Jacobi Iteration: send guard values
DO WHILE (err > tol)
CALL MPI_SEND(u(1,1),
nxblock,MPI_REAL,nbrleft,0,comm2d, ierr)
CALL MPI_RECV(u(1,nylocal),
nxblock,MPI_REAL,nbrright,0,comm2d, ierr)
CALL MPI_SEND(u(1,nylocal),
nxblock,MPI_REAL, nbrright,1,comm2d, ierr)
CALL MPI_RECV(u(1,0),
nxblock,MPI_REAL,nbrleft,1,comm2d, ierr)
CALL MPI_SEND(u(1,1),
1,rowtype,nbrtop,2,comm2d, ierr)
CALL MPI_RECV(u(nxlocal,1),
1,rowtype,nbrbottom,2,comm2d, ierr)
CALL MPI_SEND(u(nxlocal-1,1),
1,rowtype,nbrbottom,3,comm2d, ierr)
CALL MPI_RECV(u(0,1),1,rowtype,nbrtop,3,comm2d, ierr)