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OpenMP is
• An application programming interface (API) that supports shared-memory programming for C/C++ and Fortran
• Pros:• Simple• Cross-platform• Small overhead• Data parallelism support
Usage
Compiler directives:C/C++
#pragma omp directive [clause, …]
Fortran!$OMP directive [clause, …]
C$OMP directive [clause, …]
*$OMP directive [clause, …]
Parallel execution
Parallel Regions
Main OpenMP directive
#pragma omp parallel
#pragma omp parallel{
printf( “hello world from thread %d of %d\n”,
omp_get_thread_num(),omp_get_num_threads() );
}
Параллельное исполнение
• Most of the OpenMP instructions are preprocessor directives
• Main construction is “omp parallel [smth]”
OpenMP parallel model
• Memory is shared• Task is divided into the threads.
– Variables can be• shared by the threads• private, available only for one thread
• Uncareful or wrong variable usage can lead to wrong execution results.
OpenMP parallel model
Fork-join model• Program execution starts from the master thread• With OpenMP directive master thread creates the additional threads• After the parallel region is finished all threads are synchronized• Main thread continues to execute the sequential part
Основные конструкции OpenMP
#pragma omp for
Each thread gets its own amount of data – data parallelism
#pragma omp section
Each section will be executed in a separate thread – functional parallelism
#pragma omp single
Sequential execution. Only one thread will execute this code
OpenMP sections
#pragma omp sections [ clause [ clause ] ... ] new-line
{
[#pragma omp section new-line ]
structured-block1
[#pragma omp section new-line
structured-block2 ]
...
}
OpenMP sections
#pragma omp parallel#pragma omp sections nowait{
thread1_work();#pragma omp section
thread2_work();#pragma omp section
thread3_work();#pragma omp section
thread4_work();}
Functional Parallelism
OpenMP for directive
#pragma omp for [ clause [ clause ] ...
Following loop will be executed in parallel (the iterations will be divided by the execution threads)
OpenMP for directive
#pragma omp parallel private(f)
{
f=7;
#pragma omp for
for (i=0; i<20; i++)
a[i] = b[i] + f * (i+1);
} /* omp end parallel */
OpenMP for directive
Available definitions: private( list ) reduction( operator: list ) schedule( type [ , chunk ] ) nowait (для #pragma omp for)
At the end of the loop all threads will be synchronized unless “nowait” directive is mentioned
schedule defines iteration space scattering method (default behaviour depend on OpenMP version)
OpenMP variables
private ( list )Each of the listed variables will have the local copy for each exection thread
shared ( list )All the thread will share the same instance of the variable
firstprivate ( list )
All the local copies will be initialized by master thread value
lastprivate ( list )
The resulting master thread value will be taken from the last thread executed
…All the variables are shared by default, except the local variables inside a function calls and the loop iterators
Example
int x;x = 0; // Initialize x to zero
#pragma omp parallel for firstprivate(x) // Copy value // of x
// from masterfor (i = 0; i < 10000; i++) { x = x + i;}printf( “x is %d\n”, x ); // Print out value of x
/* Actually needs lastprivate(x) to copy value back out to master */
OpenMP schedule clause
schedule( type [ , chunk ] ) static: Every thread gets fixed
amount of datadynamic: Amount of data will depend on the
thread execution speed
guided: Threads will get decreased amounts of data
dymamicallyruntime: Schedule type will be defined at
runtime
Main OpenMP functions
int omp_get_num_threads(void);
int omp_get_thread_num(void);
…
http://www.openmp.org/
OpenMP synchronization
Implicit sunchrionization is performed at the end of any parallel section(unless nowait option is mentioned)
OpenMP synchroniztion
сritical – can be executed only by one thread at a time.
atomic – Special critical section version for the atomic operations
barrier – synchronization point
ordered – sequential execution
master – only the main thread will execute the following code
…
OpenMP critical
cnt = 0;f=7;#pragma omp parallel{#pragma omp for
for (i=0; i<20; i++) {if (b[i] == 0) {
#pragma omp critical cnt ++;} /* endif */
a[i] = b[i] + f * (i+1);} /* end for */
} /*omp end parallel */
More information
OpenMP Homepage: http://www.openmp.org/
Introduction to OpenMP - tutorial from WOMPEI 2000 (link) Writing and Tuning OpenMP Programs on Distributed Shared Memory
Machines (link)
R.Chandra, L. Dagum, D. Kohr, D. Maydan, J. McDonald, R. Menon:
Parallel programming in OpenMP.
Academic Press, San Diego, USA, 2000, ISBN 1-55860-671-8
R. Eigenmann, Michael J. Voss (Eds):
OpenMP Shared Memory Parallel Programming.
Springer LNCS 2104, Berlin, 2001, ISBN 3-540-42346-X