# Lecture 3: Parallel Algorithm Design

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Lecture 3: Parallel Algorithm Design

Techniques of Parallel Algorithm Design
Balanced binary tree Pointer jumping Accelerated cascading Divide and conquer Pipelining Multi-level divide and conquer

Balanced binary tree Processing on binary tree: Let the leaves correspond to input and internal nodes to processors. Example Find the sum of n integers (x1, x2, ... , xn).

Problem of finding Prefix Sum
Balanced binary tree Problem of finding Prefix Sum Definition of Prefix Sum Input：　n integers put in array A[1..n] on the shared memory Output： array B[1..n], where for each B[i] (1≦i≦n) B[i] = A[1] + A[2] A[i] Example　Input: A[1..5] = (5, 8, -7, -10, 3)， 　　 Output: B[1..5] = (5, 13, 6, -4, -1) Sequential algorithm for Prefix Sum main (){ B[1] = A[1]; for (i = 2; i≦n; i++) { B[i] = B[i-1] + A[i]; }

Solving Prefix Sum problem on balanced binary tree (1)
Outline of the parallel algorithm for prefix sum To simplify the problem, let n = 2k (k is an integer) Calculate the sub-sum from the leaves to the root in bottom up style. Using the sub-sum obtained in (1) , calculate the prefix sum from the root to the leaves in up down style.

Solving Prefix Sum problem on balanced binary tree (2)
First read the input at the leaves. Then, calculate the sub-sum from the leaves to the root in bottom up style. (2) From the root to the leaves, do the following: send the right son its sub-sum obtained in (1), and send the left son the value of (its sub-sum) – the right son’s sub-sum). 12-(-2) P 4 3 2 1 6 7 =14 12 12-5 =7 -(-3) =10 -(-4) =6 -5 =-3 -2 =4 12-10 =2

Solving Prefix Sum problem on balanced binary tree (3)
Correctness of the algorithm 　When step (1) finished, the sub-sum in each internal node is the sum of its subtree.

Solving Prefix Sum problem on balanced binary tree (4)
Correctness of the algorithm - Continue In step (2), at each internal node The sub-sum sent to the right son is the summation of its subtree. (b) The sub-sum sent to the left son is the sum of its subtree subtracted by the sum of its right son’s subtree. P 4 3 2 1 12-10 =2 7 5 (b) (a) 12

Solving Prefix Sum problem on balanced binary tree (5)
Algorithm Parallel-PrefixSum (EREW PRAM algorithm) main (){ if (number of processor == i) B[0, i] = A[i]; for (h=1; h≦log n; h++) { if (number of processor j ≦ n/2h) { B[h, j] = B[h-1, 2j-1] + B[h-1, 2j]; } C[log n, 1] = B[log n, 1] for (h = (log n) - 1; h≧0; h--) { if (j is even) C[h, j] = C[h+1, j/2]; if (j is odd) C[h, j] = C[h+1, (j+1)/2] - B[h, j+1];

Solving Prefix Sum problem on balanced binary tree (6)
(First step) B[3,1] B[0,4] B[0,3] B[0,2] B[0,1] A[1] A[2] A[3] A[4] B[1,2] B[1,1] B[2,1] B[0,8] B[0,7] B[0,5] B[0,6] A[5] A[6] A[7] A[8] B[1,4] B[1,3] B[2,2]

Solving Prefix Sum problem on balanced binary tree (7)
(Second step) Ｃ[3,1] C [0,4] [0,3] [0,2] [0,1] [1,2] [1,1] [2,1] [0,8] [0,7] [0,5] [0,6] [1,4] [1,3] [2,2]

Solving Prefix Sum problem on balanced binary tree (8)
Analysis of the algorithm 　Computing time: for loop repeated log n times and each loop can be executed in O(1) time →　O(log n) time 　Number of processors: Not larger than n →　n processors 　Speed up: O(n/log n) 　Cost: O(n log n) It is not cost optimal since the running time of the optimal Θ(n).

