Parallel Programming in Fortran 95 using OpenMP - People

4.3. Timing routines 65
!---------------------------------------------------------!
subroutine substract(x,d)
use omp_lib
use data_types
implicit none
type(number) :: x
integer :: d
call OMP_set_nest_lock(x%lck)
call add(x,-d)
call OMP_unset_nest_lock(x%lck)
end subroutine substract
The subroutine add is called once by each thread. In both calls the updating of the
variable x is correctly done, since a lock is allowing the access to only one thread at a
time. The reason why substract is imposing the lock relays on the fact that substract
is not obliged to know what the subroutine add internally does.
4.3 Timing routines
The last set of routines in the OpenMP Fortran API run-time library allows to measure
absolute values of wallclock times with the idea to serve as an evaluation tool for programs.
4.3.1 OMP get wtime
This function returns the absolute wallclock time in seconds. By two calls to this function,
it is possible to measure the time required to compute the commands enclosed between
the two calls, like in the following example:
start = OMP_get_wtime()
... !work to be timed
end = OMP_get_wtime()
time = end - start
The interface declaration for this function is:
function OMP_get_wtime()
real(8) :: OMP_get_wtime
end function OMP_get_wtime
The times returned by this function are ”per-thread times” by which is meant they are
not required to be globally consistent across all the threads participating in a program.