Yes, this does sound like the classic "assuming MPI buffering" case.
Check out this magazine column that I wrote a long time ago about this
topic:
http://cw.squyres.com/columns/2004-08-CW-MPI-Mechanic.pdf
It's #1 on the top 10 list of All-Time Favorite Evils to Avoid in
Parallel. :-)
One comment on Mattijs's email: please don't use bsend. Bsend is
evil. :-)
On Jun 13, 2008, at 5:27 AM, Mattijs Janssens wrote:
Sounds like a typical deadlock situation. All processors are waiting
for one
another.
Not a specialist but from what I know if the messages are small
enough they'll
be offloaded to kernel/hardware and there is no deadlock. That why
it might
work for small messages and/or certain mpi implementations.
Solutions:
- come up with a global communication schedule such that if one
processor
sends the receiver is receiving.
- use mpi_bsend. Might be slower.
- use mpi_isend, mpi_irecv (but then you'll have to make sure the
buffers stay
valid for the duration of the communication)
On Friday 13 June 2008 01:55, zach wrote:
I have a weird problem that shows up when i use LAM or OpenMPI but
not
MPICH.
I have a parallelized code working on a really large matrix. It
partitions the matrix column-wise and ships them off to processors,
so, any given processor is working on a matrix with the same number
of
rows as the original but reduced number of columns. Each processor
needs to send a single column vector entry
from its own matrix to the adjacent processor and visa versa as part
of the algorithm.
I have found that depending on the number of rows of the matrix -or,
the size of the vector being sent using MPI_Send, MPI_Recv, the
simulation will hang.
It is only until i reduce this dimension to a certain max number will
the sim run properly. I have also found that this magic number
differs
depending on the system I am using, eg my home quad-core box or
remote
cluster.
As i mentioned i have not had this issue with mpich. I would like to
understand why it is happening rather than just defect over to mpich
to get by.
Any help would be appreciated!
zach
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Jeff Squyres
Cisco Systems
