(1) Nested parallelism in OpenMP: http://docs.oracle.com/cd/E19205-01/819-5270/aewbc/index.html You need to turn on nested parallelism by setting OMP_NESTED or omp_set_nested because many implementations turn off this feature by default, even some implementations didn’t support nested parallelism fully. If turned on, whenever you meet parallel for, OpenMP will create the number of threads as defined in OMP_NUM_THREADS. So, … Read more
You can install llvm using brew since it now includes openmp. brew install llvm You can make a symlink if you want ln -s /usr/local/opt/llvm/bin/clang /usr/local/bin/clang-omp My makefile looks like this CPP = /usr/local/opt/llvm/bin/clang CPPFLAGS = -I/usr/local/opt/llvm/include -fopenmp LDFLAGS = -L/usr/local/opt/llvm/lib example: example.c $(CPP) $(CPPFLAGS) $^ -o $@ $(LDFLAGS)
user 0m45.735s clock() measures CPU time the process used (as good as it can) per 7.27.2.1 The clock function returns the implementation’s best approximation to the processor time used by the program since the beginning of an implementation-defined era related only to the program invocation. and not wall clock time. Thus clock() reporting a time … Read more
With gcc, you need to compile and link with -fopenmp to enable OpenMP. Other compilers have different options; with intel it’s -openmp, with pgi it’s -mp, etc.
OK, this is about the difference between the lexical and dynamic extent of OpenMP directives and the interaction with variable scoping. The lexical extent of a directive is the text between the beginning and the end of the structured block following a directive. The dynamic extent is the lexical extent plus statements executed as part … 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.
Do not use OpenMP with Rcpp‘s *Vector or *Matrix objects as they mask SEXP functions / memory allocations that are single-threaded. OpenMP is a multi-threaded approach. This is why the code is crashing. One way to get around this limitation is to use a non-R data structure to store the results. One of the following … Read more
Cython Cython has OpenMP support: With Cython, OpenMP can be added by using the prange (parallel range) operator and adding the -fopenmp compiler directive to setup.py. When working in a prange stanza, execution is performed in parallel because we disable the global interpreter lock (GIL) by using the with nogil: to specify the block where … Read more
Actually, it is quite simple to implement that cleanly with tasks using a recursive divide-and-conquer approach. This is almost textbook code. void operation(int* p1, int* p2, size_t bins) { for (int i = 0; i < bins; i++) p1[i] += p2[i]; } void reduce(int** arrs, size_t bins, int begin, int end) { assert(begin < end); … Read more
You want the OMP_SET_LOCK/OMP_UNSET_LOCK functions: https://hpc.llnl.gov/tuts/openMP/#OMP_SET_LOCK Basically: omp_lock_t writelock; omp_init_lock(&writelock); #pragma omp parallel for for ( i = 0; i < x; i++ ) { // some stuff omp_set_lock(&writelock); // one thread at a time stuff omp_unset_lock(&writelock); // some stuff } omp_destroy_lock(&writelock); Most locking routines such as pthreads semaphores and sysv semaphores work on that … Read more