Let me clarify one thing, When I said "there are q-1 groups that can communicate in parallel at the same time" I meant that this is possible at any particular time. So at the beginning we have q-1 groups that could communicate in parallel, then another set of q-1 groups and so on until we exhaust all groups. My hope is that the speedup can be such that the total number of broadcasts i.e. [q^(k-1)]*(q-1)*k to be executed in time equivalent to only [q^(k-1)]*k broadcasts.
Cheers, Kostas. On Tue, Oct 31, 2017 at 10:42 PM, Konstantinos Konstantinidis < kostas1...@gmail.com> wrote: > Assume that we have K=q*k nodes (slaves) where q,k are positive integers > >= 2. > > Based on the scheme that I am currently using I create [q^(k-1)]*(q-1) > groups (along with their communicators). Each group consists of k nodes and > within each group exactly k broadcasts take place (each node broadcasts > something to the rest of them). So in total [q^(k-1)]*(q-1)*k MPI > broadcasts take place. Let me skip the details of the above scheme. > > Now theoretically I figured out that there are q-1 groups that can > communicate in parallel at the same time i.e. groups that have no common > nodes and I would like to utilize that to speedup the shuffling. I have > seen here https://stackoverflow.com/questions/11372012/mpi-severa > l-broadcast-at-the-same-time that this is possible in MPI. > > In my case it's more complicated since q,k are parameters of the problem > and change between different experiments. If I get the idea about the 2nd > method that is proposed there and assume that we have only 3 groups within > which some communication takes places one can simply do: > > *if my rank belongs to group 1{* > * comm1.Bcast(..., ..., ..., rootId);* > *}else if my rank belongs to group 2{* > * comm2.Bcast(..., ..., ..., rootId);* > *}else if my rank belongs to group3{* > * comm3.Bcast(..., ..., ..., rootId);* > *} * > > where comm1, comm2, comm3 are the corresponding sub-communicators that > contain only the members of each group. > > But how can I generalize the above idea to arbitrary number of groups or > perhaps do something else? > > The code is in C++ and the MPI installed is described in the attached file. > > Regards, > Kostas > >
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