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
Thread and Data Management: It's Time to Blow Your Mind
With both the recent NIVIDA and AMD graphics hardware launches, we spent quite a bit of time talking about thread management. Since Larrabee is designed to be more of a collection of general purpose scalar and vector processing units, and vertex, primitive and pixel data (along with associate shader programs) are software managed. As we discussed what a context is for AMD and NVIDIA graphics hardware, a true context is going to be a different thing altogether on Larrabee.
We do have to make a point of saying before proceeding that NVIDIA and AMD are under no obligation to actually tell us how their architecture is physically implemented. It is entirely possible that much of the attributes of the hardware are not actually attributes of the hardware but simply reflections of how hardware resources are used. In recent discussions with both companies about certain realities of their hardware revealed to us that the belief is if the system behaves like a specific physical implementation then it effectively is the same as that physical implementation.
Of course, we disagree. And it is possible that some of this has more similarity with NVIDIA and AMD than they are letting on. But we'll go on what we've got for now, and assume that what Intel is doing is as divergent as it sounds.
Each Larrabee core on a chip (of which it seems likely there will be some multiple of 8 in the final product) can maintain 4 simultaneous software threads (4 contexts are kept active at a time). This gives the appearance of 4 virtual physical processors to software running directly on the hardware even though all four threads are sharing a single resource. It is very likely that the major purpose of this is to hide some of the long latency we hit when going to memory for texture data and the like.
Now, for the purpose of graphics rendering using Intel's software rendering library or as it emulates DirectX and OpenGL, a thread is set up to manage the resources for a larger group of instructions and data that Intel calls a "fiber". Normally a thread will manage 8 fibers at a time. The hardware thread maintains a context in software for the fiber. The fiber's job is to manage the execution data parallel kernels on multiple groups of 16 "strands" (because the vector processor is 16-wide). A strand is what we have traditionally called a thread on other graphics hardware. The problem here is that Intel hardware is actually executing threads in a way that emulates hardware features of other architectures.
To put it together a little better, imagine one of Intel's threads as one of NVIDIA's TPCs, a fiber as an SM, and a strand as a thread. Okay, so it isn't that simple (simple?). But it is a sort of rough way of looking at it and a quick way of understand why naming is different here.
Let's take a deeper look at what goes on. With 4 threads per core (with at least 8 and hopefully something more like 32 cores), 8 fibers per thread, and some multiple of 16 strands per fiber, we could end up with a huge number of strands being managed simultaneously. This is active, running threads we are looking at as well. Since Larrabee will be a CPU in a true sense of the term, we can have as many threads as necessary live and waiting for a time slice. In the context of a normal CPU, this would be managed by the operating system, but as Larrabee will see the light of day as a graphics card, the driver will probably be managing timesharing issues an OS would normally perform.
While running ridiculous numbers of threads per core at a time might kill performance, unlike current GPUs, resource availability doesn't disrupt the creation of threads. Six of one, half dozen of the other? Maybe, and maybe not. Having active threads with data available to context switch to is key to hiding latency in NVIDIA and AMD hardware. If enough threads cannot be actively maintained, stalls happen and kill performance. Similar issues will impact Intel, and keeping dual-issue in-order hardware busy with multiple threads might be more easily managed if it can fall back on traditional CPU thread management paradigms to handle an abundance of threads that manage software that manages data parallel kernels.
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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.