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OpenMP Lab IntroductionPowerPoint Presentation

OpenMP Lab Introduction

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Compiling for OpenMP

Open project Properties dialog box

Select OpenMP Support from C/C++ -> Language

Setting the Number of Threads

set OMP_NUM_THREADS=4

The default number of threads should be the number of cores available on the system.

In command window, use the environment variable, OMP_NUM_THREADS, to set the number of threads desired

General Instructions

- Use OpenMP to parallelize the given serial codes
- More detailed instructions given in lab directory
- Consult the OpenMP 3.0 Specification document for more detailed information and additional constructs
- http://www.openmp.org for download
- API functions listed and explained
- intomp_get_thread_num()
- intomp_get_num_threads()

Convolution

Filter

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4

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indata

outdata

Apply filter masks to a grayscale image

Code uses 5-point stencil to simulate shifted image as filters

Matrix-Matrix Multiplication

- Several different formulations of dense matrix-matrix multiplication
- Triple-nested loop
- Blocked
- Recursive

- Try OpenMP in one or more
- Is one version easier to use with OpenMP?
- Is there a noticeable difference in execution speed in one version over the others?

Sparse Matrix, Dense Vector

- Multiply a sparse matrix by a dense vector
- Matrix is stored in compressed sparse row (CSR) format
- Multiply only non-zero elements with corresponding vector elements

Prefix Scan

- Alternate Algorithm:

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- for j := 1 to log2n do
- for all k in parallel do
- if (k ≥ 2j-1) then
- x[k] := x[k – 2j-1] + x[k]
- fi
- od
- od

Prefix scan

Compute the inclusive prefix sum from input array; store results in output array

Quicksort

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041

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526

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739

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- Recursive version of Quicksort algorithm
- Consider using OpenMP tasks
- At some point (size of partition), overhead to create new task become prohibitive; switch to serial Quicksort or other sort algorithm

Minimum Spanning Tree

- Construct the Minimum Spanning Tree from a weighted graph using Prim’s Algorithm
- Start with arbitrary node in MST
- While more nodes not in MST do
- Find smallest weight edge from node in MST to some node not in MST
- Add new edge and node to MST

- Caution: first nested loop finds the index of the minimum

http://www.intel.com/go/threadingchallenge2010/

Apprentice Level

Master Level

Phase 1: May 31, 2010 to July 12, 2010

Phase 2: August 9, 2010 to November 1, 2010

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