[gmx-developers] MPI Datatypes and TMPI

[Roland Schulz]

Hi,

we are looking how to best tackle the issue of packing data for
MPI communication in 5.0. We believe this is an important issue because the
two current approaches (serializing in serial buffer (e.g. global_stat) or
sending in many small messages (e.g. initial bcast)) are both slow and
produces difficult to read code code (global_stat). The many
small messages is for large systems a serious issue for scaling and even
the serializing is unnecessary slow because it means
a potentially unnecessary copy.

We first looked into Boost::MPI but while it is very nice in some aspects,
it also has disadvantages. Thus we're looking at alternatives.
Most interesting alternatives use MPI Datatypes to get high performance and
avoid the unnecessary copy of serialization. The problem is that TMPI
doesn't support MPI Datatypes.

Thus my question: Is it planned to add Datatypes to TMPI? If not is TMPI
still required in 5.0? Would it be sufficient to support OpenMP for non-MPI
multi-core installations in 5.0? What was the reason for TMPI in the first
place? Why did we not just bundle e.g. OpenMPI for those users missing an
MPI implementation?

Roland

-- 
ORNL/UT Center for Molecular Biophysics cmb.ornl.gov
865-241-1537, ORNL PO BOX 2008 MS6309
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