Andy

Further to my previous posting on this matter, I have just tried my shiny
new Intel 7.0 compiler on my version of the three examples, and found that
all three give similar performance and that the performance was much the
same as the quickest time of the Compaq (6.6) compiler.  So it looks to me
as if the Intel compiler is smart and can do the right thing quickly.

Does anyone have anything else to say about this?  Each of my examples runs
in just over 6 seconds on my Pentium 4, 2.2 GHz.

Alistair

-----Original Message-----
From: Fortran 90 List [mailto:[log in to unmask]] On Behalf Of
Andy Leonard
Sent: 27 January 2003 15:39
To: [log in to unmask]
Subject: Performance penalties for allocatable arrays?


Can anyone explain why the use of allocatable arrays in the first example
program below causes a 10% increase in the CPU time spent in the subroutine,
compared to the second example program where the arrays are in a common
block?  (When the allocatable is changed to pointer, it adds another 10%).

Both codes were compiled with the IBM xlf90 compiler (with the same compiler
options) and the cpu time determined by profiling with gprof.

We have noticed some significant increases in CPU time in our software since
we started dynamically allocating arrays.  The trivial examples below are
representative of our code, as we frequently use common blocks (and now
modules) rather than passing data through the arguments.

Thanks,

Andy



Program 1:
c==============================
       program main
       use A_MOD

       na = 100000
       ALLOCATE(A(na))
       ALLOCATE(dAdt(na))

       do n=1,1000
         call SUB
       enddo

       stop
       end
c==============================
       subroutine SUB
       use A_MOD

       do n=1,na
         A(n) = A(n) + dAdt(n)*dt
       enddo

       return
       end
c==============================
       module A_MOD

       integer :: na
       real :: dt = 0.001
       real, allocatable, dimension(:) :: A, dAdt

       end module A_MOD
c==============================

Program 2:

c==============================
       program main
       parameter (na = 100000)
       common /A_MOD/ dt,A(na),dAdt(na)

       dt = 0.001
       do n=1,1000
         call SUB
       enddo

       stop
       end
c==============================
       subroutine SUB
       parameter (na = 100000)
       common /A_MOD/ dt,A(na),dAdt(na)

       do n=1,na
         A(n) = A(n) + dAdt(n)*dt
       enddo

       return
       end

  Andy Leonard
  Gamma Technologies, Inc.
  601 Oakmont Lane, Suite 220
  Westmont, IL 60559
  Tel:    (630) 325-5848
  Fax:   (630) 325-5849
  E-mail:  [log in to unmask]
  Web:     http://www.gtisoft.com