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: are local variables automatically private?

#pragma omp parallel { int x; // private to each thread ? } #pragma omp parallel for for (int i = 0; i < 1000; ++i) { int x; // private to each thread ? } Thank you! P.S. If local variables are automatically private, what is the point of…
pic11
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Undefined reference to `omp_get_max_threads_'

I'm getting the following errors trying to compile a project: (fortran, using gfortran) undefined reference to `omp_get_max_threads_' undefined reference to `omp_get_thread_num_' Problem is, my GCC version is 4.4.3, which was suppose to support…
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How to check the version of OpenMP on Linux

I wonder how to check the version of OpenMP on a Linux remote machine? I don't know where it is installed either.
Tim
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difference between omp critical and omp single

I am trying to understand the exact difference between #pragma omp critical and #pragma omp single in OpenMP: Microsoft definitions for these are: Single: Lets you specify that a section of code should be executed on a single thread, not…
Amir
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C++ Parallelization Libraries: OpenMP vs. Thread Building Blocks

I'm going to retrofit my custom graphics engine so that it takes advantage of multicore CPUs. More exactly, I am looking for a library to parallelize loops. It seems to me that both OpenMP and Intel's Thread Building Blocks are very well suited for…
Adrian Grigore
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Enable OpenMP support in clang in Mac OS X (sierra & Mojave)

I am using Mac OS X Sierra, and I found that clang (LLVM version 8.1.0 (clang-802.0.38)) does not support OpenMP: when I run clang -fopenmp program_name.c, I got the following error: clang: error: unsupported option '-fopenmp' It seems that clang…
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How does the SECTIONS directive in OpenMP distribute work?

In OpenMP when using omp sections, will the threads be distributed to the blocks inside the sections, or will each thread be assigned to each sections? When nthreads == 3: #pragma omp sections { #pragma omp section { printf ("id =…
kar
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OpenMP set_num_threads() is not working

I am writing a parallel program using OpenMP in C++. I want to control the number of threads in the program using omp_set_num_threads(), but it does not work. #include #include #include "mpi.h" using namespace std; int…
Nurlan
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Parallelization: pthreads or OpenMP?

Most people in scientific computing use OpenMP as a quasi-standard when it comes to shared memory parallelization. Is there any reason (other than readability) to use OpenMP over pthreads? The latter seems more basic and I suspect it could be…
hanno
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Can I safely use OpenMP with C++11?

The OpenMP standard only considers C++ 98 (ISO/IEC 14882:1998). This means that there is no standard supporting usage of OpenMP under C++03 or even C++11. Thus, any program that uses C++ >98 and OpenMP operates outside of standards, implying that…
Walter
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Understanding the collapse clause in openmp

I came across an OpenMP code that had the collapse clause, which was new to me. I'm trying to understand what it means, but I don't think I have fully grasped it's implications; One definition that I found is: COLLAPSE: Specifies how many loops in…
iomartin
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Reducing on array in OpenMP

I am trying to parallelize the following program, but don't know how to reduce on an array. I know it is not possible to do so, but is there an alternative? Thanks. (I added reduction on m which is wrong but would like to have an advice on how to do…
user2891902
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Does multithreading emphasize memory fragmentation?

Description When allocating and deallocating randomly sized memory chunks with 4 or more threads using openmp's parallel for construct, the program seems to start leaking considerable amounts of memory in the second half of the test-program's…
Byron
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Helgrind (Valgrind) and OpenMP (C): avoiding false positives?

The documentation for the Valgrind thread error detection tool Helgrind, found here warns that, if you use GCC to compile your OpenMP code, GCC's OpenMP runtime library (libgomp.so) will cause a chaos of false positive reports of data races, because…
Amittai Aviram
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OpenMP Dynamic vs Guided Scheduling

I'm studying OpenMP's scheduling and specifically the different types. I understand the general behavior of each type, but clarification would be helpful regarding when to choose between dynamic and guided scheduling. Intel's docs describe dynamic…
Matt Goodrich
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