NVIDIA annonce par voie de communiqué de presse que sa technologie de rendu à plusieurs processeurs graphiques, le SLI, est désormais proposée sous licence pour les futures plates-formes Intel à base de chipset P55. Rappelons-nous que pour le chipset Intel X58, NVIDIA avait du ouvrir sa technologie SLI pour voir cette dernière proposée sur les premières machines Core i7...

PCGH: Intel made a lot of fuzz about ray tracing in the last 18 months or so. Do you think that's going to be a major part of computer and especially gaming graphics in the foreseeable future, until 2015 maybe?
 
Bill Dally: It's interesting that they've made a big fuzz about it while we've had a demonstration of real-time ray tracing at Siggraph last year. It's one thing making a fuzz, it's another thing demonstrating it running real-time on GPUs.  
 
But to answer that, what I see as most likely for game graphics going forward is hybrid graphics. Where you start out by rasterizing the scene and then you make a decision at each fragment, whether that fragment can be renderer just with a shader calculating using local information or if it's a specular surface or if it's transparent surface or if there's is a silhouette edge and soft-shadows are important. Then you may need cast rays to compute a very accurate and photo realistic color for that point. So I think it's gonna be a hybrid version where some pixels are rendered conventionally and some pixels involve ray tracing and that gives us the most efficient use of our computational resources - using ray tracing where it does the most good.

PCGH: While we're at it. Intel also made a big fuzz about Larrabee.
Bill Dally: M-hm.
 
PCGH: They are aiming for a mostly programmable architecture there. They state that they have only 10 percent dedicated to graphics of the whole die, the rest being completely programmable according to Intel. And still they want to compete in the high-end with your GPUs. Do you think that's feasible right now?
 
Bill Dally: First of all, right now, Larrabee is a bunch of View-graphs. So, until they actually have a product, it's difficult to say how good it is or what it does. You have to be careful to read to much into View-graphs - it's easy to be perfect, when you have to do is be a View-Graph. It's much harder when you have to deliver a product that actually works.  
 
But to the question of the degree of fixed function hardware: I think it puts them at a very serious disadvantage. Our understanding of Larrabee, which is based on their paper at Siggraph last summer and the two presentations at the Game Developers Conference in April, is that they have fixed function hardware for texture filtering, but they do not have any fixed function hardware either for rasterization or compositing and I think that that puts them at a very serious disadvantage. Because for those parts of the graphics pipeline they're gonna have to pay 20 times or more energy than we will for those computations. And so, while we also have the option of doing rasterization in software if we want - we can write a kernel for that running on our Streaming Multiprocessors - we also have the option of using our rasterizer to do it and do it far more efficiently. So I think it puts them at a very big disadvantage power-wise to not have fixed function hardware for these critical functions. Because everybody in a particular envelope is dominated by their power consumption. It means that at a given power value they're going to deliver much lower performance graphics.  
 
I think also that the fact that they've adopted an x86-instruction set puts them at a disadvantage. It's a complex instruction set, it's got instruction prefixes, it only has eight registers and while they claim that this gives them code compatibility, it gives them code compatibility only if they want to run one core without the SIMD extension. To use the 32 cores or use the 16-wide SIMD extension , they have to write a parallel program, so they have to start over again anyway. And they might as well have started over with a clean instruction set and not carry the area and power cost of interpreting a very complicated instruction set - that puts them at a disadvantage as well.  
 
So while we're very concerned about Larrabee, Intel is a very capable company, and you always worry, when a very capable company starts eating your lunch, we're not too worried about Larrabee at least based on what they disclosed so far.

PCGH: Will it be a major contributor or limiting factor: The driver team? Intel's integrated graphics have not a very good reputation for their drivers and it seems that both AMD and Nvidia are putting real loads of effort into their drivers.
 
Bill Dally: I think that you have to deliver a total solution. But if any part of the solution is not competitive, then the whole solution is not competitive. And our view based on what's been disclosed [until] today, is that the hardware itself is not going to be competitive and if they have a poor driver as well, that only makes it worse. But even a good driver won't save the hardware

PCGH: Recently, you have introduced new mobile Geforce parts which support DX10.1 compliance. Did your work already have an influence on those parts or where they completed already when you joined Nvidia?
 
Bill Dally: That was completely before my time. Those were already in the pipe. I'm tending to look a bit further out, so...

PCGH: When do you think we're going to see products on the shelves, that were influenced by your work?
 
Bill Dally: I've had small influences on some of the products that are going to be coming out towards the end of this year but those products were largely defined and it was just little tweaks toward the end. It's really gonna be the products in about the 2011 time frame that I will be involved in from the earlier stages.

PCGH: With Microsoft's Windows 7, and thus DirectX 11, expected on the shelves from October 22nd, do you expect a large impact on graphics cards sales from that?
 
Bill Dally: I actually don't know what drives the sales that much, but I would hope that people appreciate the GPU a lot more with Windows 7 because of DirectX Compute and the fact that the operating system both makes every use of the GPU itself and also exposes it in it's APIs for applications to use.

PCGH: Independently whether it's a DX11 or a DX10 / 10.1 GPU?
 
Bill Dally: If it supports DirectX Compute, then it doesn't need to be DX 11.

PCGH: No, but there's a different level of DX Compute, if I'm not mistaken. It's the DX11 Compute, then the downlevel shader called DX Compute 4.0 and 4.1?
 
Bill Dally: No, you're exceeding my knowledge a bit right now.

