O uso do paralelismo deve levar a minimizar o tempo de um programa, mas isso não aconteceu comigo. Quando programei meu código em paralelo usando o OpenMP, o tempo de execução é aumentado, ou seja, PARALLEL TIME> SERIAL TIME.

Meu código:

    PROGRAM MAIN
    use omp_lib
    implicit none
    REAL*8 Times1,Times2
    INTEGER I,J
    real, allocatable, dimension(:) :: a
    allocate(a(1000))
    DO J = 1, 1000
    a(j)=j  
    ENDDO
!    ***************NO PARALLEL CODE ************************************
    call CPU_TIME(Times1)
    write(*,*) 'CPU NO PARALLEL STARTED:',Times1
    DO I = 1, 1000
    DO J = 1, 500000
    a(I)=a(I)+0.0001
    end do 
    a(I)=a(I)+a(I)+a(I)
    ENDDO
    call CPU_TIME(Times2)
    write(*,*) 'CPU CPU NO PARALLEL finished:',Times2
    write(*,*) 'NO PARALLEL TIMES:',Times2-Times1
    write(*,*) '---------------------------------------------------'
!    ***************PARALLEL CODE ************************************
    call CPU_TIME(Times1)
    write(*,*) 'CPU PARALLEL STARTED:',Times1
!$OMP PARALLEL DEFAULT(shared), private(I,J)
!$OMP DO
    DO I = 1, 1000
    DO J = 1, 500000
    a(I)=a(I)+0.0001
    end do 
    a(I)=a(I)+a(I)+a(I)
    ENDDO
!$OMP END DO
!$OMP END PARALLEL
    call CPU_TIME(Times2)
    write(*,*) 'CPU PARALLEL finished:',Times2
    write(*,*) 'PARALLEL TIMES:',Times2-Times1
    deallocate(a)
    STOP
    END

e o resultado :

 CPU NO PARALLEL STARTED:  1.560010000000000E-002
 CPU CPU NO PARALLEL finished:   4.86723120000000
 NO PARALLEL TIMES:   4.85163110000000

 CPU PARALLEL STARTED:   4.86723120000000
 CPU PARALLEL finished:   9.89046340000000
 PARALLEL TIMES:   5.02323220000000

Por que meu tempo medido por CPU_TIME () aumentou com o OpenMP?