[mpich-discuss] Bcast question?

[John Chludzinski]

I got rid of the distinction between the last process and the others.  All
processes now use:

*int number_of_bytes = ceil((double)total_number_of_bytes /pool_size);
*
read_buffer = (char*) calloc(number_of_bytes, 1);
...
my_offset = (MPI_Offset) my_rank * number_of_bytes;
...
MPI_File_seek(fh, my_offset, MPI_SEEK_SET);
...
MPI_File_read(fh, read_buffer, number_of_bytes_2, MPI_BYTE, &status);
...

This seems to work.  Look reasonable?

---John


On Sat, Aug 11, 2012 at 5:06 PM, Rajeev Thakur <thakur at mcs.anl.gov> wrote:

> You can simply have each process read the entire file using a single
> MPI_File_read_all. No need for Gather or Gatherv.
>
> Rajeev
>
> On Aug 11, 2012, at 5:47 AM, John Chludzinski wrote:
>
> > I followed up on your suggestion to look into MPI-IO - GREAT suggestion.
> >
> > I found an example at http://beige.ucs.indiana.edu/I590/node92.html and
> added code to gather the pieces of the file read in by each process:
> >
> > MPI_Gather( read_buffer, number_of_bytes, MPI_BYTE, rbuf,
> number_of_bytes, MPI_BYTE, MASTER_RANK, MPI_COMM_WORLD);
> >
> > All process execute this line.  The problem is that number_of_bytes
> maybe different for the last process if  total_number_of_bytes is not a
> multiple of pool_size (i.e., total_number_of_bytes % pool_size != 0).  And
> if the value isn't the same for all processes, you get:
> >
> > Fatal error in PMPI_Gather: Message truncated
> >
> > If I set pool_size (the number of processes) so that
> total_number_of_bytes is a multiple of it (i.e., total_number_of_bytes %
> pool_size == 0), the code executes without error.
> >
> > I thought I read in Peter Pacheco's book that this need not necessarily
> be required?
> >
> > ---John
> >
> >
> > On Fri, Aug 10, 2012 at 9:58 AM, William Gropp <wgropp at illinois.edu>
> wrote:
> > The most likely newbe mistake is that you are timing the time time that
> the MPI_Bcast is waiting - for example, if your code looks like this:
> >
> > if (rank == 0) { tr = MPI_Wtime(); read data tr = MPI_Wtime()-tr; }
> > tb = MPI_Wtime(): MPI_Bcast(…); tb = MPI_Wtime() - tb;
> >
> > then on all but rank 0, you are timing the time that MPI_Bcast is
> waiting for the read data step to finish.  Instead, consider adding an
> MPI_Barrier before the MPI_Bcast:
> >
> > if (rank == 0) { tr = MPI_Wtime(); read data tr = MPI_Wtime()-tr; }
> > MPI_Barrier();
> > tb = MPI_Wtime(): MPI_Bcast(…); tb = MPI_Wtime() - tb;
> >
> > *Only* do this when you are trying to answer such timing questions.
> >
> > You may also want to consider using MPI-IO to parallelize the read step.
> >
> > Bill
> >
> > William Gropp
> > Director, Parallel Computing Institute
> > Deputy Director for Research
> > Institute for Advanced Computing Applications and Technologies
> > Paul and Cynthia Saylor Professor of Computer Science
> > University of Illinois Urbana-Champaign
> >
> >
> >
> > On Aug 10, 2012, at 3:03 AM, John Chludzinski wrote:
> >
> > > I have a problem which requires all process to have a copy of an array
> of data that is read in from a file (process 0).  I Bcast the array to all
> processes (using MPI_COMM_WORLD).
> > >
> > > I instrumented the code with some calls to Wtime to find the time
> consumed for different actions.  In particular, I was interested in
> comparing the time required for Bcast's vs. fread's.  The size of the array
> is 1,200,000 of type MPI_DOUBLE.
> > >
> > > For a 3 process run:
> > >
> > > RANK = 0      fread_time = 2.323575
> > >
> > > vs.
> > >
> > > RANK = 2      bcast_time = 2.361233
> > > RANK = 0      bcast_time = 0.081910
> > > RANK = 1      bcast_time = 2.399790
> > >
> > > These numbers seem to indicate that Bcast-ing the data is as slow as
> reading the data from a file (on my Western Digital Passport USB drive).
>  Am I making a newbie mistake?
> > >
> > > ---John
> > >
> > > PS> I'm using Fedora 16 (32 bit) notebook with a dual core AMD Phenom
> processor.
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