C OpenMP并行for循环未在多个线程上运行

Question

I am trying a simple example of Open MP, to parallelize a for loop, but I cannot see that the for loop is being executed on more than one core.

Here is the C program:

#include </usr/local/Cellar/gcc/5.1.0/lib/gcc/5/gcc/x86_64-apple-darwin14.5.0/5.1.0/include/omp.h>
#include <stdio.h>
#include <unistd.h>
int main() {
    int n;
    #pragma omp parallel
    {
        #pragma omp for
        for(n = 0; n < 10; n++)
            printf(" Thread %d: %d\n", omp_get_thread_num(), n);

        printf("Number of threads: %d\n", omp_get_num_threads());
    }
    printf("Total number of cores in the CPU: %ld\n", sysconf(_SC_NPROCESSORS_ONLN));
}

The above code is very similar to this example .

When I execute this program and print out the total number of threads being used by it (by default it should be the total number of CPU cores), I find the result 1 . Here is the output:

10:pavithran$ gcc -fopenmp openMPExample.c -o openMPExample.o
10:pavithran$ ./openMPExample.o 
 Thread 0: 0
 Thread 0: 1
 Thread 0: 2
 Thread 0: 3
 Thread 0: 4
 Thread 0: 5
 Thread 0: 6
 Thread 0: 7
 Thread 0: 8
 Thread 0: 9
Number of threads: 1
Total number of cores in the CPU: 8
10:pavithran$ 

Why is it that I do not find the numbers being printed by different threads? Why is it that the total number of threads is always 1? Someone please help me in getting the parallel for loop to work.

PS: The first "#include ..." statement contains the explicit path because the simple form: #include <omp.h> does not seem to work .

Answer 1

You are compiling your program with Apple's clang version of gcc that does not support OpenMP. This is why you get the output

$ gcc main.c -fopenmp
main.c:1:10: fatal error: 'omp.h' file not found
#include <omp.h>
         ^
1 error generated.

when using

#include <omp.h>

as opposed to (the montrosity)

#include </usr/local/Cellar/gcc/5.1.0/lib/gcc/5/gcc/x86_64-apple-darwin14.5.0/5.1.0/include/omp.h>

Using the Correct Version of gcc

Note: Based on the include path I'm assuming use of Homebrew to install gcc . If that is not the case there is a section at the end on installing gcc via Homebrew with OpenMP support.

Homebrew installs gcc as gcc-4.9 or gcc-5 (for both GCC 5.1 and GCC 5.2) depending on the version you have installed. It does not create a symlink for gcc in /usr/local/bin and so when you execute gcc from the command line you will be calling Apple's clang version. This is what we must correct for and there are a multitude of ways in which we can use the Homebrew version of gcc over Apple's clang version

1. Explicitly execute gcc via

    $ gcc-5 main.c -fopenmp 

2. Create a symlink for gcc in /usr/local/bin with the following commands

    $ cd /usr/local/bin $ ln -s gcc-5 gcc 

   Now when you execute gcc -v you should see something similar to the below (providing your $PATH variable is set up correctly, see below, and you've installed gcc with OpenMP support via Homebrew, see below)

    $ gcc -v Using built-in specs. COLLECT_GCC=gcc-5 COLLECT_LTO_WRAPPER=/usr/local/Cellar/gcc/5.2.0/libexec/gcc/x86_64-apple-darwin14.4.0/5.2.0/lto-wrapper Target: x86_64-apple-darwin14.4.0 Configured with: ../configure --build=x86_64-apple-darwin14.4.0 --prefix=/usr/local/Cellar/gcc/5.2.0 --libdir=/usr/local/Cellar/gcc/5.2.0/lib/gcc/5 --enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-5 --with-gmp=/usr/local/opt/gmp --with-mpfr=/usr/local/opt/mpfr --with-mpc=/usr/local/opt/libmpc --with-isl=/usr/local/opt/isl --with-system-zlib --enable-libstdcxx-time=yes --enable-stage1-checking --enable-checking=release --enable-lto --with-build-config=bootstrap-debug --disable-werror --with-pkgversion='Homebrew gcc 5.2.0 --without-multilib' --with-bugurl=https://github.com/Homebrew/homebrew/issues --enable-plugin --disable-nls --disable-multilib Thread model: posix gcc version 5.2.0 (Homebrew gcc 5.2.0 --without-multilib) 

3. Create an alias for the Homebrew version of gcc . You can do this by adding a line similar to

    alias hgcc='gcc-5' 

   to your .bashrc or .bash_profile . What is even better is to add it to a file called .bash_aliases and then include it in your .bashrc or .bash_profile with

    [[ -f ~/.bash_aliases ]] && . ~/.bash_aliases 

   We would then execute it via hgcc as in

    $ hgcc main.c -fopenmp 

Aside: Installing gcc with OpenMP support on OS X

To install gcc with OpenMP support the easiest route to take is via Homebrew . To do this you need to use the following command as per one of my previous answers

brew install gcc --without-multilib

or if you have already installed gcc via Homebrew use

brew reinstall gcc --without-multilib

This installs gcc as gcc-4.9 or gcc-5 (for both GCC 5.1 and GCC 5.2) depending on the version you have installed. It does not create a symlink for gcc in /usr/local/bin and so when you execute gcc from the command line you will be calling Apple's clang version. You can verify this by running gcc -v which should produce a similar output to the below

$ gcc -v
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.4.0
Thread model: posix

Checking Path is Set Correctly

On (my Mac system at least) the default $PATH includes /usr/local/bin before /usr/bin as in

$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

If this is not the case for you you will need to edit your .bashrc or .bash_profile and add

export PATH=/usr/local/bin:$PATH

This ensures that the Homebrew version of gcc will be found before any of the Apple versions, when we correct for Homebrew installing it as gcc-4.9 or gcc-5 .

Answer 2

What about setting OMP_NUM_THREADS environment variable? What would give you something like this?

 > OMP_NUM_THREADS=8 ./openMPExample.o

(BTW, having a binary which name ends with '.o' might not be such a good idea)