OpenMP Lab Introduction
upcrc.illinois.edu
Compiling for OpenMP
• Open project Properties dialog box
• Select OpenMP Support from C/C++ -> Language
Setting the Number of Threads
• 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
set OMP_NUM_THREADS=4
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
• int omp_get_thread_num()
• int omp_get_num_threads()
Convolution
• Apply filter masks to a grayscale image
• Code uses 5-point stencil to simulate shifted image as
filters
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Filter
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indata
outdata
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
• Compute the inclusive prefix sum from input array; store
results in output array
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Prefix scan
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Alternate Algorithm:
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
Quicksort
• Recursive version of Quicksort algorithm
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• 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/
Phase 1: May 31, 2010 to July 12, 2010
Phase 2: August 9, 2010 to November 1, 2010
Apprentice Level
Master Level