How do I initialize two different blacs contexts?
I have a computer with processors nproc
and I would like to initialize two blacs, one of the sizes p x q = nprocs
and one of the sizes 1 x 1
.
Suppose MPI is allready initialized and routine search for good block sizes, the first grid is initialized with
call blacs_get( -1, 0, self%context )
call blacs_gridinit( self%context, 'R', self%nprows, self%npcols )
call blacs_gridinfo( self%context, self%nprows, self%npcols, self%myrow, self%mycol )
But how do I set up the second one? Do I have to enter another mpi communicator first?
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As an answer and example, I share this implementation:
call blacs_get( -1, 0, self%context )
call blacs_gridinit( self%context, 'R', self%nprows, self%npcols )
call blacs_gridinfo( self%context, self%nprows, self%npcols, self%myrow, self%mycol )
print*, "A ", self%context, self%nprows, self%npcols, self%myrow, self%mycol
call sleep(1)
call blacs_get( -1, 0, val )
call blacs_gridinit( val, 'R', 1, 1 )
call blacs_gridinfo( val, self%nprows, self%npcols, self%myrow, self%mycol )
call sleep(1)
print*, "B ", val, self%nprows, self%npcols, self%myrow, self%mycol
call sleep(1)
call blacs_get( -1, 0, val2 )
call blacs_gridinit( val2, 'R', 2, 2 )
call blacs_gridinfo( val2, self%nprows, self%npcols, self%myrow, self%mycol )
call sleep(1)
print*, "C ", val2, self%nprows, self%npcols, self%myrow, self%mycol
Which adds three blacs contexts, no need to initialize another MPI communicator and composes the following output on four cores:
A 0 2 2 1 1
A 0 2 2 0 0
A 0 2 2 1 0
A 0 2 2 0 1
B -1 -1 -1 -1 -1
B -1 -1 -1 -1 -1
B -1 -1 -1 -1 -1
B 1 1 1 0 0
C 1 2 2 1 0
C 1 2 2 1 1
C 1 2 2 0 1
C 2 2 2 0 0
So the crucial point is that the first argument to blacs_gridinit is an I / O argument that needs the global blacs context of all processes as input. It is returned to a new variable by calling blacs_get, the third argument.
In this case, I found a completely opposite intuitive fact: the context value seems to follow some kind of sum rule, so after initializing the 1x1 grid and again the 4x4 grid, the 4x4 grid values for all processes do not match.
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