Missing get-*-point (i.e. get_val) methods with MPI

[thchr]

The otherwise very useful (get-*-point r) methods, e.g. (get-field-point r), do not work with MPI since the method get_val doesn't have an MPI implementation.

This is already nicely noted by an error message:

mpb/mpb/fields.c

Lines 571 to 572 in 47f68c2

	#ifdef HAVE_MPI
	CHECK(0, "get-*-point not yet implemented for MPI!");

How painful would it be to implement this functionality? I'm assuming the difficulty lies in locating which local process owns the ix index?

This is pretty much guesswork on my part, as I don't know MPI, but, conceptually, would this involve something like:

int ixp = ix - local_x_start
if (ixp >= 0 && ixp < local_nx)
     // assign data[(((ixp * ny) + iy) * nz + iz) * stride] to a process buffer
else
     // assign zero to the (same) process buffer
// reduce over buffer and return value

?

Best regards,
Thomas

[stevengj]

Yes, it would basically look like that. Conceptually, it's not difficult, but it's a little annoying to get the coordinates right and debug, so I never got around to it, especially since I don't use the MPI version myself very much.

[stevengj]

Note, however, that you need to use local_y_start etcetera because the position-space fields in MPI are transposed for performance reasons (x and y are swapped), so the code in the MPI case will be disjoint from the serial case (which is not transposed).

[stevengj]

Closed by #112.