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.

Definitive Book Guide

Using OpenMP: Portable Shared Memory Parallel Programming - Barbara Chapman et al.
Using OpenMP - The Next Step: Affinity, Accelerators, Tasking, and SIMD - Ruud van der Pas et al.
Parallel Programming in OpenMP - Rohit Chandra.
An Introduction to Parallel Programming - Peter Pacheco.
Parallel Programming in C with MPI and OpenMP - Michael J. Quinn.

Helpful links

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omp parallel vs. omp parallel for

What is the difference between these two? [A] #pragma omp parallel { #pragma omp for for(int i = 1; i < 100; ++i) { ... } } [B] #pragma omp parallel for for(int i = 1; i < 100; ++i) { ... }

multithreading parallel-processing openmp

asked Sep 19 '09 at 10:52

Hyunjik Bae

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What is the difference between atomic and critical in OpenMP?

What is the difference between atomic and critical in OpenMP? I can do this #pragma omp atomic g_qCount++; but isn't this same as #pragma omp critical g_qCount++; ?

openmp atomic critical-section

asked Oct 17 '11 at 18:35

codereviewanskquestions

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How are firstprivate and lastprivate different than private clauses in OpenMP?

I've looked at the official definitions, but I'm still quite confused. firstprivate: Specifies that each thread should have its own instance of a variable, and that the variable should be initialized with the value of the variable, because it…

openmp

asked Mar 08 '13 at 22:59

SaiyanGirl

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Using OpenMP with C++11 range-based for loops?

Is there any counter-indication to doing this ? Or is the behavior well specified? #pragma omp parallel for for(auto x : stl_container) { ... } Because it seems that OpenMP specification is only valid for c++98 but I guess there might be more…

c++11 openmp

asked Jul 25 '13 at 03:29

Jean-Michaël Celerier

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memory bandwidth for many channels x86 systems

I'm testing the memory bandwidth on a desktop and a server. Sklyake desktop 4 cores/8 hardware threads Skylake server Xeon 8168 dual socket 48 cores (24 per socket) / 96 hardware threads The peak bandwidth of the system is Peak bandwidth desktop =…

c x86 openmp avx512 memory-bandwidth

asked Jun 28 '19 at 09:05

Z boson

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Is armadillo solve() thread safe?

In my code I have loop in which I construct and over determined linear system and try to solve it: #pragma omp parallel for for (int i = 0; i < n[0]+1; i++) { for (int j = 0; j < n[1]+1; j++) { for (int k = 0; k < n[2]+1; k++) { …

c++ multithreading openmp blas armadillo

asked May 05 '15 at 21:05

maxdebayser

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7 answers

OpenMP and Python

I have experience in coding OpenMP for Shared Memory machines (in both C and FORTRAN) to carry out simple tasks like matrix addition, multiplication etc. (Just to see how it competes with LAPACK). I know OpenMP enough to carry out simple tasks…

python parallel-processing openmp

asked Jul 06 '12 at 19:24

user1132648

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5 answers

Pthreads vs. OpenMP

I'm creating a multi-threaded application in C using Linux. I'm unsure whether I should use the POSIX thread API or the OpenMP API. What are the pros & cons of using either? Edit: Could someone clarify whether both APIs create kernel-level or…

c pthreads openmp

asked Oct 16 '10 at 17:16

user191776

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5 answers

Why is the != operator not allowed with OpenMP?

I was trying to compile the following code: #pragma omp parallel shared (j) { #pragma omp for schedule(dynamic) for(i = 0; i != j; i++) { // do something } } but I got the following error: error: invalid controlling predicate. The…

c++ c multithreading parallel-processing openmp

asked Nov 09 '12 at 17:08

dreamcrash

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What's the difference between "static" and "dynamic" schedule in OpenMP?

I started working with OpenMP using C++. I have two questions: What is #pragma omp for schedule? What is the difference between dynamic and static? Please, explain with examples.

c++ multithreading openmp

asked Jun 01 '12 at 12:21

Lücks

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What are the differences between MPI and OpenMP?

I would like to know (in a few words) what are the main differences between OpenMP and MPI.

mpi openmp difference

asked Sep 08 '15 at 17:42

elli

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Difference between section and task openmp

What is the difference in OpenMP between : #pragma omp parallel sections { #pragma omp section { fct1(); } #pragma omp section { fct2(); } } and : #pragma omp parallel { #pragma omp single { …

c parallel-processing openmp

asked Dec 09 '12 at 15:09

Arkerone

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Parallel for vs omp simd: when to use each?

OpenMP 4.0 introduces a new construct called "omp simd". What is the benefit of using this construct over the old "parallel for"? When would each be a better choice over the other? EDIT: Here is an interesting paper related to the SIMD directive.

c++ c performance openmp simd

asked Feb 03 '13 at 15:29

zr.

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How does OpenMP handle nested loops?

Does the following code just parallelize the first (outer) loops, or it parallelize the entire nested loops? #pragma omp parallel for for (int i=0;i

c++ loops parallel-processing openmp

asked Nov 13 '12 at 07:42

user0002128

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4 answers

How to set linker flags for OpenMP in CMake's try_compile function

I would like to verify that the current compiler can build with openmp support. The application has do deploy across a wide variety of unix systems, some of which might have old versions of OpenMP, and I would like to test for important OpenMP…

cmake openmp

asked Sep 13 '12 at 04:28

dusktreader

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