Fortran: dynamic arrays vs. automatic array Avoiding Memory Allocation

Question

Profiling one of our fortran codes, there are two subroutines that are taking up most of the computational time (22.1% and 17.2%). In each routines, ~5% of the time is spent allocating and freeing memory. These routines look like

MODULE foo

CONTAINS

SUBROUTINE bar( ... )
...
IMPLICIT NONE
...
REAL, ALLOCATABLE, DIMENSION(:,:) :: work
...
ALLOCATE (work(size1,size2))
...
DEALLOCATE (work)
END SUBROUTINE bar
...
END MODULE foo

These subroutines get called on the order of ~4000-5000 times in my bench mark so I would like to get rid of ALLOCATE and DEALLOCATE. Changing these to automatic arrays changes to profiler output to.

MODULE foo

CONTAINS

SUBROUTINE bar( ... )
...
IMPLICIT NONE
...
REAL, DIMENSION(size1,size2) :: work
...
END SUBROUTINE bar
...
END MODULE foo

Changes the resulting profile to

Running Time        Symbol Name
20955.0ms  17.0%    __totzsp_mod_MOD_totzsps
    7.0ms   0.0%        malloc
    5.0ms   0.0%        free
    2.0ms   0.0%        user_trap

16192.0ms  13.2%    __tomnsp_mod_MOD_tomnsps
   20.0ms   0.0%        free
    3.0ms   0.0%        malloc
    1.0ms   0.0%        szone_size_try_large

I looks like gfortran is allocating these on the stack and not that heap but I'm concerned about when happens when these arrays become too large.

The second approach that I'm taking is to allocate and deallocate these arrays once.

work_array.f

MODULE work_array
IMPLICIT NONE

REAL(rprec), ALLOCATABLE, DIMENSION(:,:) :: work

END MODULE work_array

I allocate these once in a different part of the code. Now my subroutine looks like

MODULE foo

CONTAINS

SUBROUTINE bar( ... )
...
USE work_array
IMPLICIT NONE
...
END SUBROUTINE bar
...
END MODULE foo

However when I run the code now the profile get worse.

Running Time        Symbol Name
30584.0ms  21.6%    __totzsp_mod_MOD_totzsps
 3494.0ms   2.4%        free
 3143.0ms   2.2%        malloc
   27.0ms   0.0%        DYLD-STUB$$malloc_zone_malloc
   19.0ms   0.0%        szone_free_definite_size
    6.0ms   0.0%        malloc_zone_malloc

24325.0ms  17.1%    __tomnsp_mod_MOD_tomnsps
 2937.0ms   2.0%        free
 2456.0ms   1.7%        malloc
   23.0ms   0.0%        DYLD-STUB$$malloc_zone_malloc
    3.0ms   0.0%        szone_free_definite_size

Where are these extra mallocs, and frees coming from? How can I set this up so I allocate these arrays once?

Answer 1

Since the work array is only used inside the bar subroutine, you could add the save attribute to it and allocate it when the subroutine is called for the first time. If work1 or work2 is different compared to previous calls, you can just reallocate the array in that case.

This does leave the problem of deallocation once the subroutine is no longer needed. If you need to call it during the whole life-time of the program, it's no problem since the OS should deallocate the memory when the program quits. On the other hand, if you only need it during initialization, the memory will remain allocated even when not needed. Maybe you can add an argument to the subroutine which tells it to deallocate the work array, if memory usage is a problem.