(keeping the user's list in the CC)
> On Mar 24, 2017, at 4:05 AM, gzzh...@buaa.edu.cn wrote:
>
> hi jeff:
> I tried to call MPI_Comm_spawn("./child", MPI_ARGV_NULL, 1,
> MPI_INFO_NULL, root, MPI_COMM_WORLD, &newcomm, &errs)
> in order every MPI process in MPI_COMM_WORLD can spawn one child process.
The MPI_COMM_SPAWN call is collective, meaning that all processes in the source
communicator have to invoke the call with the same parameters. Meaning: in
your example, this will launch exactly one additional process. Hence, if after
MPI_INIT MPI_COMM_WORLD has N processes, if you immediately make this call to
MPI_COMM_SPAWN, you will have a total of (N+1) processes.
> Then all of these children processes are expected
> to form their own MPI_COMM_WORLD even they are located on multiple nodes.
In your example, there will only be 1 child process launched.
> I found if the parameter "root" was a same value in every MPI process, for
> example root =1, there is just one MPI process "rank=1"
> can really spawn its child process, although other MPI processes call this
> function but didn't create their child process.
That is correct. That is the nature of a collective MPI call.
> When I changed the value of root to every MPI process itself's rank, like
> this:
> MPI_Comm_spawn("./child", MPI_ARGV_NULL, 1, MPI_INFO_NULL, rank,
> MPI_COMM_WORLD, &newcomm, &errs)
> some wrong occured. I don't quite understand.
That is also correct behavior. MPI-3.1 section 2.4, p11:42-45 initially
defines the term "collective":
-----
collective A procedure is collective if all processes in a process group need
to invoke the procedure. A collective call may or may not be synchronizing.
Collective calls over the same communicator must be executed in the same order
by all members of the process group.
-----
MPI_COMM_SPAWN is a collective operation in that all processes in the
communicator come together to perform one action (i.e., spawn one or more
children). Put differently: you cannot invoke MPI_COMM_SPAWN in one process in
a communicator without also invoking it (with the same parameters) in all
others processes in the same communicator.
--
Jeff Squyres
jsquy...@cisco.com
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