[mpich-discuss] Bcast question?

John Chludzinski john.chludzinski at gmail.com
Fri Aug 10 04:03:36 CDT 2012

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


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?


PS> I'm using Fedora 16 (32 bit) notebook with a dual core AMD Phenom
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