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>Even for highly parallel workloads like ray tracing, the performance increase levels off at about 20 times. “No matter how many processors I apply, ray tracing

>Even for highly parallel workloads like ray tracing, the performance increase levels off at about 20 times. “No matter how many processors I apply, ray tracing ain’t going to go any faster than 20 times faster,” What? That's just not true. Matrix multiplication is one such embarrassingly parallel workload that can go much faster than 20 times. Ray tracing very probably too.

randcraw 10y ago
I suspect Wilson was saying "Since 5% of the runtime of the raytracing algorithm cannot be sped up through parallelism, even if the time needed to run the other 95% were reduced to zero due to parallelism (or some other magic), the speedup of raytracing could never exceed 100/(100-95), or 20X." In essence, Amdahl's Law trumps Moore's Law.
- marcosdumay 10y ago
  Moore's Law trumps Amdahl's Law when you decide to simply solve two problems at the same time. Instead of a 19X speedup on rendering, you get an 18X speedup on both rendering, physics, and AI, and gain some extra responsivity on the mean-time.
- Dylan16807 10y ago
  And that's true with some workloads, and maybe some systems that use raytracing, but not raytracing itself. The only overhead is combining the final data from each processor, and that's log(n) in the number of processors with a very small constant. A system with a million independent processors can raytrace very nearly a million times faster.
Qantourisc 10y ago
This can simple not be true: proof: take 2 computers render in parallel, combine images at the end when happy with the result. The combined images will look better. The 2 computers shared no resources. As such, ray-tracing scales. Luxrender for example has a problem here: all threads are writing to the same output buffer, causing bottlenecks and non perfect scaling. (Might be corrected by now.) The advise then was: run 2 or more Luxrenders, and combine the output image (luxrenders flm file).

randcraw 10y ago

I suspect Wilson was saying "Since 5% of the runtime of the raytracing algorithm cannot be sped up through parallelism, even if the time needed to run the other 95% were reduced to zero due to parallelism (or some other magic), the speedup of raytracing could never exceed 100/(100-95), or 20X." In essence, Amdahl's Law trumps Moore's Law.

marcosdumay 10y ago
Moore's Law trumps Amdahl's Law when you decide to simply solve two problems at the same time. Instead of a 19X speedup on rendering, you get an 18X speedup on both rendering, physics, and AI, and gain some extra responsivity on the mean-time.
Dylan16807 10y ago
And that's true with some workloads, and maybe some systems that use raytracing, but not raytracing itself. The only overhead is combining the final data from each processor, and that's log(n) in the number of processors with a very small constant. A system with a million independent processors can raytrace very nearly a million times faster.

Qantourisc 10y ago

This can simple not be true: proof: take 2 computers render in parallel, combine images at the end when happy with the result. The combined images will look better. The 2 computers shared no resources. As such, ray-tracing scales. Luxrender for example has a problem here: all threads are writing to the same output buffer, causing bottlenecks and non perfect scaling. (Might be corrected by now.) The advise then was: run 2 or more Luxrenders, and combine the output image (luxrenders flm file).