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Semantics Consistent Parallelism

Semantics Consistent Parallelism. Li-Yi Wei Microsoft Research. Parallelism. parallel. parallel. parallel. Traditional parallelization. Sequential consistency [Lampert] parallel algorithms do NOT change result sorting, FFT, matrix, etc. sort. New parallelization (cheating).

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Semantics Consistent Parallelism

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  1. Semantics Consistent Parallelism Li-Yi Wei Microsoft Research

  2. Parallelism parallel parallel parallel

  3. Traditional parallelization • Sequential consistency [Lampert] • parallel algorithms do NOT change result • sorting, FFT, matrix, etc. sort

  4. New parallelization (cheating) • Not all algorithms need to be seq-consistent • perception (graphics/vision/image/video), statistics • approximate solutions might suffice → more parallel • Semantic consistency

  5. Pseudo Random Number Generator • The main source of randomness in programs • e.g. rand() in c/c++ • Desirable properties • white noise statistics • repeatable, fast computation • Traditional sequential method • e.g. xn = (a xn-1 + b) mod c

  6. input trivially prepared in parallel, e.g. linear ramp feed input value into hash, independently & in parallel output white noise key idea: borrow cryptographic hash! [Tzeng and Wei I3D 2008] Parallel PRNG input hash output

  7. How to start a bar flight

  8. How to stop the bar flight Parallel GPU run time (slow motion) 4M Poisson disk samples / sec in parallel!

  9. Poisson disk sampling • A set of samples that are as random as possible remain a minimum distance r away from each other • Why pick this problem? important algorithm - sampling, graphics, statistics seemly non-parallelizable

  10. Loop: random sample from the entire domain accept sample if not in conflict with existing ones O High quality ground truth X Slow speed inherently sequential Dart throwing [Cook 1986]

  11. Parallel Poisson disk sampling[Wei SIGGRAPH 2008] • Samples from a grid • 1 sample per grid cell • Sample grid cells far apart in parallel • Watch out for bias! • Tricks to avoid bias

  12. Image cloning

  13. http://www.freewebs.com/cheesesandwiches/cloning.jpg http://mahboubian.googlepages.com/dog-bird.jpg

  14. Poisson image editing[Perez et al. SIGGRAPH 2003] • State-of-art for image cloning + other stuff • Solving Poisson equation • interior detail (source) • boundary condition (target) • Heavy computation

  15. Coordinate interpolation • [Farbman et al. SIGGRAPH 2009] • p = ∑ wibi • Easy computation • Run parallel on a GPU

  16. Summary • Conclusion • sequential consistency too strict • semantic consistency (perceptual, statistical, etc) • Future work • parallelism via semantic consistency • individual algorithms / applications • general methodology?

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