1 / 12

OpenMP (Open Multi-Processing)

OpenMP (Open Multi-Processing). David Valentine Computer Science Slippery Rock University. The buzz for OpenMP. There are more than a dozen events at SC12 with “OpenMP” in their titles. OpenMP celebrating 15 years: booth #2237 API designed for C/C++ and FORTRAN

xanti
Download Presentation

OpenMP (Open Multi-Processing)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. OpenMP(Open Multi-Processing) David Valentine Computer Science Slippery Rock University

  2. The buzz for OpenMP • There are more than a dozen events at SC12 with “OpenMP” in their titles. • OpenMP celebrating 15 years: booth #2237 • API designed for C/C++ and FORTRAN • Shared memory parallelism, in multicore world • As such, is an incremental learning curve for current programmers: • Start with serial code • Grab the obvious parallelizable sections to get the quickest results (Amdahl’s Law).

  3. Shared Memory Parallelism • Our world has already gone multicore • How best can we take advantage of the cores already on the desktop without jumping into the weeks of low-level thread manipulation? • There are several choices: • openMP • Cilk • Threaded Building Blocks (TBB)

  4. OpenMP (open multi-processing)OpenMP.org • Started in 1997, as continuation of ANSI X3H5  • Supported by industry (HP, IBM, Intel, Sun, et al) and government (DoE) • Designed for shared memory, multicore • Thread based parallelism • Explicit programmer control • Fork-join model

  5. OpenMP Fork-Join Model • Explicit programmer control • Can use thread number (omp_get_thread_num()) to set different tasks per thread in parallel region Fork Join wikipedia.com

  6. OpenMP • Made of 3 components: • Compiler Directives (20 as of 3.1) • #pragmaomp parallel will spawn parallel region • Run time library routines (32) • intmyNum = omp_get_thread_num( ); • Environment Variables (9) • setenv OMP_NUM_THREADS 8

  7. OpenMP Goals • Their 4 stated goals are: • Standardization • Lean and Mean • Ease of Use • Portability • CS2 students see their programs “go parallel” with just 2 or 3 lines of additional code! • At this level we are just exposing them to the concept of mulitcore, shared memory parallelism

  8. General Code Structure(from https://computing.llnl.gov/tutorials/openMP/#Abstract) #include <omp.h> main () { int var1, var2, var3; Serial code … Beginning of parallel section. Fork a team of threads. Specify variable scoping #pragmaomp parallel private(var1, var2) shared(var3) { Parallel section executed by all threads Other OpenMP directives Run-time Library calls All threads join master thread and disband } //parallel block Resume serial code … }//main

  9. The obligatory Hello World example #include <omp.h> #include <stdio.h> int main() { printf("Getting started...\n\n"); #pragmaomp parallel printf("Hello World from thread %i of %i\n", omp_get_thread_num(), omp_get_num_threads()); printf("\nThat's all Folks!\n"); return 0; } • Compile with OpenMP enabled • Project-Properties-Configuration Properties-C/C++ -Language – OpenMP Support – YES • Or gcc uses -fopenmp

  10. For CS1/CS2 • NB most programs are severely I/O bound • But, we are looking for only: • A simple counting loop (FOR) • where each iteration is independent, and • has enough work to distribute across our cores • The first two requirements are easy- the third one can involve “handicapping” the loop work • We won’t show them nearly all of OpenMP; we just want to whet their appetites here • Tell them the Truth, tell them nothing but the Truth, but for heaven’s sake don’t tell them ALL the Truth!

  11. eg. Trapezoidal Rule float trap(float xLo, float xHi, intnumIntervals) { float area; //area under the curve (the integral) float width; //width of each trapezoid float x; //our points of evaluation float sum; //sum up all our f(x)’s sum= 0.0; //init our summing var width = (xHi-xLo)/numIntervals; //width of each trap for(inti=1; i<numIntervals; i++) {//get the interior points x = xLo + i*width; //each iter. independent of others sum += f(x); //add the interior value }//for sum += (f(xLo) + f(xHi))/2.0; //add the endpoints area = width * sum; //calc the total area return area; //return the approximation }//trap

  12. eg. Trapezoidal Rule • Students add two lines: • #include <omp.h> • #pragma • When they see the cores all “redline” @100%, they are hooked. #pragmaomp parallel for private(x) reduction(+:sum) for(inti=1; i<numIntervals; i++) {//get the interior points x = xLo + i*width; //each iteration independent of others sum += f(x); //add the interior value }//for

More Related