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Example Applications
partitioning method with OPENMP thread directives. In the first case, the data dependency is easily
satisfied because each thread computes down a different set of columns. In the second case we still want
to compute down the columns for cache reasons, but to satisfy the data dependency, each thread computes
a different portion of the same column and the threads work left to right across the rows together.
implicit none
include 'mpif.h'
integer nrow
! # of rows
integer ncol
! # of columns
parameter(nrow=1000,ncol=1000)
double precision array(nrow,ncol) ! compute region
integer blk
! block iteration counter
integer rb
! row block number
integer cb
! column block number
integer nrb
! next row block number
integer ncb
! next column block number
integer rbs(:)
! row block start subscripts
integer rbe(:)
! row block end subscripts
integer cbs(:)
! column block start subscripts
integer cbe(:)
! column block end subscripts
integer rdtype(:)
! row block communication datatypes
integer cdtype(:)
! column block communication datatypes
integer twdtype(:)
! twisted distribution datatypes
integer ablen(:)
! array of block lengths
integer adisp(:)
! array of displacements
integer adtype(:)
! array of datatypes
allocatable rbs,rbe,cbs,cbe,rdtype,cdtype,twdtype,ablen,adisp,adtype
integer rank
! rank iteration counter
integer comm_size
! number of MPI processes
integer comm_rank
! sequential ID of MPI process
integer ierr
! MPI error code
integer mstat(mpi_status_size)
! MPI function status
integer src
! source rank
integer dest
! destination rank
integer dsize
! size of double precision in bytes
double precision startt,endt,elapsed ! time keepers
external compcolumn,comprow
! subroutines execute in threads
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MPI initialization
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Data initialization and start up
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call mpi_init(ierr)
call mpi_comm_size(mpi_comm_world,comm_size,ierr)
call mpi_comm_rank(mpi_comm_world,comm_rank,ierr)
if (comm_rank.eq.0) then
write(6,*) 'Initializing',nrow,' x',ncol,' array...'
call getdata(nrow,ncol,array)
write(6,*) 'Start computation'
endif
call mpi_barrier(MPI_COMM_WORLD,ierr)
startt=mpi_wtime()
Compose MPI datatypes for row/column send-receive
Note that the numbers from rbs(i) to rbe(i) are the indices
of the rows belonging to the i'th block of rows. These indices
specify a portion (the i'th portion) of a column and the
datatype rdtype(i) is created as an MPI contiguous datatype
to refer to the i'th portion of a column. Note this is a
contiguous datatype because fortran arrays are stored
column-wise.
For a range of columns to specify portions of rows, the situation
is similar: the numbers from cbs(j) to cbe(j) are the indices
of the columns belonging to the j'th block of columns. These
indices specify a portion (the j'th portion) of a row, and the
datatype cdtype(j) is created as an MPI vector datatype to refer
to the j'th portion of a row. Note this a vector datatype
Platform MPI User's Guide 201