- OpenMP is a portable, parallel programming model
for shared memory multiprocessor architectures,
developed in collaboration with a number of computer vendors.
- OpenMP API are available for C/C++
(and Fortran)
- For more information on the OpenMP developer community, including tutorials and other resources, see their web site at: http://www.openmp.org
- OpenMP programs are ordinary Fortran or C/C++ proograms
that include
compiler INSTRUCTIONS (directives)
to tell the Fortran or C/C++ compiler to generate parallel
execution code (using threads)
-
Syntax of OpenMP compiler directive for C/C++ compilers:
- Program statements between the { and } are executed by multiple threads
#pragma omp parallel [Options...] { ... ... Parallel region ... ... Program statements between the braces ... are executed in parallel by all threads ... } - The number of threads that will be created to execute parallel sections is controlled by the environment variable OMP_NUM_THREADS
- C/C++ on Linux
(Lab machines)
- Compile: g++ -O -c
-fopenmp
Prog.C
- Link: g++ -O -o Executable -fopenmp Prog1.o Prog2.o ....
- Compile: g++ -O -c
-fopenmp
Prog.C
- Before I continue, I like to show you an example of
an Open MP program so you have some idea what Open MP
can do for you....
-
Example Program:
(OpenMP Hello World) ---
click here
- Compile with:
-
g++ -fopenmp openMP01.C
- Run with:
export OMP_NUM_THREADS=8
a.outYou will see "Hello World !!!" printed EIGHT times !!! (Remove the #pragma line and you get ONE line)....
#include <iostream.h> #include <opm.h> int main(int argc, char *argv[]) { #pragma omp parallel { cout << "Hello World !!!" << endl; } }
-
Example Program:
(OpenMP Hello World) ---
click here
- OpenMP on Linux systems:
- The GNU compiler uses different header files:
- The compile option is: -fopenmp
#include <iostream> // NO: .h !!! #include <opm.h> using namespace std; // You need to use this namespace int main(int argc, char *argv[]) { #pragma omp parallel { cout << "Hello World !!!" << endl; } }
- The GNU compiler uses different header files:
- What is happening in an OpenMP program ?
- Before the directive #pragma omp parallel
the program is single-threaded
- this thread is also known as
the
MASTER THREAD
- The directive #pragma omp parallel
tells the compiler to create a thread team.
The number of threads in the team is determined by
the environment variable
OMP_NUM_THREADS
- At the end of the parallel section,
ALL threads
are
implicitly joined
(i.e., the master thread calls "pthread_join" on all worker threads)
Then the master thread T0 continue with the single-threaded execution following the PARALLEL section.
- Before the directive #pragma omp parallel
the program is single-threaded
- this thread is also known as
the
MASTER THREAD