TABLE OF CONTENTS

cgca_m2red/cgca_redand [ Subroutines ]

[ Top ] [ cgca_m2red ] [ Subroutines ]

NAME

cgca_redand

SYNOPSIS

subroutine cgca_redand(coarray,p)

INPUTS

logical(kind=ldef),intent(inout) :: coarray[*]
integer(kind=idef),intent(in) :: p

SIDE EFFECTS

coarray values change

DESCRIPTION

This routine does collective AND operation over coarray values across all images. The result is returned in coarray on every image. The result is TRUE if coarray values on all images are TRUE, and FALSE otherwise. The algorithm implements a divide and conquer scheme that works only when the number of images, n, is a power of 2 - n=2**p. p is the input to this routine.

If the number of images is 2**p, then reduction takes p iterations. In this example I have 2**4=16, so it takes 4 iterations. 

     img1 img2 img3 img4 img5 img6 img7 img8 img9 img10 img11 img12 img13 img14 img15 img16
  1. img1 _/   img3 _/   img5 _/   img7 _/   img9 _/    img11 _/    img13 _/    img15 _/
  2. img1 ______/        img5 ______/        img9 _______/          img13 _______/
  3. img1 ________________/                  img9 ___________________/
  4. img1 ____________________________________/
     img1

NOTE

For efficiency no check is made that n = 2**p. This check must be made in the calling routine or the main program.

USES

none

USED BY

cgca_m2red

SOURCE

integer(kind=idef) :: i, img, step, stepold

img = this_image()

step    = 2
stepold = 1

! do the reduction

redu: do i = 1,p

 if (mod(img,step)-1 .eq. 0) then
  sync images (img+stepold)
  coarray = coarray .and. coarray[img+stepold]
 else if (mod(img+stepold,step)-1 .eq. 0) then
  sync images (img-stepold)
 end if

 stepold = step
 step = step * 2

end do redu

! now send the result, which is in z[1] to all images.
! all images wait for image 1, so can use sync images(*),
! but, as the standard suggests, sync images is probably faster.

sync all

coarray = coarray[1]

end subroutine cgca_redand

CGPACK/cgca_m2red [ Modules ]

[ Top ] [ Modules ]

NAME

cgca_m2red

SYNOPSIS

!$Id: cgca_m2red.f90 380 2017-03-22 11:03:09Z mexas $

module cgca_m2red

DESCRIPTION

Module dealing with collective reduction operations, including all required image syncronisation.

AUTHOR

Anton Shterenlikht

COPYRIGHT

See LICENSE

CONTAINS

cgca_redand

USES

cgca_m1co

USED BY

cgca

SOURCE

use cgca_m1co
implicit none

private
public :: cgca_redand

contains