[MPICH] Changing the comm size at runtime

[Rajeev Thakur]

If the slaves can connect to the master with a single collective
MPI_Comm_connect (i.e. the communicator includes all slaves), then it is
easy: A single MPI_Intercomm_merge will create the giant intracommunicator.
If the slaves are connecting one by one, it is much more difficult. Here is
an example of how to do it:

Let's say Process A is the master and processes B, C, and D are slaves. B,
C, D individually connect to A, resulting in 3 intercommunicators: AB_inter,
AC_inter, and AD_inter. To merge them all into a single intracommunicator:

* begin by doing an MPI_Intercomm_merge on AB_inter, resulting in an
intracommunicator AB_intra.

* then create an intercommunicator between AB on one side and C on the other
by using MPI_Intercomm_create. Pass AB_intra as the local_comm on A and B,
MPI_COMM_WORLD as the intracomm on C, and AC_inter as the peer_comm. This
results in the intercommunicator AB_C_inter. 

* then call MPI_Intercomm_merge on it to create the intracommunicator
ABC_intra. 

* then call MPI_Intercomm_create to create an intercommunicator between ABC
and D just as you did with AB and C above. 

* then call MPI_Intercomm_merge to get a single intracommunicator containing
all.

Rajeev 


> -----Original Message-----
> From: owner-mpich-discuss at mcs.anl.gov 
> [mailto:owner-mpich-discuss at mcs.anl.gov] On Behalf Of Patrick Gräbel
> Sent: Sunday, March 18, 2007 2:02 PM
> To: mpich-discuss at mcs.anl.gov
> Subject: Re: [MPICH] Changing the comm size at runtime
> 
> At the moment the master iterates over a set of Intracomm objects that
> are created by calling MPI_Intercomm_Merge on the accepted Intercomm
> objects in order to communicate. This works but...
> 
> ...is there a way to form a larger Intracomm (or Intercomm) 
> object that
> contains all slaves at once? This would make the programm more elegant
> and smaller.
> 
> Greetings
> Patrick
> 
> Rajeev Thakur wrote:
> > I should point out that you can also create an 
> intracommunicator from an
> > intercommunicator by using MPI_Intercomm_merge and then use 
> the regular
> > intracommunicator collectives if that's what you need.
> > 
> > Rajeev
> 
>