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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6462 questions
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Is collapse clause with non-rectangular loops allowed by the OpenMP 5.1 Spec?

Consider the following OpenMP code: #pragma omp target teams distribute parallel for collapse(4) map(tofrom: a) private(i,j,k,l) for (i = 0; i < SIZE_N; i++) { for (j = 0; j < SIZE_M; j++) { for (k = i; k < SIZE_N; k++) { for (l = 0; l…
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What loop size to multithread?

Imagine a simple loop: constexpr int N; // some big number #pragma omp parallel for for(int i=0; i
Bbllaaddee
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Running Fortran OpenMP codes with OpenMP Tools interface

I'm trying to run a simple Fortran OpenMP code with a library using the OpenMP Tools Interface (OMPT). I have this working with a C++ code using clang + llvm openmp runtime, just by doing OMP_TOOL_LIBRARIES=/home/path/to/libotter.so…
LonelyCat
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Where to see what OMP schedule(auto) picks?

Is there a way to find out what scheduling scheme the OMP runtime chooses for schedule(auto)? I found that (and intuitvely it makes sense) for my problemschedule(static) is the fastest, so I am wondering if that's what the runtime chooses when is…
Marcel Braasch
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Can I deallocate a shared variable by a single thread using OpenMP?

I am using OpenMP in order to parallelize a code. Here is the most important part of the code according to the question that I will ask: !$OMP PARALLEL PRIVATE(num_thread) & !$OMP…
hakim
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How to use two nodes for one OpenMp Fortran90 code in SLURM Cluster?

I am freshly new to using SLURM in CLUSTER. I am now struggling with OpenMP fortran 90. I try to calculate integrals using two nodes (node1 and node2) through SLURM. What I want is to return one value by combining the calculations of node 1 and node…
Goring
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Speed up and scheduling with OpenMP

i'm using OpenMP for a kNN project. The two parallelized for loops are: #pragma omp parallel for for(int ii=0;ii
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error: reduction variable is private in outer context (omp reduction)

I am confused about the data sharing scope of the variable acc in the flowing two cases. In the case 1 I get following compilation error: error: reduction variable ‘acc’ is private in outer context, whereas the case 2 compiles without any…
Misslinska
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Confused about OMP_NUM_THREADS and numactl NUMA-cores bindings

I'm confused about how multiple launches of same python command bind to cores on a NUMA Xeon machine. I read that OMP_NUM_THREADS env var sets the number of threads launched for a numactl process. So if I ran numactl --physcpubind=4-7 --membind=0…
Joe Black
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Parallel code with OpenMP takes more time to execute than serial code

I'm trying to make this code to run in parallel. It's a chunk of code from a big project. I thought I started parallelizing slowly to see if there is a problem step by step (I don't know if that's a good tactic so please let me know). double…
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How to parallelise a code inside a while using OpenMP

I am trying to parallelise the heat_plate algorithm but I am stuck at this bit of code inside my while: while(1) { ..... ..... #pragma omp parallel shared(diff, u, w) private(i, j, my_diff) { my_diff = 0.0; #pragma omp for for (i = 1; i <…
D K
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openmp Linker flags in MSVC

when I try to compile my project in MSVC2008 with the linker flag (Configuration properties>>Linker>>Command line>> Additional options) set to : "/STACK:10000000 /machine:x64 /openmp" it warns me that the /openmp flag is unknown. "LINK : warning…
Nima Nouri
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When should I overlook critical sections and when nowait is needed ? OpenMp

I am studying OpenMP and I have some questions that I believe will clear up my thoughts. I have a small example of a matrix multiplication A*B where A,B,C are global variables. I know how we can parallelize the for loops one at a time or both…
gregni
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Why would executing a function in parallel significantly slowdown the program?

I am trying to parallelize a code using OpenMP, the serial time for my current input size is around 9 seconds, I have a code of the following form: int main() { /* do some stuff*/ myfunction(); } void myfunction() { for (int i=0; i
Sergio
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C++ call to LAPACKE run on a single thread while NumPy uses all threads

I wrote a C++ code whose bottleneck is the diagonalization of a possibly large symmetric matrix. The code uses OpenMP, CBLAS and LAPACKE C-interfaces. However, the call on dsyev runs on a single thread both on my local machine and on a HPC cluster…
Toool
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