openmp | 易学教程

【并行计算】基于OpenMP的并行编程

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我们目前的计算机都是基于冯偌伊曼结构的，在MIMD作为主要研究对象的系统中，分为两种类型：共享内存系统和分布式内存系统，之前我们介绍的基于MPI方式的并行计算编程是属于分布式内存系统的方式，现在我们研究一种基于OpenMP的共享内存系统的并行编程方法。OpenMP是一个什么东东？首先我们来看看来之百度百科中的定义：OpenMp是由OpenMP Architecture Review Board牵头提出的，并已被广泛接受的，用于共享内存并行系统的多处理器程序设计的一套指导性的编译处理方案(Compiler Directive)。OpenMP支持的编程语言包括C语言、C++和Fortran；而支持OpenMp的编译器包括Visual studio，Sun Compiler，GNU Compiler和Intel Compiler等。OpenMP功能中最强大的一个功能是：在我们之前串行程序的源码基础上，只要进行少量的改动，就可以并行化许多串行的for循环，达到明显提高性能的效果。 1.OpenMP环境配置与例子废话不多少，首先上一个例子，由于本文以Vs 2015作为IDE，C++作为开发语言，在正式进行OpenMP编码之前，需要对编译器稍微配置一下。启动VS2015新建一个C++的控制台应用程序，如下图所示：然后在项目解决方案资源管理器上选择项目名称，点击右键，选择“属性”

OpenMP divide for loop over cores

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问题 i am trying to execute some application in parrallel using sse instructions and openmp. Concerning the openmp part i have code like: for(r=0; r<end_condition; r++){ .. several nested for loops inside .. } i want to divide this loop over r over multiple cores, and for example when using two cores one core should execute r=0 .. r=end_condition/2-1 and the other r=end_condition/2 .. r=end_condition-1. There is no communication between iterations of the loop so they can be ran in parallel, at the

gfortran linking flag for openmp

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问题 I'm trying to link multiple .o files using gfortran . I've compiled the files like so (in a makefile): gfortran -c -fopenmp file1.f gfortran -c -fopenmp file2.f Now I'd like to link the files with an option for OpenMP. I know with the Intel compiler the linking flag is -liomp5 , so to link the files with the Intel compiler one would call: ifort -o a.out file1.o file2.o -liomp5 This is obviously not the correct flag for the GNU compiler. What is the correct OpenMP linking flag for gfortran ?

OpenMP benchmark: Am I doing it right?

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问题 I made a program which calculates the fibonacci sequence. I executed it with different numbers of threads (eg. 1, 2, 10) but the execution time remained almost the same (about 0.500 seconds). I'm using CodeBlocks on Ubuntu and the GNU GCC compiler. In CodeBlocks I linked the library gomp and defined the flag -fopenmp for the compiler. #include <omp.h> #include <stdio.h> #include <stdlib.h> int main() { int i, n=1000, a[n]; omp_set_num_threads(4); for(i=0; i<n; i++) { a[i] = 1 + (rand() % ( 50

Parallel for inside a while using OpenMP on C

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问题 I'm trying to do a parallel for inside a while, somothing like this: while(!End){ for(...;...;...) // the parallel for ... // serial code } The for loop is the only parallel section of the while loop. If I do this, I have a lot of overhead: cycles = 0; while(!End){ // 1k Million iterations aprox #pragma omp parallel for for(i=0;i<N;i++) // the parallel for with 256 iteration aprox if(time[i] == cycles){ if (wbusy[i]){ wbusy[i] = 0; wfinished[i] = 1; } } // serial code ++cycles; } Each

unbalanced nested for loops in openmp

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问题 I've been trying to parallelize an algorithm with unbalanced nested for loops using OpenMP. I can't post the original code as it's a secret project of an unheard government but here's a toy example: for (i = 0; i < 100; i++) { #pragma omp parallel for private(j, k) for (j = 0; j < 1000000; j++) { for (k = 0; k < 2; k++) { temp = i * j * k; /* dummy operation (don't mind the race) */ } if (i % 2 == 0) temp = 0; /* so I can't use openmp collapse */ } } Currently this example is working slower

Morphing multiple executables into a single application

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问题 I have three legacy applications that share a lot of source code and data. Multiple instances of each of these applications be executed by a user at any time, e.g. a dozen mixed application executions can be active at a time. These applications currently communicate through shared memory and messaging techniques so that they can maintain common cursor positioning, etc. The applications are written primarily in C++, use Qt and run in total to about 5 million lines of code. Only some of the

openmp not linking correctly when compiling with clang

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问题 I have built clang 4.0 from source on Ubuntu 16.04 and am trying to compile a simple OpenMP program but receive the following errors. /tmp/test-7f2c7c.o: In function `main': /home/me/sf_shared/test.c:(.text+0x52): undefined reference to `__kmpc_fork_call' /tmp/test-7f2c7c.o: In function `.omp_outlined.': /home/me/sf_shared/test.c:(.text+0xd9): undefined reference to `__kmpc_for_static_init_4' /home/me/sf_shared/test.c:(.text+0x16d): undefined reference to `__kmpc_for_static_fini' clang-4.0:

Sorting an array in openmp

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问题 I have an array of 100 elements that needs to be sorted with insertion sort using OpenMP. When I parallelize my sort it does not give correct values. Can some one help me void insertionSort(int a[]) { int i, j, k; #pragma omp parallel for private(i) for(i = 0; i < 100; i++) { k = a[i]; for (j = i; j > 0 && a[j-1] > k; j--) #pragma omp critical a[j] = a[j-1]; a[j] = k; } } 回答1: Variables "j" and "k" need to be private on the parallel region. Otherwise you have a data race condition. 回答2:

Using openmp on windows with mingw. Cannot find -lpthread

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问题 I have a CMake project which is using OpenMP and works on linux. When I went to compile it on my windows machine it looked like CMake was having trouble finding the openmp flags for mingw's gcc. I decided to try a smaller test case and just compile main_openmp.c #include <omp.h> #include <stdio.h> int main(int argc, char* argv[]) { int id; #pragma omp parallel private(id) { id = omp_get_thread_num(); printf("%d: Hello World!\n", id); } return 0; } Then when I try to compile gcc -o OpenMPTest2