Re: [Gretl-devel] MPI doc
On Tue, 17 Feb 2015, Sven Schreiber wrote:
Am 17.02.2015 um 14:23 schrieb Allin Cottrell:
> On Tue, 17 Feb 2015, Sven Schreiber wrote:
>
>> In the last line the gretl interpreter would just have to know that
>> the "b" to be broadcast lives in rank 0. I don't see why it would
have
>> to know about non-existing b's in other ranks.
>
> You have n copies of the gretl interpreter running.
This is something I wasn't yet fully aware of I guess...
A little more info on MPI, in case it's helpful. MPI provides an
infrastructure that lets you launch n copies of a program on
different cores or different machines, and a mechanism whereby these
processes can communicate with each other (broadcast, scatter and
friends). MPI is not a programming language as such, but it provides
APIs in both C and Fortran. Gretl's implementation rests on the C
API.
Ok, so if I understand correctly, without pre-declaration it's
not
possible for the rank-0-interpreter to tell the other interpreters
"watch out there's a 'b' coming your way" because there's
currently no mechanism that allows the rank-0-interpreter to
enclose the necessary information "hey guys, 'b' is a matrix (with
dimensions such-and-such)"?
In principle, rank 0 can send any information you want to the other
processes, but only via broadcast/scatter.
When you do mpibcast() on a matrix in gretl, libgretl first
broadcasts the dimensions, "in the background", so that each process
can allocate space for the content. If that goes OK then the actual
content is broadcast and copied into the memory of each process.
We don't currently support broadcast of string variables but we
could if that seemed useful. In that case rank 0 could send out the
name of a matrix to be received. However, this would generate a
regress, in that you'd then need a pre-existing string variable at
each rank to receive the name (plus string substitution to make use
of it), as in
string mname
if $mpirank == 0
mname = "b"
matrix b = {whatever}
endif
mpibcast(&mname)
if $mpirank > 0
matrix @mname
endif
mpibcast(&@mname)
This would be more cumbersome that just declaring b in all
processes.
Allin