The following code (built with ifx 2025.2.1 on Windows and run with 6 processes) produce somewhat puzzling information, and we would like to get some clarity on the behavior we observe.
The code uses BLACS_GRIDMAP to create a custom 1x4 grid from processes 2, 3, 4 and 5 (thus, process 0 and 1 are not part of that grid). Then a process that is in that grid (process 2) calls BLACS_PNUM to retrieve the numbers of all the processes that are in that grid.
Instead of the expected output of -1, -1, 2, 3, 4, 5, we get 0, 1, 2, 3, -1, -1.
Can somebody explain this? Are process numbers defined within the context of a particular grid, or are they global?
PROGRAM P
IMPLICIT NONE (TYPE, EXTERNAL)
EXTERNAL :: BLACS_GET, BLACS_PINFO, BLACS_PNUM, BLACS_GRIDMAP, BLACS_GRIDINFO
INTEGER(KIND = 8) :: BLACS_PNUM, CTXT_SYS, C_TXT, MY_PROC, N_PROCS, UMAP(1, 4), I, NPROW, NPCOL, MYROW, MYCOL
CALL BLACS_PINFO(MY_PROC, N_PROCS)
IF (N_PROCS /= 6) THEN
WRITE(*, '(A)') 'Run this program with 6 processes!'
STOP
END IF
CALL BLACS_GET(0, 0, CTXT_SYS)
C_TXT = CTXT_SYS
UMAP(1, :) = [2, 3, 4, 5]
CALL BLACS_GRIDMAP(C_TXT, UMAP, 1, 1, 4)
IF (MY_PROC == 2) THEN
CALL BLACS_GRIDINFO(C_TXT, NPROW, NPCOL, MYROW, MYCOL)
WRITE(*, *) 'Process number : ', MY_PROC, 'Process row: ', MYROW, 'Process column: ', MYCOL
WRITE(*, *) 'Other processes: ', (BLACS_PNUM(C_TXT, 0, I), I = 0, N_PROCS - 1)
END IF
END PROGRAM P
