Questions tagged [openmp]

OpenMP is a cross-platform multi-threading API which allows fine-grained task parallelization and synchronization using special compiler directives.

OpenMP is a cross-platform multi-threading API which allows fine-grained task parallelization and synchronization using special compiler directives. OpenMP offers easy access to multi-threading without requiring knowledge of system-dependent details. At the same time, it is reasonably efficient compared to fine-tuned implementations with the bonus of being easiest to write multi-threads code. Forums and complete information on OpenMP is at https://openmp.org/.

OpenMP is based on multi-thread model, and offers Shared Memory parallelism and heterogeneous programming for coprocessors through compiler directives, library routines and environment variables. It is restricted to C/C++ and Fortran applications, however provides portability across different Shared Memory architectures.

It is through directives, added by the programmer to the code, that the compiler adds parallelism in the application. OpenMP can be used in single or multi-cores machines, in the first architecture the compiler directives are ignored, thus the application is executed in a sequential manner, promoting portability between the two architectures.

Latest version is 5.2 (November 2021): Official OpenMP specifications.

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OpenMP and STL vector

I've got some code for which I'd like to use OpenMP in the following way: std::vector v(1000); # pragma omp parallel for for (int i = 0; i < 1000; ++i) { v[i] = i; } I have read that STL vector container is not thread-safe in the situation…
cbd
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Set number of threads using omp_set_num_threads() to 2, but omp_get_num_threads() returns 1

I have the following C/C++ code using OpenMP: int nProcessors=omp_get_max_threads(); if(argv[4]!=NULL){ printf("argv[4]: %s\n",argv[4]); nProcessors=atoi(argv[4]); printf("nProcessors: %d\n",nProcessors); } …
Eamorr
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OpenMp Coding: warning: ignoring #pragma omp parallel

I am getting this warning during compilation of a C code with OpenMP directives on Linux: warning: ignoring #pragma omp parallel Gcc version is 4.4. Is it only a warning I should not care about? Will the execution be in parallel?. I would like a…
anum
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Why doesn't this code scale linearly?

I wrote this SOR solver code. Don't bother too much what this algorithm does, it is not the concern here. But just for the sake of completeness: it may solve a linear system of equations, depending on how well conditioned the system is. I run it…
lvella
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Difference between linking OpenMP with -fopenmp and -lgomp

I've been struggling a weird problem the last few days. We create some libraries using GCC 4.8 which link some of their dependencies statically - eg. log4cplus or boost. For these libraries we have created Python bindings using boost-python. Every…
duselbaer
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Difference between OpenMP threadprivate and private

I am trying to parallelize a C program using OpenMP. I would like to know more about: The differences between the threadprivate directive and the private clause and In which cases we must use any of them. As far as I know, the difference is the…
L30nardo SV.
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Thread-safety of writing a std::vector vs plain array

I've read on Stackoverflow that none of the STL containers are thread-safe for writing. But what does that mean in practice? Does it mean I should store writable data in plain arrays? I expect concurrent calls to std::vector::push_back(element)…
clstaudt
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Is it possible to do a reduction on an array with openmp?

Does OpenMP natively support reduction of a variable that represents an array? This would work something like the following... float* a = (float*) calloc(4*sizeof(float)); omp_set_num_threads(13); #pragma omp parallel…
Andrew Wagner
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library isn't found in the GCC version (4.2.1) in Mavericks

I have a problem with GCC. I want to update it to a new version, from the 4.2.1, to program with parallel programming. However, in this version there is no library . How can I download an updated version? The error that the terminal give me…
Mattia Baldari
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C++ OpenMP Parallel For Loop - Alternatives to std::vector

Based on this thread, OpenMP and STL vector, which data structures are good alternatives for a shared std::vector in a parallel for loop? The main aspect is speed, and the vector might require resizing during the loop.
Armin Meisterhirn
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May compiler optimizations be inhibited by multi-threading?

It happened to me a few times to parallelize portion of programs with OpenMP just to notice that in the end, despite the good scalability, most of the foreseen speed-up was lost due to the poor performance of the single threaded case (if compared to…
Massimiliano
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Reduction with OpenMP

I am trying to compute mean of a 2d matrix using openmp. This 2d matrix is actually an image. I am doing the thread-wise division of data. For example, if I have N threads than I process Rows/N number of rows with thread0, and so on. My question…
mkuse
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openmp parallel for loop with two or more reductions

Hi just wondering if this is the right way to go going about having a regular for loop but with two reductions , is this the right approach below? Would this work with more then two reductions as well. Is there a better way to do this? also is there…
pyCthon
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How do I parallelize a for loop through a C++ std::list using OpenMP?

I would like to iterate through all elements in an std::list in parallel fashion using OpenMP. The loop should be able to alter the elements of the list. Is there a simple solution for this? It seems that OpenMP 3.0 supports parallel for loops when…
mshang
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How to disable OpenMP directives in a nice way?

I have C++ code with OpenMP pragmas inside. I want to test this code both for multithread mode (with OpenMP) and in single thread mode (no OpenMP). For now, to switch between modes I need to comment #pragma omp (or at least parallel). What is the…
Jakub M.
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