# the study of cache oblivious algorithms

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The Study of Cache Oblivious Algorithms. Prepared by Jia Guo. Cache-Oblivious Algorithms by Matteo Frigo, Charles E. Leiserson, Harald Prokop, and Sridhar Ramachandran . In the 40th Annual Symposium on Foundations of Computer Science, FOCS '99, 17-18 October, 1999, New York, NY, USA . - PowerPoint PPT PresentationTRANSCRIPT

The Study of Cache Oblivious AlgorithmsPrepared by Jia Guo

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Cache-Oblivious Algorithms by Matteo Frigo, Charles E. Leiserson, Harald Prokop, and Sridhar Ramachandran. In the 40th Annual Symposium on Foundations of Computer Science, FOCS '99, 17-18 October, 1999, New York, NY, USA.

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OutlineCache complexity Cache aware algorithmsCache oblivious algorithmsMatrix multiplicationMatrix transposition FFTConclusion

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AssumptionOnly two levels of memory hierarchies: An ideal cacheFully associativeOptimal replacement strategyTall cache A very large memory

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An Ideal Cache ModelAn ideal cache model (Z,L) Z: Total words in the cacheL: Words in one cache line

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Cache ComplexityAn algorithm with input size n is measured by:Work complexity W(n)Cache complexity: the number of cache misses it incurs. Q(n; Z, L)

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OutlineCache complexity Cache aware algorithmsCache oblivious algorithmsMatrix multiplicationMatrix transposition FFTConclusion

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Cache Aware AlgorithmsContain parameters to minimize the cache complexity for a particular cache size (Z) and line length (L).Need to adjust parameters when running on different platforms.

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Example:A blocked matrix multiplication algorithm

s is a tuning parameter to make the algorithm run fast

A11ssnA

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Example (2)Cache complexityThe three s x s sub matrices should fit into the cache so they occupy cache linesOptimal performance is obtained when Z/L cache misses needed to bring 3 sub matrices into cachen2/L cache misses needed to read n2 elements It is

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OutlineCache complexity Cache aware algorithmsCache oblivious algorithmsMatrix multiplicationMatrix transposition and FFTConclusion

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Cache Oblivious AlgorithmsHave no parameters about hardware, such as cache size (Z), cache-line length (L).No tuning needed, platform independent.The following algorithms introduced are proved to have the optimal cache complexity.

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Matrix MultiplicationPartition matrix A and B by half in the largest dimension. A: n x m, B: m x p

Proceed recursively until reach the base case - one element.

n max (m, p)m max (n, p)p max (n, m)

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Matrix Multiplication (2)A*BA1*B1A2*B2A11*B11A12*B12A21*B21A22*B22Assume Sizes of A, B are nx4n, 4nxn+++

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Matrix Multiplication (3)Intuitively, once a sub problem fits into the cache, its smaller sub problems can be solved in cache with no further misses.

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Matrix Multiplication (4)Cache complexityCan achieve the same as the cache complexity of Block-MULT algorithm (cache aware)For a square matrix, the optimal cache complexity is achieved.

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OutlineCache complexity Cache aware algorithmsCache oblivious algorithmsMatrix multiplicationMatrix transposition FFTConclusion

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Matrix Transposition

If n is very large, the access of B in column will cause cache miss every time! (No spatial locality in B)

A

ATfor i 1 to m for j 1 to n B( j, i ) = A( i, j )m x nBn x m

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Matrix Transposition (2)Partition array A along the longer dimension and recursively execute the transpose function.A11A12A21A22A11TA21TA12TA22T

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Matrix Transposition (3)Cache complexityIt has the optimal cache complexityQ(m, n) = (1+mn/L)

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Fast Fourier Transform

Use Cooley-Tukey algorithmCooley-Tukey algorithms recursively re-express a DFT of a composite size n = n1n2 as:Perform n2 DFTs of size n1. Multiply by complex roots of unity called twiddle factors. Perform n1 DFTs of size n2.

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n2n1

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Assume X is a row-major n1 n2 matrixSteps:Transpose X in place.Compute n2 DFTsMultiply by twiddle factorsTranspose X in placeCompute n1 DFTsTranspose X in-place

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Fast Fourier Transform*twiddle factorTranspose to select n2 DFT of size n1Call FFT recursively with n1=2, n2=2 Reach the base case, returnTranspose to select n1 DFT of size n2 Transpose and return n1=4, n2=2

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Fast Fourier TransformCache complexityOptimal for a Cooley-Tukey algorithm, when n is an exact power of 2Q(n) = O(1+(n/L)(1+logzn)

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Other Cache Oblivious AlgorithmsFunnelsort Distribution sortLU decomposition without pivots

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OutlineCache complexity Cache aware algorithmsCache oblivious algorithmsMatrix multiplicationMatrix transpositionFFTConclusion

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QuestionsHow large is the range of practicality of cache-oblivious algorithms?What are the relative strengths of cache-oblivious and cache-aware algorithms?

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Practicality of Cache-oblivious AlgorithmsAverage time to transpose an NxN matrix, divided by N2

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Practicality of Cache-oblivious Algorithms (2)Average time taken to multiply two NxN matrices, divided by N3

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Question 2Do cache-oblivious algorithms perform as well as cache-aware algorithms?FFTW libraryNo answer yet.

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References Cache-Oblivious Algorithms by Matteo Frigo, Charles E. Leiserson, Harald Prokop, and Sridhar Ramachandran. In the 40th Annual Symposium on Foundations of Computer Science, FOCS '99, 17-18 October, 1999, New York, NY, USA.Cache-Oblivious Algorithms by Harald Prokop. Master's Thesis, MIT Department of Electrical Engineering and Computer Science. June 1999. Optimizing Matrix Multiplication with a Classifier Learning System by Xiaoming Li and Mara Jesus Garzarn. LCPC 2005.

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