seen in Pixar’s feature films. In 2004 Hanrahan received an Oscar for Technical Achievement in Computer Graphics for his role in modeling the ways light scatters below translucent surfaces such as skin.
Tom Duff, who works at Pixar, conducted our interview with Akeley and Hanrahan at Pixar’s studios in Emeryville, California. A graphics-computing veteran himself, Duff has spent more than 30 years writing software for feature films, from Star Trek II to Ratatouille. His current passion is building robots and their software for theme parks and other entertainment uses. Duff has a lengthy resume of publications and patents and has received two Academy Awards for his scientific and technical achievements.
TOM DUFF What are GPUs and how have they evolved over the years?
PAT HANRAHAN Graphics requires a lot of computation. People who want realtime graphics want as many cycles as they can get as cheaply as they can get them. GPUs were developed in response to the demand for creating a lot of inexpensive cycles that you can use for graphics. KURT AKELEY It used to be that there was so little compute power available for the amount of money somebody could spend on a desktop, that we really couldn’t do very high-quality graphics at all. What that meant was the problem had to be reduced to something simple enough that you could build hardware that was very specific and solved a very specific set of problems.
The graphics hardware pipeline, the abstraction that shows up in some of the articles in this issue of Queue, was an answer to that: an architecture for solving a very small subset of what’s required to do movie-quality graphics, but solving it efficiently enough that you could get it to work interactively 30 years ago.
What’s happened is that over those 30 years, the Moore’s law increase in transistor counts by 50 percent a year or so, and clock increases by maybe 20 percent a year over most of that period, resulted in a fantastic amount of computing power. This means the GPUs are actually solving a much more general problem now. There’s enough compute power that you don’t have to do just a little bit to a vertex and barely compute a pixel and stick it in a frame buffer—that was the whole story 30 years ago. Now you can do really advanced shading. Of course, that involves much more general operations.
So, where GPUs used to be quite specific and very distinct from CPUs, today we’re having this collision in terms of architecture; what a GPU is and what a CPU is are no longer disjoint sets.
What we’re talking about isn’t just whether we can use graphics processors to do general-purpose computing, but in the bigger sense, how will general-purpose computing be done? How will graphics processing and other technologies that have evolved influence the way computing is done in general? That’s a big issue that the world’s going to be working through for the next five or ten years. PH One way to think about it is that GPU architects, because they had this huge problem to solve—that is, making pictures in realtime—had to come up with some very innovative techniques to develop multicore chips. In the process of innovating, they’ve actually created things that are more general than they might have thought. I mean, they knew they were general, but now people are starting to discover them, or at least the ideas behind them.
Several years ago the major CPU chip vendors weren’t interested in parallel computers. They would just say, “Clock rates will continue to increase; don’t worry about parallelism.” But then they decided a couple of years ago that they can’t keep making these things faster and faster without using parallelism. Now everybody realizes that converting your programs to run on multicores is a big thing, and you have to do it or you won’t get more performance.
You can view GPUs as a couple of steps ahead of the game. They were out there maybe a little bit further, and they still are out there further than the dual-core or quad-core CPUs. GPUs will have 16 or 32 cores, and they’re specialized for certain different classes of workloads that are more related to graphics.
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