Balanced binary tree Accelerated cascading To reduce the cost, solve the problem sequentially when the size of the problem is small. Accelerated cascading is used, usually, with balanced binary tree and divide and conquer techniques.

Balanced binary tree Accelerated cascading for Prefix Sum problem
Policy for improving the algorithm To make the algorithm cost optimal, we decrease the number of processors from n to n/logn. (Note: Computing time of the algorithm is O(logn).) Steps: Instead of processing n elements in parallel, divide n elements into n/logn groups with logn elements each. To each group assign one processor and solve the problem for the group sequentially.

Balanced binary tree Accelerated cascading for Prefix Sum problem
Improved algorithm Parallel-PrefixSum Divide n elements in A[1..n] in to n/log n groups with log n elements each. 　（O(1) time，O(n/log n) processors） (2)　Assign each group one processor and find the prefix sum for each group. （O(log n) time，O(n/log n) processors） A[1..n] log n elements

Balanced binary tree Accelerated cascading for Prefix Sum problem (3)
Improved algorithm Parallel-PrefixSum - continue (3)　Let S be the set of the last element in each group (it is the sum of the group). Use algorithm Parallel-PrefixSum to find the prefix sum of S. ( O(log (n/log n) ) = O(log n) time，O(n/log n) processors) Algorithm Parallel-PrefixSum Last element in each group

Balanced binary tree Accelerated cascading for Prefix Sum problem (4)
Improved algorithm Parallel-PrefixSum - continue (4)　Use the prefix sum of S to find the prefix sum of the input A[1..n]. （O(log n) time，O(n/log n) processor） Result of (3)

Balanced binary tree Accelerated cascading for Prefix Sum problem (5)
Analysis of the improved algorithm Computing time and the number of processors：　 Each step: O(log n) time, O(n/log n) → Totally, O(log n) time, O(n/log n) processors 　Speed up = O(n/log n) 　Cost：　O(log n × n/log n) = O(n) 　It is cost optimal. 　It is also time optimal ( Don’t show the proof here) 　 It is optimal algorithm.

Divide and Conquer Divide and conquer (1) 2 divide and conquer
(2) n divide and conquer（ε＜１） ε

Divide and Conquer Divide and conquer technique
Well known technique in algorithm design 　Solving problems recursively 　Used very often in both sequential and parallel algorithms How to divide and conquer Dividing step: dividing the problem into a number of subproblems. Conquering step: solving each subproblem recursively. (3) Merging step: merging the solutions of subproblems to the solution of the original problem.

Divide and Conquer Convex hull problem
Input: a set of n points in the plane. Output: the smallest convex polygon which contains all points of the input. (The convex polygon is represented by the list of its vertices in order of clockwise.) 　Basic problems in computational geometry. 　A lot of applications. 　Solved in O(nlogn) time sequentially. In the following we only consider the upper convex hull. （Upper convex hull: ( P9, P8, P1, P0 ) ） P 1 2 3 9 4 5 6 7 8 Output: ( P ,P ,P ,P ,P ,P ) Lower convex hull Upper convex hull

Merging of two upper convex hulls
Divide and Conquer Merging of two upper convex hulls Finding the upper common tangent p 1 2 3 9 4 5 6 7 8 10 Common upper tangent = (p ,p ) It is known that common tangents can be found in O(log n) time sequentially.

Divide and Conquer 2 divide and conquer (1)
Outline of the algorithm Parallel-UpperConvexHull Preprocessing　Sort all the points according to their x coordinates, and let the result is the sequence (p1, p2, p3, ... , pn). If the size of sequence is 2, return the sequence. Divide (p1, p2, p3, ... , pn) to the left half part and the right half part, and find the upper convex hull of each recursively. (3) Find the upper common tangent of two upper convex hulls obtained in (2), and output the solution of the problem.

