Besides calling omp_get_num_threads() outside of the parallel region in your case, calling omp_set_num_threads() still doesn’t guarantee that the OpenMP runtime will use exactly the specified number of threads. omp_set_num_threads() is used to override the value of the environment variable OMP_NUM_THREADS and they both control the upper limit of the size of the thread team that … Read more
The code posted by the OP will never execute in parallel, because the parallel keyword does not appear. The fact that the OP got ids different from 0 shows that probably his code was embedded in a parallel directive. However, this is not clear from his post, and might confuse beginners. The minimum sensible example … Read more
Try using Homebrew’s llvm: brew install llvm You then have all the llvm binaries in /usr/local/opt/llvm/bin. Compile the OpenMP Hello World program. Put omp_hello.c /****************************************************************************** * FILE: omp_hello.c * DESCRIPTION: * OpenMP Example – Hello World – C/C++ Version * In this simple example, the master thread forks a parallel region. * All threads in … Read more
I haven’t used TBB extensively, but my impression is that they complement each other more than competing. TBB provides threadsafe containers and some parallel algorithms, whereas OpenMP is more of a way to parallelise existing code. Personally I’ve found OpenMP very easy to drop into existing code where you have a parallelisable loop or bunch … Read more
single and critical are two very different things. As you mentioned: single specifies that a section of code should be executed by single thread (not necessarily the master thread) critical specifies that code is executed by one thread at a time So the former will be executed only once while the later will be executed … Read more
It appears that the C/C++ specification for OpenMP provides no direct way of doing this programmatically. So you have to check the docs for your compiler version. gcc –version ## get compiler version For GCC, this is a good resource (does not mention newest versions of GCC): http://gcc.gnu.org/wiki/openmp: As of GCC 4.2, the compiler implements … Read more
Yes, local variables are automatically private. The reason for the existence of the private clause is so that you don’t have to change your code. The only way to parallelize the following code without the private clause int i,j; #pragma omp parallel for private(j) for(i = 0; i < n; i++) { for(j = 0; … Read more
The lines you have written will parallelize only the outer loop. To parallelize both you need to add a collapse clause: #pragma omp parallel for collapse(2) for (int i=0;i<N;i++) { for (int j=0;j<M;j++) { //do task(i,j)// } } You may want to check OpenMP 3.1 specifications (sec 2.5.1) for more details.
CMake has a standard module for testing if the compiler supports OpenMP: find_package(OpenMP) if (OPENMP_FOUND) set (CMAKE_C_FLAGS “${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}”) set (CMAKE_CXX_FLAGS “${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}”) set (CMAKE_EXE_LINKER_FLAGS “${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}”) endif() Note: This answer is not recommended to be used anymore for including OpenMP in the project for current CMake versions. Refer to the other answers.
A simple answer: OpenMP only used to exploit multiple threads for multiple cores. This new simd extention allows you to explicitly use SIMD instructions on modern CPUs, such as Intel’s AVX/SSE and ARM’s NEON. (Note that a SIMD instruction is executed in a single thread and a single core, by design. However, the meaning of … Read more