User Guide

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OpenMP Critical Sections

Use OpenMP critical sections to prevent multiple threads from accessing the critical section's code at the same time, thus only one active thread can update the data referenced by the code. Critical sections are useful for a non-nested mutex.
Unlike OpenMP atomic operations that provide fine-grain synchronization for a single operation, critical sections can provide course-grain synchronization for multiple operations.
Use:
  • #pragma omp critical
    with C/C++.
  • !$omp critical
    and
    !$omp end critical
    with Fortran.
If the optional
(name)
is omitted, it locks an unnamed global mutex. The easiest approach is to use the unnamed form unless this shared mutex is causing unacceptable performance delays.
After you rewrite your code to use OpenMP* parallel framework, you can analyze its performance with
Intel® Advisor
perspectives. Use the
Vectorization and Code Insights
perspective to analyze how well you OpenMP code is vectorized or use the
Offload Modeling
perspective to model its performance on a GPU.
For example, consider this annotated C/C++ serial program:
int count; void Tick() { ANNOTATE_LOCK_ACQUIRE(0); count++; ANNOTATE_LOCK_RELEASE(0); } . . .
The parallel C/C++ code after adding
#include <omp.h>
and
#pragma omp critical
:
#include <omp.h> //prevents a load-time problem with a .dll not being found int count; void Tick() { // Replace Lock annotations #pragma omp critical count++; } . . .
Consider this annotated Fortran serial code:
program ABC integer(kind=4) :: count = 0 . . . contains subroutine Tick call annotate_lock_acquire(0) count = count + 1 call annotate_lock_release(0) end subroutine Tick . . . end program ABC
The parallel Fortran code after adding
use omp_lib
,
!$omp critical
, and
!$omp end critical
:
program ABC use omp_lib integer(kind=4) :: count = 0 . . . contains subroutine Tick !$omp critical count = count + 1 !$omp end critical end subroutine Tick . . . end program ABC

Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.