[gmx-developers] mpi and thread command line options

[Roland Schulz]

On Tue, Jul 10, 2012 at 8:09 PM, Szilárd Páll <szilard.pall at cbr.su.se> wrote:
> On Tue, Jul 10, 2012 at 11:15 PM, Berk Hess <hess at kth.se> wrote:
>> Hi,
>>
>> We are working on the final part of the 4.6 release, which is making the MPI
>> and OpenMP thread setup automated, fully checked and user friendly.
>> We have to decide on the naming of the options.
>> Roland has an implementation of mpi spawn ready. This would allow to do
>> mdrun -np #processes instead of using mpirun (at least with openmpi).
>
> Would this feature add anything but the convenience of being able to
> run without mpirun on a single node? Without MPI spawning working
> reliably in most cases (or with the ability to detect with a high
> certainty when it does not), enabling an -np mdrun option would just
> lead to confusion when mdrun exits with cryptic MPI error due to not
> being able to spawn.
The idea was to make mdrun behave the same whether it is compiled with
real MPI or tMPI. Thus also only support a single node. But MPICH is
behaving quite stupid and they also don't seem to care. And only
supporting it for OpenMPI is probably also more confusing then helpful
(then tmpi+OpenMPI would behave the same but MPICH/MVAPICH would
behave different). So you are probably right that it is better to not
add spawn at all.

> Therefore, I'd be OK with a new *hidden* -np option that only works in
> single-node case, but not with a non-hidden one advertised in the
> documentation/wiki.
As a hidden option it would only help for testing. But I don't think
it is worth adding it for just that.

Roland