OpenMP 4.5 won't offload to GPU with target directive

问题

I am trying to make a simple GPU offloading program using openMP. However, when I try to offload it still runs on the default device, i.e. my CPU.

I have installed a compiler, g++ 7.2.0 that has CUDA support (is in on a cluster that I use). When I run the below code it shows me that it can see the 8 GPUs but when I try to offload it says that it is still on the CPU.

#include <omp.h>
#include <iostream>
#include <stdio.h>
#include <math.h>
#include <algorithm> 
#define n 10000
#define m 10000

using namespace std;

int main()
{

    double tol = 1E-10;
    double err = 1;
    size_t iter_max = 10;
    size_t iter = 0;
    bool notGPU[1] = {true};

    double Anew[n][m];
    double A[n][m];

   int target[1];
   target[0] = omp_get_initial_device();
   cout << "Total Devices: " << omp_get_num_devices() << endl;
   cout << "Target: " << target[0] << endl;


    for (int iter = 0; iter < iter_max; iter++){
        #pragma omp target 
        {
        err = 0.0;
        #pragma omp parallel for reduction(max:err)
        for (int j = 1; j < n-1; ++j){
            target[0] = omp_is_initial_device();
            for (int i = 1; i < m-1; i++){
                Anew[j][i] = 0.25 * (A[j][i+1] + A[j][i-1] + A[j-1][i] + A[j+1][i]);
                err = fmax(err, fabs(Anew[j][i] - A[j][i]));
            }

        }
        }

    }
    if (target[0]){
       cout << "not on GPU" << endl;
} else{
    cout << "On GPU" << endl;}



    return 0;
}

When I run this I always get that it is not on the GPU, but that there are 8 devices available.

回答1:

This is not a well documented process!

You have to install some packages which look a little like:

sudo apt install gcc-offload-nvptx

You also need to add additional flags to your compilation string. I've globbed together a number of them below. Mix and match until something works, or use them as the basis for further Googling.

gcc -fopenmp -foffload=x86_64-intelmicemul-linux-gnu="-mavx2" -foffload=nvptx-none -foffload="-O3" -O2 test.c -fopenmp-targets=nvptx64-nvidia-cuda

When I last tried this with GCC in 2018 it just didn't work. At that time target offloading for OpenMP only worked with the IBM XL compiler and OpenACC (a similar set of directives to OpenMP) only worked on the Nvidia's PGI compiler. I find PGI to do a worse job of compiling C/C++ than the others (seems inefficient, non-standard flags), but a Community Edition is available for free and a little translating will get you running in OpenACC quickly.

IBM XL seems to do a fine job compiling, but I don't know if it's available for free.

The situation may have changed with GCC. If you find a way to get it working, I'd appreciate you leaving a comment here. My strong recommendation is that you stop trying with GCC7 and get ahold of GCC8 or GCC9. GPU offloading is a fast-moving area and you'll want the latest compilers to take best advantage of it.

回答2:

Looks like you're missing a device(id) in your #pragma omp target line:

#pragma omp target device(/*your device id here*/)

Without that, you haven't explicitly asked OpenMP to run anywhere but your CPU.

来源：https://stackoverflow.com/questions/57206552/openmp-4-5-wont-offload-to-gpu-with-target-directive

标签

c++

gpu

openmp