Re: [Gretl-devel] MPI doc

Am 16.02.2015 um 18:42 schrieb Allin Cottrell: > On Mon, 16 Feb 2015, Sven Schreiber wrote: > >> Am 16.02.2015 um 17:58 schrieb Allin Cottrell: > >>> The trouble here is that the matrices a and b exist only in the rank 0 >>> process; they need to exist everywhere. >>> >>> <hansl> >>> mpi --np=2 >>> matrix a >>> matrix b >>> if $mpirank == 0 >>> b = ones(1000,1) >>> a = zeros(1000,1) >>> endif >>> mpiscatter(&b, byrows) >>> </hansl> >>> >> >> Hm, ok I understand. My spontaneous reaction would be: "but isn't this >> what mpiscatter (or mpibroadcast) is supposed to achieve as part of >> their job?". >> What's the value added of requiring the pre-existence of the recipient >> objects? > > It's not a question of value added, it's just how MPI works! Remember > that each "rank" corresponds to a distinct instance of gretlmpi. > Consider your original code, without the prior declaration of the > matrices, from the point of view of the rank 1 process: the first line > rank 1 sees is: > > mpiscatter(&b, byrows) > > So it's like, "mpiscatter what?? I don't know anything about this b". > Yeah, I actually understood that :-) But since we're on the gretl list my comment was aimed at the hansl wrapper implementation: Why can't hansl see the &b in mpiscatter(&b, byrows) and shout out: "Hey I've seen this b before, it was created in line xyz, and now everybody's getting it, too." I mean this look-up of existing objects is what the interpreter does all the time, no?