Re: [Libmesh-users] How to broadcast a scalar value to all the processors in libmesh?

[Benjamin K. <ben...@na...>]

As Roy mentioned, there is no wrapper for this.  The right way to do it, for
example, is MPI_Bcast and may be seen in mesh_communiation.C:

http://libmesh.sourceforge.net/doxygen/mesh__communication_8C-source.php

This code broadcasts several vectors.

If we implement a broadcast wrapper I would rather do it for the general
case of a vector.  This raises an observation about message-passing
programming: in general it is better to have fewer, large messages than
more, small ones since latency is a considerable factor in initiating
communication.

As a result, you may come across weird, non-obvious hacks.  Say I want to
compute a global min and max of scalar values...

This would work

MPI_Allreduce (my_min, global_min, ..., MPI_MIN)
MPI_Allreduce (my_max, global_max, ..., MPI_MAX)

But this will perform better:

MPI_Allreduce ([-my_min my_max], [global_min, global_max], ..., MPI_MAX);
global_min = -global_min;

-Ben




On 9/26/07 1:40 PM, "Bin" <xub...@gm...> wrote:

> Hi,
> 
> In my codes, I have to broadcast a scalar value to all the processors in
> order to form the scaled element stiffness matrix. Can you tell me how to do
> it in libmesh? Is there any build-in function in libmesh which can do it?
> 
> Thank you.