The collective in Open MPI works differently than in MPICH. They are dynamically selected based on the number of processes involved and the amount of data to be exchanged. Therefore, it is difficult to answer your question without knowing this information.
There are 4 algorithms for MPI_Allgather in Open MPI:
- recursive doubling
- neighbor exchange
I think their complexity is described in "Performance analysis of MPI collective operations" (http://www.springerlink.com/content/542207241006p64h/).
On Oct 29, 2010, at 15:42 , Tim Stitt wrote:
> Dear OpenMPI Developers,
> I would be grateful if someone could briefly describe the cost (complexity) for the allgatherv() collective operation in the current release of OpenMPI.
> For MPICH2 I believe the cost is ceiling(lg p). Can anyone comment on the algorithms and cost used in the OpenMPI implementation?
> Thanks in advance,
> devel mailing list