were used to design the Core2Duo. Antitrust suits aside, it's clear that Intel is similar to MS in that they just bully, bribe or outright steal to get ahead then pay whatever fines are levied because in the end they can never fine them enough to not make it worthwhile for Intel or MS to break the law.
The 65nm Core2Duo is amazing. The 45nm E8400 I just bought is even more so. However the more I think about Intel's past failures as well as how they operate as a company the more far fetched this whole thing becomes.
IMO they should have tried to compete in the dedicated GPU market before trying something like this. From a purely marketing standpoint Intel and graphics just don't go together. To come in to a new field in which they are unproven (I would bet Intel executives believe that building IGP's have somehow given them experience) and make outrageous claims such as the GPU is dead and Intel will now be the leader, is absurd.
JarredWalton - Tuesday, August 5, 2008 - link
I think a lot of you are missing the point that we fully understand this is all on paper and what remains to be seen is how it actually pans out in practice. Without the necessary drivers to run DirectX and OpenGL at high performance, this will fail. How many times was that mentioned? At least two or three.Now, the other thing to consider is that in terms of complexity, a modern Core 2 core is far more complex to design than any of the GPUs out there. You have all sorts of general functions that need to be coded. A GPU core these days consists of a relatively simple core that you then repeat 4, 8, 16, 32, etc. times. Intel is doing exactly that with Larrabee. They went back to a simple x86 core and tacked on some serious vector processing power. Sounds a lot like NVIDIA's SP or ATI's SPU really.
Fundamentally, they have what is necessary to make this work, and all that remains is to see if they can pull off the software side. That's a big IF, but then Intel is a big company. We have reached the point where GPUs and CPUs are merging - CUDA and GPGPU aim to do just that in some ways - so for Intel to start at the CPU side and move towards a GPU is no less valid an approach than NVIDIA/ATI starting at GPUs and moving towards general purpose CPUs.
Midwayman - Monday, August 4, 2008 - link
I not interested in the graphics so much. It may or may not compete with the the top end nvidia chips if released on time. What is more interesting is if this can easily be integrated as a general purpose cpu for non-graphics work? Imagine getting a benefit out of your gpu 100% of the time, not just when you're gaming. I know its possible to use more modern GPU's this way if you code specifically for them, but with its x86 architecture, it might be able to do it without having apps specifically coded for it.