openmp | 易学教程

openMp optimisation of dynamic array access

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问题 I am trying to measure the speedup in parallel section using one or four threads. As my parallel section is relatively simple, I expect a near-to-fourfold speedup. ( This is following my question: openMp: severe perfomance loss when calling shared references of dynamic arrays ) As my parallel sections runs twice as fast on four cores compared to only one, I believe I have still not found the reason for the performance loss. I want to parallelise my function iter as well as possible. The

Qimage setPixel with openmp parallel for doesn't work

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问题 The code works without parallelism, but when I add pragma omp parallel, it doesn't work. Furthermore, the code works perfectly with pragma omp parallel if I don't add setPixel. So, I would like to know why the parallelism doesn't work properly and exits the program with code 255 when I try to set pixel in the new image. This code wants to change an image doing two loops to change every pixel using a Gauss vector. If something can't be understood I'll solve it inmediately. for (h = 0; h <

Set custom compiler in eclipse (omp4j)

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问题 So I am trying to use omp4j with the eclipse IDE. The problem is, that omp4j needs to replace the javac command to work (see http://www.omp4j.org/download). And I don't know how I can accomplish that in eclipse other than renaming the omp4j.jar to javac.jar and replacing my JDKs javac.jar and that seems like a wrong solution. 回答1: omp4j is a preprocessor . If omp4j is called without --no-compile , the preprocessed Java source code will be automatically compiled via javac , so omp4j can be

-fopenmp does not include omp.h on amazon linux?

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问题 I'm trying to compile a test openmp program on an Amazon AWS t2.micro instance. It seems to have trouble. Upon trying to compile this OpenMP hello world program, the compiler fails to find omp.h despite using gcc hello_world.c -fopenmp . After that, I tried running locate omp.h and found it in /usr/lib/gcc/x86_64-amazon-linux/4.8.5/include . I next attempted to compile by including that directory with gcc -I . Then, the compiler still needed libgomp.spec , which has been encountered and

How to combine boost odeint with OpenMP and boost multiprecision?

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问题 I am asking a question, which is related to the last comments in this post: Using openmp with odeint and adaptative step sizes In the end, the original poster asked whether or not OpenMP is compatible with boost multiprecision. I guess this problem has been solved in the meanwhile but I could not find the answer. Hence, I tried to figure it out on my own and implemented some coupled ODEs. #include <iostream> #include <vector> #include <omp.h> #include <boost/numeric/odeint.hpp> #include

OpenMP creates only one thread

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问题 I use Ubuntu and write several lines of code.But it creates only one thread. When I run on my terminal the nproc command, the output is 2. My code is below int nthreads, tid; #pragma omp parallel private(tid) { tid = omp_get_thread_num(); printf("Thread = %d\n", tid); /* for only main thread */ if (tid == 0) { nthreads = omp_get_num_threads(); printf("Number of threads = %d\n", nthreads); } } The output: Thread = 0 Number of threads = 1 How can I do parallelism? 回答1: If you are using gcc/g++

OpenMP/gcc on macOS : gcc --without-multilib not available

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问题 Last year I had a school project that used the OpenMP API for parallel computing. I installed gcc-6 --without-multilib with the Homebrew (brew) tool. It worked like charm. This year I had to make a new clean install of macOS High Sierra because of a software issue. Now I can't seem to install gcc without multilib with brew. When I type " brew info gcc ", I can see the different install flags, and --without-multilib in not in this list. (I tried gcc@5, @6 and @7). I tried installing gcc with

OpenMP atomic _mm_add_pd

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问题 I'm trying to use OpenMP for parallelization of an already vectorized code with intrinsics, but the problem is that I'm using one XMM register as an outside 'variable' that I increment each loop. For now I'm using the shared clause __m128d xmm0 = _mm_setzero_pd(); __declspec(align(16)) double res[2]; #pragma omp parallel for shared(xmm0) for (int i = 0; i < len; i++) { __m128d xmm7 = ... result of some operations xmm0 = _mm_add_pd(xmm0, xmm7); } _mm_store_pd(res, xmm0); double final_result =

How to parallel nested loop to find the nearest two point in OpenMP? [duplicate]

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问题 This question already has answers here : How does OpenMP handle nested loops? (3 answers) Closed 5 years ago . This question is not a duplicate of fusing nested loops. The OP wants to do a reduction of a maximum value and at the same time store two indices. Fusing the loops will not fix the OPs problem. The OP would still have race conditions on the shared indices and would be accessing the reduced value which does not get merged until the end of the reduction (see my answer for one solution

OpenMp多线程编程计时问题

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在做矩阵乘法并行化测试的时候，在利用<time.h>的clock()计时时出现了一点问题。首先看串行的程序： // matrix_cpu.c #include <stdio.h> #include <stdlib.h> #include <time.h> #define NUM 2048 void matrixMul(float *A, float *B, float *C, int M, int K, int N) { int i, j, k; for(i = 0; i < M; i++) { for(j = 0; j < N; j++) { float sum = 0.0f; for(k = 0; k < K; k++) { sum += A[i*k+k] * B[k*N+j]; } C[i*N+j] = sum; } } } int main(int argc, char* argv[]) { float *A, *B, *C; clock_t start, finish; double duration; A = (float *) malloc (sizeof(float) * NUM * NUM); B = (float *) malloc (sizeof(float) * NUM * NUM); C = (float *) malloc (sizeof(float)