[mvapich-discuss] Stupid question about MPI Communicators

[Mike Heinz]

Jim,

If I have an MPI job running across multiple physical nodes, how do they ensure they get the same context id?

-----Original Message-----
From: mvapich-discuss-bounces at cse.ohio-state.edu [mailto:mvapich-discuss-bounces at cse.ohio-state.edu] On Behalf Of James Dinan
Sent: Thursday, June 09, 2011 3:37 PM
To: mvapich-discuss at cse.ohio-state.edu
Subject: Re: [mvapich-discuss] Stupid question about MPI Communicators

Hi Mike,

Nope, communication cannot pass from one communicator into another.
Communicators contain both a group and a context ID and this context ID is effectively used as a second tag when matching communication operations.  If you were to call MPI_Comm_compare on such communicators, MPI will tell you MPI_CONGRUENT if both communicators were built with the same MPI group or MPI_SIMILAR, but not MPI_IDENT(ical).

Best,
  ~Jim.

On 6/9/11 1:28 PM, Mike Heinz wrote:
> If I create two communicators with the exact same members, are they considered to be the same communicator?  That is, if I create Communicator A and Communicator B with the same members, and Rank-a sends via A, will Rank b receive it while listening to Communicator B?
>
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