Some additional comments: 1) You need to use -fopenmp=libomp to enable OpenMP in clang. -fopenmp just links libgomp but ignores all the pragmas. Weird, I know — and will be changed in the trunk soon. 2) 3.7 is the first version that supports OpenMP. 3.6 doesn’t. 3) clang is only able to work with libomp. … Read more
. I sent an email to OpenMP developers about this subject, the answer I got: For signed int, the wrap around behavior is undefined. If we allow !=, programmers may get unexpected tripcount. The problem is whether the compiler can generate code to compute a trip count for the loop. For a simple loop, like: … Read more
You can extend your strategy to an arbitrary number of sub-regions, and reduce them recursively, using tasks: #include<vector> #include<iostream> using namespace std; const int n = 10000; const int baseLength = 100; int f(int ii) { return ii; } int recursiveSumBody(int * begin, int * end){ size_t length = end – begin; size_t mid = … Read more
You should use reduction like this: #pragma omp parallel for reduction (+:sum) for (int i=0;i<n;i++) sum=sum+a[i];
Yes, named calls will work to set thread affinity. The only problem is to fix thread number and to set right affinity in right thread (you can try using static scheduling of for loop for known number of threads). As I know, almost every openmp allows to set affinity via environment. The name of environment … Read more
If your compiler supports OpenMP 3.0, you can use the collapse clause: #pragma omp parallel for schedule(dynamic,1) collapse(2) for (int x = 0; x < x_max; ++x) { for (int y = 0; y < y_max; ++y) { //parallelize this code here } //IMPORTANT: no code in here } If it doesn’t (e.g. only OpenMP … Read more
Pthreads and OpenMP represent two totally different multiprocessing paradigms. Pthreads is a very low-level API for working with threads. Thus, you have extremely fine-grained control over thread management (create/join/etc), mutexes, and so on. It’s fairly bare-bones. On the other hand, OpenMP is much higher level, is more portable and doesn’t limit you to using C. … Read more
These are equivalent. #pragma omp parallel spawns a group of threads, while #pragma omp for divides loop iterations between the spawned threads. You can do both things at once with the fused #pragma omp parallel for directive.
Standard Apple’s clang supports OpenMP. They just disabled the driver option. But you can use the frontend option instead this way: clang -Xclang -fopenmp <you_program> -I <path to omp.h> -L <path to libomp.dylib> -lomp Also, you need to set DYLD_LIBRARY_PATH environmental variable: export DYLD_LIBRARY_PATH=<path to libomp.dylib> How to get/build libomp. $ cd $ svn co … Read more
EDIT: As of 13 Aug 2017 the –without-multilib option is no longer present in Homebrew and should not be used. The standard installation brew install gcc will provide a gcc installation that can be used to compile OpenMP programs. As below it will be installed into /usr/local/bin as gcc-<version>. The current gcc version available from … Read more