> On Feb 23, 2015, at 10:20 , Harald Servat <harald.ser...@bsc.es> wrote:
>
> Hello list,
>
> we have several questions regarding calls to collectives using
> intercommunicators. In man for MPI_Bcast, there is a notice for the
> inter-communicator case that reads the text below our questions.
>
> If an I is an intercomunicator for communicattors
Not for communicator but for groups (slight technical difference)
> C1={p1,p2,p3} and C2={p4,p5,p6}, and a process p3 (from C1) wants to
> broadcast a message to C2. Is it mandatory that p1 and p2 have to call to
> MPI_Bcast? Or can the user skip adding these calls?
The MPI_Bcast is collective over all the processes involved in the
communicator. As you pointer out in the text you cited, this is indeed required
by the MPI standard (MPI standard 3.0 page 148 starting from line 43). To be
clear this strictly means that all processes (no exceptions) should call the
MPI_Bcast.
>
> BTW, what is the behavior for the broadcast for p1 and p2 in this case,
> simply return?
It is implementation dependent.
>
> Will MPI fail if MPI_PROC_NULL is not given for the parameter root in p1 and
> p2?
In the best case it will fail. However, as figuring out that multiple roots
exists on the sam group requires communications, I would guess that most of the
MPI implementations will simply have some random unexpected behaviors.
George.
>
> Thank you very much in advance.
>
> ----
>
> ** When Communicator is an Inter-communicator
>
> When the communicator is an inter-communicator, the root process in the first
> group broadcasts data to all the processes in the second group. The first
> group defines the root process. That process uses MPI_ROOT as the value of
> its root argument. The remaining processes use MPI_PROC_NULL as the value of
> their root argument. All processes in the second group use the rank of that
> root process in the first group as the value of their root argument. The
> receive buffer arguments of the processes in the second group must be
> consistent with the send buffer argument of the root process in the first
> group.
>
>
>
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