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OpenMP: Introduction

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OpenMP: Introduction

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    1. OpenMP: Introduction Greg Wolffe / Christian Trefftz Grand Valley State University Supercomputing 2008 Education Program

    2. Supercomputing 2008 Education Program 2 Moores Law: Value ramp

    3. Supercomputing 2008 Education Program 3 Performance: Inflection Point

    4. Supercomputing 2008 Education Program 4 Multi-Core Architecture

    5. Supercomputing 2008 Education Program 5 Power Consumption: Superlinear

    6. Supercomputing 2008 Education Program 6 Power Efficiency

    7. Supercomputing 2008 Education Program 7 Conclusion ? Parallelism Hardware Going parallel (many-core) Servers / laptops / devices Software? Herb Sutter of Microsoft in Dr. Dobbs Journal: The free lunch is over. Software performance will no longer increase from one generation to the next as hardware improves unless it is parallel software.

    8. Supercomputing 2008 Education Program 8 Threads and Parallel Programming Process: A unit of work managed by an OS with its own address space and OS managed resources. Thread: resources within a process that execute the instructions in a program. Threads have their own program counter and a private memory region (a stack), but share the other resources within the process including the heap. Threads are the natural unit of execution for parallel programs on shared memory hardware.

    9. Supercomputing 2008 Education Program 9 Demo: Multiple Threads Multi-core in action Multi-threaded program OpenMP-multipleThreads.cpp OpenMP-multipleThreads.exe C++ w/OpenMP using Visual Studio Multi-core CPU in laptops Windows Performance Monitor Visualization

    10. Supercomputing 2008 Education Program 10 OpenMP Portable API for shared memory thread-based parallelism C/C++ and Fortran Fork-Join model

    11. Supercomputing 2008 Education Program 11 OpenMP Fundamentals Environment variables export OMP_NUM_THREADS=4 Library functions omp_set_num_threads (4); Compiler directives (#pragmas) #pragma omp parallel num_threads (4)

    12. Supercomputing 2008 Education Program 12 OpenMP Programming Source code #include omp.h #pragma omp parallel { parallel region } Compiling g++ -fopenmp filename.cc o filename

    13. Supercomputing 2008 Education Program 13 Directive Responsibility Work-sharing Data scoping Synchronization Scheduling

    14. Supercomputing 2008 Education Program 14 OpenMP: Work Sharing Parallel region: partition work Each thread executes same code Parallel for loop: partition iterations Threads share iterations of loop Parallel section: functional parallelism Threads perform different tasks

    15. Supercomputing 2008 Education Program 15 Demo: Hello, World Sequential code Uh, well skip that Multi-threaded code OpenMP-helloWorld.cc Compile/Execute Uses default number of threads Note: I/O not protected!

    16. Supercomputing 2008 Education Program 16 OpenMP: Data Shared: threads access a single copy of the data object Private: each thread gets volatile copy Firstprivate: initialized from master Lastprivate: masters copy updated with last value of last thread

    17. Supercomputing 2008 Education Program 17 Problem: Numerical Integration Mathematically: Approximately: Rectangle: Height f(xi) Width ?x

    18. Supercomputing 2008 Education Program 18 Demo: Parallel Loop Sequential code pi.cc Multi-threaded code OpenMP-forLoop.cc Compile/Execute Note: some data should not be shared! Compute Speedup Vary problem size Use more threads

    19. Supercomputing 2008 Education Program 19 Critical Section Problem Shared memory system ? shared data Shared data ? concurrent access Concurrent access ? corrupted variables Critical section problem: ensure correct access to shared data

    20. Supercomputing 2008 Education Program 20 Data Corruption

    21. Supercomputing 2008 Education Program 21 Concurrency Control Synchronization Mutex ensures exclusive access to critical section of code Barrier causes a group of threads to pause until all have reached a defined point Signalling Conditional Wait waits for some event; signals when it occurs Broadcasting signals a group of waiting threads

    22. Supercomputing 2008 Education Program 22 Mutual Exclusion // program code entry // access shared data exit // program code

    23. Supercomputing 2008 Education Program 23 Barrier

    24. Supercomputing 2008 Education Program 24 Conditional Wait

    25. Supercomputing 2008 Education Program 25 Broadcast

    26. Supercomputing 2008 Education Program 26 OpenMP: Synchronization #pragma omp master {} #pragma omp critical {} #pragma omp atomic count++; #pragma omp barrier reduction (+: sum)

    27. Supercomputing 2008 Education Program 27 Demo: Critical Section Unprotected code OpenMP-criticalSection.cc Execute Note: use synchronization constructs to solve critical section problem Verify correctness

    28. Supercomputing 2008 Education Program 28 OpenMP: Scheduling Static: splits iteration space into blocks of size chunk Dynamic: assign blocks to threads as they become idle (uneven workloads) Guided: adjusts chunk-size exponentially until all assigned

    29. Supercomputing 2008 Education Program 29 Problem: Mandelbrot Set Set of points in the complex plane which remain bounded Varying number of iterations = varying workload

    30. Supercomputing 2008 Education Program 30 Demo: Uneven Workload Graphical code OpenMP-Mandelbrot.cc Execute and Time Sequential and parallel Experiment with various scheduling strategies to improve performance

    31. Supercomputing 2008 Education Program 31 Acknowledgements Multi-core www.intel.com/multi-core Multi-Core Programming Shameem Akhter and Jason Roberts Intel Press, 2006. OpenMP www.openmp.org Parallel Programming in OpenMP Chandra, Dagum, Kohr, Maydan, McDonald, and Menon Morgan Kaufmann Publishers, 2001.

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