PCGH: Speaking of DirectX 11: Were you personally surprised when AMD was showing DX11 hardware at Computex?
 
Bill Dally: No, not particularly. We had had some advanced word of that.

PCGH: Ok, going back to the new mobile parts: They have a very much improved GFLOPS/watt ratio, almost double over the previous generation. Is this the way to go for the future? To squeeze out the maximum number of FLOPS per watt?
 
Bill Dally: We consider power-efficiency a first-class problem. And it's driven starting from our mobile offerings, but it's actually important across the product line. I mean at every power-point - even  at the 225-watt-top-of-the-line GPUs, we absolutely have to deliver as much performance as we can in that power-envelope. So a lot of the techniques that we use in our mobile devices, things like very aggressive clock-gating, power-gating are being used across the product line.

PCGH: Do you think that DX Compute is going to facilitate a more rapid increase of GFLOPS per square millimeter than was seen in the past?  
 
Bill Dally: We try to deliver as many GFLOPS per square millimeter as we can, regardless of how people program it. So I think DirectX Compute will enable both within Microsoft's software and also what third party windows software for more applications to use the power of the GPU. We're gonna deliver the absolute best performance per square millimeter regardless of how people program it. That's something that we constantly, in our engineering effort, are striving to improve.

PCGH: How fast is the ALU/FLOP-ratio evolving? Is the move towards more FLOPS accelerating in the future?
 
Bill Dally: The texturing and FLOPS actually tends to hold a pretty constant ratio and that's driven by what the shaders we consider important are using. We're constantly benchmarking against different developers‘ shaders and see what our performance bottlenecks are. If we're gonna be texture limited on our next generation, we pop another texture unit down. Our architecture is very modular and that makes it easy to re-balance.  
 
The ratio of FLOPS to bandwidth, off-chip bandwidth is increasing. This is, I think, driven by two things. One is fortunately the shaders are becoming more complex. That's what they want anyway. The other is, it's just much less expensive to provide FLOPS than it is [to provide] bandwidth. So you tend to provide more of the thing which is less expensive and then try to completely saturate the critical expensive resource which is the memory bandwidth.

PCGH: Do you think a large leap in available bandwidth would be necessary for next generation hardware - like for DirectX 11 with it's focus on random R/W (Scatter, Gather operations etc.) which should benefit greatly from more or at least more granular memory access.
 
Bill Dally: Almost everything would benefit from more bandwidth and being able to do it at a finer grain. But I don't think that there's gonna be any large jumps. I think we're gonna evolve our memory bandwidth as the GDDR memory components evolve and track that increase .

PCGH: Another topic: In contrast to your competitor, Nvidia's GPUs, at least the high-end ones, have in the last couple of years always been very large, physically. AMD is going the route of having a medium-sized die and scale it with X2 configurations for high-end needs; Nvidia is producing very large GPUs. Is that a trend which could change in the future or don't you think you have reached the limits of integration in single-chips, the "Big Blocks of Graphics"?
 
Bill Dally: We're trying to always deliver the best performance and value to our customers and we're gonna continue doing that. And for any given generation there's an economic decision that has to be made about how large to make the die. Our architecture is very scalable, so the larger we make the die, the more performance we deliver. We also deliver duplex-configurations as in GTX 295 and so, if build a very large die and then also put two of them we can deliver even more performance. And so for each generation we're gonna to the calculation side what is the most economic way of delivering the best performance for our customers.

PCGH: Starting with G80, we've seen the integration of shared memory, scratch pads and the like, for data sharing between individual SIMDs. This was obviously done specifically for GPGPU and stuff. Are we going to see more of that non-logic-area in future GPUs?
 
Bill Dally: First of all, we like to call it GPU computing, not GPGPU. GPGPU refers to using shaders to do computing, while GPU computing uses the native compute capability of the hardware. To answer that particular question, the answer is yes, although it's gonna be over many generations of future hardware. We see improving the on-chip memory system as a critical technology [in order] to enabling more performance where the off-chip bandwidth is scaling at a rate that's slower than the amount of floating point that we can put on the die. So we need to do more of things like the shared memory that's exposed to the multiple threads within a cooperative thread array.

PCGH: What do you think: What is the area, current GPUs, throughput processors or however you may call them, are lacking most? Which is the area which should be improved at the forefront?
 
Bill Dally: Well, they're actually pretty good, so it's hard to faults with them. But there's always room for improvement. But i think it's not about wanting, but about opportunities to make them even better. The areas where there's opportunities to make them even better is mostly in the memory system. I think that we're increasingly becoming limited by memory bandwidth on both the graphics and the compute side. And I think there's an opportunity from the hundreds of processors we're at today to the thousands of cores we're gonna be at in the near future to build more robust memory hierarchies on chip to make better use of the off-chip bandwidth.

PCGH: Do you envision future GPUs more like the current approach, where you have one large scheduling block and then the work gets distributed to each cluster or is it going to be a more independent collaboration of work units where each rendering cluster, each SIMD unit gets more and more independent?
 
Bill Dally: In the immediate future, I think things are gonna wind up very much the way they are today. I'm viewing it as a flat set of resources where the work gets spread uniformly across them. I think ultimately, they are going forward, we are going to have a need to build a more hierarchical structure into our GPUs both in the memory system and in how work gets scheduled.

PCGH: So that's a more distributed approach then for computing in graphics or computing in whatever the operating system uses the GPU for.  
 
Bill Dally: Yeah.