topic Why is code slower with when allocate is used vs. static arrays in Intel® Fortran Compiler

Why is code slower with when allocate is used vs. static arrays

crtierney42 — Thu, 01 Apr 2010 21:55:38 GMT

I have talked to a few F90 experts but have not found a good answer to my problem.. Why is the code below run twice as slow when the arrays are dynamically allocated vs. statically? I tried a few compiler options (-fno-alias, -static) and tried
using VECTOR NONTEMPORAL but nothing helps.

In this case on a 2.8GHz Nehalem (not shared), the performance difference is 2x.

Is using allocatable arrays such as this (typical use in numerical models) just slower? If so, why?

I compiled the code with ifort -O3 -xSSE4.2 -fno-alias -fpp relax.f -o relax, and added -DDYNAMIC to enable
allocatable memory. The compiler version is 11.1.038.

Thanks,
Craig

program relax

integer :: mbyte=262144
integer :: nb=100

#if defined(DYNAMIC)
real,allocatable :: a(:,:),b(:,:)
allocate(a(0:mbyte+1,nb))
allocate(b(0:mbyte+1,nb))
#else
real :: a(0,262145),b(0,262145)
#endif

a=100
b=100

do iter=1,100
!$OMP parallel do
do n=1,nb
do k=1,mbyte
b(k,n)=(a(k+1,n)+a(k-1,n))/2.
end do
end do
!$OMP parallel do
do n=1,nb
do k=1,mbyte
a(k,n)=b(k,n)
end do
end do
call sub(a)
print *,iter
end do
stop
end
subroutine sub()
return
end

Why is code slower with when allocate is used vs. static arrays

rreis — Fri, 02 Apr 2010 09:25:24 GMT

Is the program correct? The !$OMP PARALLEL is opened but not closed... and I don't undertstand the static array declaration, shouldn't it read:

real :: a(0:262145,100),b(0:262145,100) ?

Why is code slower with when allocate is used vs. static arrays

Xiaoping_D_Intel — Fri, 02 Apr 2010 09:25:45 GMT

Why the static version and dynamic allocated version arrays have different data size and shape?

The allocatable version has shape of (262145,100). The static version has shape of (0,262145) and its data size is 0. Obviously there will be out-of-bound error for the static version in following loops. If compiled with runtime check enabled by "-C" option there will be a runtime error:

forrtl: severe (408): fort: (2): Subscript #1 of the array A has value 2 which is greater than the upper bound of 0

Why is code slower with when allocate is used vs. static arrays

crtierney42 — Fri, 02 Apr 2010 18:55:56 GMT

Ahhh, I wrote bad code. It was a sanity check that went bad. Yes the code had bugs. After fixing them,
the two cases run pretty much the same.

Sorry for the post.

Craig