Divide and Conquer 2 divide and conquer (2)
How 2 divide and conquer works Find the upper common tangent for two upper convex hulls of two vertices each. Find the upper common tangent for two upper convex hulls of four vertices each. Find the upper common tangent for two upper convex hulls of eight vertices each.

Divide and Conquer 2 divide and conquer（３） Recursive execution
When the problem is divide once, the size of the subproblem becomes half. Suppose the size of the subproblems becomes 2 when the problem is divided k times. n/2k= 2　⇒　 k = log2 n - 1 n n/2 n/4 2 Height= log

Divide and Conquer ２divide and conquer (4) Complexity of the algorithm
Preprocessing：　O(log n) time，n processors　 Steps (1)〜(3)：　each step runs O(log n) time，use n/2 processors 　 T(n) = T(n/2) + O(log n) Therefore, T(n) = O(log n) ∴　The algorithm runs in O(log n) time using O(n) processors. Computational model: There is no concurrent access　⇒　EREW PRAM Proprocessing　Sort the sequence of the points according to their x coordinates. If the size of the sequence is 2, return the sequence. Divide the sequence into the left half part and the right half part, and find the upper convex hull of each recursively. Find the upper common tangent of two upper convex hulls obtained in (2), and output the upper convex hull of the sequence. 2 2

Divide and Conquer 2divide and conquer (5)
Finding the complexity of the algorithm from recursive tree Computing time Number of processors At the level of the leaves, n/2 processors are used at the same time. ⇒　n/2 processors n n/2 n/4 2 Height log c Time Totally c log n c log n/2 c log n/4 O(log n) Processors 4 T(n)×P(n)=O(nlog n)　　 It is not cost optimal 2

Divide and Conquer n divide and conquer Outline of the algorithm
1/2 Outline of the algorithm Preprocessing Sort the sequence of the input points according to their x coordinates, and let the result be sequence (p1, p2, p3, ... , pn). (1)　If the size of the sequence is 2, return the sequence. (2)　Divide (p1, p2, p3, ... , pn) to equally-sized subsequence, and find the upper convex hull of each recursively. (3)　Merge upper convex hulls into the upper convex hull of the sequence． n 1/2 n 1/2

Divide and Conquer n Merging upper convex hull
1/2 Merging upper convex hull n 1/2 Assign each upper convex hull processors to find the upper common tangents in 　　O(log n) time, and then determine the edges which belong to the solution．　　　 Case 1 Case 2

Recursive tree of n divide and conquer
1/2 When the problem is divided once, the size of the subproblems becomes Suppose that the size of the subproblems becomes 2 when the problem is divided k times. 　 = 2　⇒　 k = log log n n 1/2 n 1/(2 ) k Height= loglog n n 1/2 1/4 2

Analysis of the algorithm
Divide and Conquer Analysis of the algorithm Preprocessing：　O(log n) time，n processors.　 Steps (1)〜(3)：　 each step O(log n) time，n processors. 　 T(n) = T(n　 ) + O(log n), therefore, T(n) = O(log n) ∴　Totally, the algorithm runs in O(log n) time using O(n) processors. Computational model Concurrent reading happens in the procedure of finding the upper common tangents ⇒　CREW PRAM Preprocessing　Sort the sequence of the points in their x coordinates. (1)　If the size of the sequence is 2，return the sequence. (2)　Divide the sequence into　 equally-sized subsequences, and find the upper convex hull of each recursively. (3)　Find the upper common tangents of the upper convex hulls obtained in (2), and determine the solution. n 1/2 1/2 T(n)×P(n)=O(nlog n)　　Optimal !!!

Exercise 1. Suppose nxn matrix A and matrix B are saved in two dimension arrays. Design a PRAM algorithm for A×B using n and nxn processors, respectively. Answer the following questions: What PRAM models that you use in your algorithms? What are the runings time? Are you algorithms cost optimal? Are your algorithms time optimal? 2. Design a PRAM algorithm for A×B using k (k <= nxn processors). Answer the same questions.

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