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Semantics Consistent Parallelism Li-Yi Wei Microsoft Research

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Parallelism Algorithm API Hardware parallel ? CUDA, BSGP CPU, GPU parallel

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Traditional parallelization Sequential consistency [Lampert] parallel algorithms do NOT change result sorting, FFT, matrix, etc. 12345678 38716524 sort

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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

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Examples

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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. x n = (a x n-1 + b) mod c

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Parallel PRNG 1. input trivially prepared in parallel, e.g. linear ramp 2. feed input value into hash, independently & in parallel 3. output white noise key idea: borrow cryptographic hash! [Tzeng and Wei I3D 2008] hash input output

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How to start a bar flight

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

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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

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Dart throwing [Cook 1986] 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

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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

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Image cloning

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

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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

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[Farbman et al. SIGGRAPH 2009] p = w i b i Easy computation Run parallel on a GPU Coordinate interpolation

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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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Parallelism Processors are becoming parallel Intel Larrabee, NVIDIA, AMD/ATI, IBM/Sony Cell, etc. So are programming interfaces BSGP, CUDA, CAL, Ct, DX, OpenGL, etc. As well as applications To take advantage of parallel environment

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Parallel Algorithm Traditional parallelization methods Sequential consistency [Lampert] - sorting, FFT, matrix, etc. Not all algorithms need to be seq-consistent perception (graphics/vision/image/video), statistics Approximate solutions might suffice Opportunities for new parallelization methods

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Image cloning

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