[mpich-discuss] Bcast question?

[John Chludzinski]

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.
> > _______________________________________________
> > mpich-discuss mailing list     mpich-discuss at mcs.anl.gov
> > To manage subscription options or unsubscribe:
> > https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/mpich-discuss/attachments/20120811/2ffd4503/attachment.html>