Parallel Programming in Fortran 95 using OpenMP - People
Parallel Programming in Fortran 95 using OpenMP - People
Parallel Programming in Fortran 95 using OpenMP - People
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24 2. <strong>OpenMP</strong> constructs<br />
thread 0 thread 1 thread N p<br />
!$OMP CRITICAL<br />
✞<br />
☎<br />
❄✝<br />
❝ ✆<br />
❄<br />
E xecution<br />
parallel region<br />
...<br />
write(1,*) data<br />
!$OMP END CRITICAL<br />
❄<br />
❄<br />
Figure 2.5: Graphical representation of the example expla<strong>in</strong><strong>in</strong>g the work<strong>in</strong>g pr<strong>in</strong>ciple of the<br />
!$OMP CRITICAL/!$OMP END CRITICAL directive-pair.<br />
2.3.3 !$OMP BARRIER<br />
This directive represents an explicit synchronization between the different threads <strong>in</strong> the<br />
team. When encountered, each thread waits until all the other threads have reached this<br />
po<strong>in</strong>t.Itssyntaxisisverysimple:<br />
!$OMP BARRIER<br />
Two restrictions need to be taken <strong>in</strong>to account when us<strong>in</strong>g this explicit synchronization:<br />
• The !$OMP BARRIER directive must be encountered by all threads <strong>in</strong> a team or by<br />
none at all. Several situations may not fulfill this condition. For example:<br />
!$OMP CRITICAL<br />
!$OMP BARRIER<br />
!$OMP END CRITICAL<br />
S<strong>in</strong>ce only one thread at a time is execut<strong>in</strong>g the content of the CRITICAL region, it is<br />
impossible for the other threads to reach the explicit synchronization. The result is<br />
that the program reaches a state without exit. This situation is called a deadlock<br />
and it is necessary to avoid it.<br />
Other examples, where also a deadlock happens, could be:
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