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: poor performance of heap arrays (stack arrays work fine)

I am a fairly experienced OpenMP user, but I have just run into a puzzling problem, and I am hopeful that someone here could help. The problem is that a simple hashing algorithm performs well for stack-allocated arrays, but poorly for arrays on the…
drlemon
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Measuring memory bandwidth from the dot product of two arrays

The dot product of two arrays for(int i=0; i
Z boson
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Iteration through std containers in openmp

I'm trying to use openmp to multithread a loop through std::set. When I write the following code - #pragma omp parallel for for (std::set::const_iterator i = s.begin(); i != s.end(); ++i) { const A a = *i; …
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OpenMP: What is the benefit of nesting parallelizations?

From what I understand, #pragma omp parallel and its variations basically execute the following block in a number of concurrent threads, which corresponds to the number of CPUs. When having nested parallelizations - parallel for within parallel for,…
Eran
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What does gcc without multilib mean?

I was trying to use the omh.h header file and I realized it was missing. I tried reinstalling gcc on my mac using brew. This is the message I got at the end of the installation. .. GCC has been built with multilib support. Notably, OpenMP may not…
Pranjal Mittal
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Difference between num_threads vs. omp_set_num_threads vs OMP_NUM_THREADS

I am quite confused about the ways to specify the number of threads in parallel part of a code. I know I can use: the enviromental variable OMP_NUM_THREADS function omp_set_num_threads(int) num_threads(int) in #pragma omp parallel for…
atapaka
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OpenMP C++ Matrix Multiplication run slower in parallel

I'm learning the basics of paralel execution of for loop using OpenMP. Sadly, my paralel program runs 10x slower than serial version. What am I doing wrong? Am I missing some barriers? double **basicMultiply(double **A, double **B, int size) { …
Hynek Blaha
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What is the benefit of '#pragma omp master' as opposed to '#pragma omp single'?

In OpenMP any code inside a #pragma omp master directive is executed by a single thread (the master), without an implied barrier at end of the region. (See section on MASTER directive in the LLNL OpenMP tutorial). This seems equivalent to #pragma…
Josh Milthorpe
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multiple threads writing to std::cout or std::cerr

I have OpenMP threads that write to the console via cout and cerr. This of course is not safe, since output can be interleaved. I could do something like #pragma omp critical(cerr) { cerr << "my variable: " << variable << endl; } It would be…
Wolfgang
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OpenMP performance

Firstly, I know this [type of] question is frequently asked, so let me preface this by saying I've read as much as I can, and I still don't know what the deal is. I've parallelized a massive outer for loop. Number of loop iterations varies,…
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C++17 parallel algorithm vs tbb parallel vs openmp performance

Since c++17 std library support parallel algorithm, I thought it would be the go-to option for us, but after comparing with tbb and openmp, I changed my mind, I found the std library is much slower. By this post, I want to ask for professional…
avocado
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MacOS, CMake and OpenMP

I am using the newest CMake (3.9.3) from Homebrew along with LLVM 5.0.0 also from Brew, because Clang here has OpenMP support. This worked in CMake 3.8.2 with LLVM 5. In my CMakeLists.txt I have find_package( OpenMP ) and later I want to do if(…
Mads Ohm Larsen
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"'omp.h' file not found" when compiling using Clang

I'm trying to set up an OpenMP project using Clang (3.7.0) on my laptop running Linux Mint. Now I've read that OpenMP is not supported right away so I followed the tutorial https://clang-omp.github.io/ to integrate OpenMP into Clang. I've cloned the…
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Using openMP in the cuda host code?

It it possible to use openMP pragmas in the CUDA-Files (not in the kernel code)? I will combine gpu and cpu computation. But nvvc compiler fails with "cannot find Unknown option 'openmp' ", if i am linking the porgram with a openmp option (under…
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loop tiling/blocking for large dense matrix multiplication

I was wondering if someone could show me how to use loop tiling/loop blocking for large dense matrix multiplication effectively. I am doing C = AB with 1000x1000 matrices. I have followed the example on Wikipedia for loop tiling but I get worse…
user2088790