Christian,
Suppose you have N processes calling the first MPI_File_get_position_shared
().
Some of them are running faster and could execute the call to
MPI_File_seek_shared() before all the other have got their file position.
(Note that the "collective" primitive is not a synchronization. In that
case, all parameters are broadcast to the process 0 and checked by process
0. All
the other processes are not blocked).
So the slow processes can get the file position that has just been
modified by the faster.
That is the reason why, in your program, It is necessary to synchronize all
processes just before the call to MPI_File_seek_shared().
Pascal
users-boun...@open-mpi.org a écrit sur 25/06/2011 12:54:32 :
> De : Jeff Squyres <jsquy...@cisco.com>
> A : Open MPI Users <us...@open-mpi.org>
> Date : 25/06/2011 12:55
> Objet : Re: [OMPI users] File seeking with shared filepointer issues
> Envoyé par : users-boun...@open-mpi.org
>
> I'm not super-familiar with the IO portions of MPI, but I think that
> you might be running afoul of the definition of "collective."
> "Collective," in MPI terms, does *not* mean "synchronize." It just
> means that all functions must invoke it, potentially with the same
> (or similar) parameters.
>
> Hence, I think you're seeing cases where MPI processes are showing
> correct values, but only because the updates have not completed in
> the background. Using a barrier is forcing those updates to
> complete before you query for the file position.
>
> ...although, as I type that out, that seems weird. A barrier should
> not (be guaranteed to) force the completion of collectives (file-
> based or otherwise). That could be a side-effect of linear message
> passing behind the scenes, but that seems like a weird interface.
>
> Rob -- can you comment on this, perchance? Is this a bug in ROMIO,
> or if not, how is one supposed to use this interface can get
> consistent answers in all MPI processes?
>
>
> On Jun 23, 2011, at 10:04 AM, Christian Anonymous wrote:
>
> > I'm having some issues with MPI_File_seek_shared. Consider the
> following small test C++ program
> >
> >
> > #include <iostream>
> > #include <mpi.h>
> >
> >
> > #define PATH "simdata.bin"
> >
> > using namespace std;
> >
> > int ThisTask;
> >
> > int main(int argc, char *argv[])
> > {
> > MPI_Init(&argc,&argv); /* Initialize MPI */
> > MPI_Comm_rank(MPI_COMM_WORLD,&ThisTask);
> >
> > MPI_File fh;
> > int success;
> > MPI_File_open(MPI_COMM_WORLD,(char *)
> PATH,MPI_MODE_RDONLY,MPI_INFO_NULL,&fh);
> >
> > if(success != MPI_SUCCESS){ //Successfull open?
> > char err[256];
> > int err_length, err_class;
> >
> > MPI_Error_class(success,&err_class);
> > MPI_Error_string(err_class,err,&err_length);
> > cout << "Task " << ThisTask << ": " << err << endl;
> > MPI_Error_string(success,err,&err_length);
> > cout << "Task " << ThisTask << ": " << err << endl;
> >
> > MPI_Abort(MPI_COMM_WORLD,success);
> > }
> >
> >
> > /* START SEEK TEST */
> > MPI_Offset cur_filepos, eof_filepos;
> >
> > MPI_File_get_position_shared(fh,&cur_filepos);
> >
> > //MPI_Barrier(MPI_COMM_WORLD);
> > MPI_File_seek_shared(fh,0,MPI_SEEK_END); /* Seek is collective */
> >
> > MPI_File_get_position_shared(fh,&eof_filepos);
> >
> > //MPI_Barrier(MPI_COMM_WORLD);
> > MPI_File_seek_shared(fh,0,MPI_SEEK_SET);
> >
> > cout << "Task " << ThisTask << " reports a filesize of " <<
> eof_filepos << "-" << cur_filepos << "=" << eof_filepos-cur_filepos <<
endl;
> > /* END SEEK TEST */
> >
> > /* Finalizing */
> > MPI_File_close(&fh);
> > MPI_Finalize();
> > return 0;
> > }
> >
> > Note the comments before each MPI_Barrier. When the program is run
> by mpirun -np N (N strictly greater than 1), task 0 reports the
> correct filesize, while every other process reports either 0, minus
> the filesize or the correct filesize. Uncommenting the MPI_Barrier
> makes each process report the correct filesize. Is this working as
> intended? Since MPI_File_seek_shared is a collective, blocking
> function each process have to synchronise at the return point of the
> function, but not when the function is called. It seems that the use
> of MPI_File_seek_shared without an MPI_Barrier call first is very
> dangerous, or am I missing something?
> >
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>
> --
> Jeff Squyres
> jsquy...@cisco.com
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