[gmx-developers] mpi and thread command line options

[Roland Schulz]

On Wed, Jul 11, 2012 at 2:33 AM, Alexey Shvetsov
<alexxy at omrb.pnpi.spb.ru> wrote:
> Hi!
>
> mvpich2/mvapich (as well as its derviations like platform
> mpi,pc-mpi,intel-mpi) also will behave differently. So user can get
> cryptic message about launching mpd, in case of launching mdrun -np
> directly
Not quite. mpich2 requires for MPI_Comm_spawn to work that the
application is run with mpirun/mpiexec. See
http://lists.mcs.anl.gov/pipermail/mpich-discuss/2012-June/012638.html
for the details. We would need to detect that and don't try to spawn
in that case (and run in serial with a warning).
Thus mpich2 would require: mpirun mdrun -np x. Of course that isn't
more convenient than mpirun -np x mdrun. The only advantage would be
that as with tmpi "mpirun mdrun" would automatically use as many cores
as are available and are useful for the system, whereas without spawn
the user needs to decides the number of cores and we can't have any
automatic mechanism helping the user.

Roland

>
> Roland Schulz писал 2012-07-11 05:09:
>> 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
>
> --
> Best Regards,
> Alexey 'Alexxy' Shvetsov
> Petersburg Nuclear Physics Institute, NRC Kurchatov Institute,
> Gatchina, Russia
> Department of Molecular and Radiation Biophysics
> Gentoo Team Ru
> Gentoo Linux Dev
> mailto:alexxyum at gmail.com
> mailto:alexxy at gentoo.org
> mailto:alexxy at omrb.pnpi.spb.ru
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