addition to language frontends, we are planning support for further backends. Because PTX serves only as an intermediate representation, it is possible to translate and execute compiled megakernels on machines other than NVIDIA GPUs. OptiX has a CPU fallback path that employs this approach.

One downside of OptiX, like any compiler, is that performance of the compiled kernel does not always match a hand-tuned kernel for a specific use-case. We continue to explore optimization techniques to close that gap. The original paper discusses performance in more detail.

We have also discovered tradeoffs in the compile-time specialization of kernels that achieve high performance, but result in small delays when assumptions are violated and a kernel must be regenerated. In the future, the system may choose to fall back to a generalized kernel to maintain slightly degraded interactivity while a new specialized kernel is compiled.

acknowledgments The car, frog, and engine model in Figure 1 are courtesy of TurboSquid. The bunny model in Figures 3 and 9 is courtesy of the Stanford University Graphics Lab. Phil Miller was instrumental in keeping the effort on track. The authors benefited greatly from groundwork and numerous conversations on ray tracing with members of NVIDIA Research and the SceniX team.

hanrahan, P. Interactive k-d tree gpu raytracing. In I3D ‘07: Proceedings of the 2007 Symposium on Interactive 3D Graphics and Games (2007), acm, new york, ny, usa, 167–174.

4. kajiya, j.t. the rendering equation. In Computer Graphics (Proceedings of ACM SIGGRAPH) (1986), 143–150.

5. lattner, c., adve, v. llvm: a compilation framework for lifelong program analysis & transformation. In CGO ‘04: Proceedings of the 2004 International Symposium on Code Generation and Optimization (2004).

6. lindholm, e., nickolls, j., oberman, s., montrym, j. nvIDIa tesla: a unified graphics and computing architecture. IEEE Micro 28 (2008), 39–55.

7. mcguire, m., luebke, D. hardware-
accelerated global illumination
by image space photon mapping.

In Proceedings of the 2009 ACM SIGGRAPH/Euro Graphics conference on High Performance Graphics (2009).

8. nvIDIa. Ptx: Parallel thread execution Isa version 2. 3 (2011). http://developer.download.nvidia.com/-compute/DevZone/docs/html/c/doc/ ptx_isa_ 2. 3.pdf.

9. nvIDIa. nvIDIa optix ray tracing
engine Programming guide version 2. 5
(2012). http://www.nvidia.com/object/
optix.html.

Morgan McGuire ([e-mail]. edu), nvIDIa and Williams college.

1. aila, t., laine, s. understanding the efficiency of ray traversal on gPus. In Proceedings of High-Performance Graphics 2009 (2009), 145–149.

2. gribble, c. P., ramani, k. coherent References

ray tracing via stream filtering.

In Proceedings of the 2006 IEEE Symposium on Interactive Ray Tracing (2008), 59–66.

3. horn, D.r., sugerman, j., houston, m.,

Steven G. Parker, heiko Friedrich,
David Luebke, Keith Morley,
James Bigler, Jared hoberock,
David McAllister, Austin Robison,
Andreas Dietrich, Greg humphreys,
and Martin Stich ({sparker, hfriedrich,
dluebke, kmorley, jbigler, jhoberock,
davemc, arobison, adietrich, ghumphreys,
mstich}@ nvidia.com), nvIDIa,
santa clara, ca.

© 2013 acm 0001-0782/13/05

You’ve come a long way.

Share what you’ve learned.

ACM has partnered with MentorNet, the award-winning nonprofit e-mentoring network in engineering, science and mathematics. MentorNet’s award-winning One-on-One Mentoring Programs pair ACM student members with mentors from industry, government, higher education, and other sectors.

• Communicate by email about career goals, course work, and many other topics.

• Spend just 20 minutes a week - and make a huge difference in a student’s life.

• Take part in a lively online community of professionals and students all over the world.

Make a difference to a student in your field.
Sign up today at: www.mentornet.net
Find out more at: www.acm.org/mentornet

MentorNet’s sponsors include 3M Foundation, ACM, Alcoa Foundation, Agilent Technologies, Amylin Pharmaceuticals, Bechtel Group Foundation, Cisco Systems, Hewlett-Packard Company, IBM Corporation, Intel Foundation, Lockheed Martin Space Systems, National Science Foundation, Naval Research Laboratory, NVIDIA, Sandia National Laboratories, Schlumberger, S.D. Bechtel, Jr. Foundation, Texas Instruments, and The Henry Luce Foundation.