Is it normal to observe such a big difference in execution time when the allocatable arrays are used vs fixed.
an example code produces
ifort pexcample.f90 -O3 ; time ./a.out
(a, fixed size) Time = 0.000004 seconds.
real 0m0.041s
user 0m0.002s
sys 0m0.004s
(b, allocatable array) Time = 14.428799 seconds.
real 0m19.532s
user 0m4.383s
sys 0m12.056s
program pexample
implicit none
integer(kind=8), parameter :: n = 1000, m = 1024, p = 1024, np = n*m*p
! real(kind=8) :: x(3,n,m,p)
real(kind=8), allocatable :: x(:,:,:,:)
real(kind=8) :: tic, toc
integer(kind=8) :: i, j, k, l, id
allocate( x(1:3,1:n,1:m,1:p) )
call cpu_time(tic)
do k = 1, p
do j = 1, m
do i = 1, n
! id = I + N*(J-1) + N*M*(K-1)
x(1,i,j,k) = 0.d0!(i-1)*1.d0/(n-1)
x(2,i,j,k) = 0.d0!(j-1)*1.d0/(m-1)
x(3,i,j,k) = 0.d0!(k-1)*1.d0/(p-1)
end do
end do
end do
call cpu_time(toc)
deallocate( x )
print '("Time = ",f10.6," seconds.")', toc-tic
end program pexample