CODEWITHSUNDEEP
cmpi
jules
jules

Reputation: 63

MPI Communicator MPI_Comm_split without Groups

I'm learning about MPI using C in class. I'm currently trying to get my head around when you'd use MPI_Comm_split and what the advantages are. I'm also trying to figure out if there are any benefits of using this over just using MPI_COMM_WORLD if I don't use groups or if all my processes are in one group.

From what I understand, MPI_Comm_split provides a unique communicator to each 'color' which is different from using MPI_COMM_WORLD (not sure if this last part is correct). MPI groups are a set of processes and communicators help you to communicate within and between these groups.

I was looking at the comment on this question

For example, given a shape e.g. square, the program splits MPI_COMM_WORLD into two groups grpArea and grpPerimeter which compute the area and perimeter of the given shape. These groups should do their private computations and return result to root(rank 0) of MPI_COMM_WORLD.

Let's say that instead of calculating the area and perimeter, we only calculated the area and so we don't use groups. Would there be any point in splitting the communicators using MPI_Comm_split so that each processor has its own communicator? Are there any benefits or drawbacks of doing so?

Upvotes: 0

Views: 765

Answers (1)

Zulan
Zulan

Reputation: 22670

You are basically asking whether it makes sense to use MPI_Comm_split without having different groups. No it does not. It doesn't provide any benefits or make any sense.

It can be useful to use separate communicators of the same group for two reasons:

  • Have private communicators for different modules in the application that may have overlapping tags. That way, you ensure that there is no mismatching. For this case, use MPI_Comm_dup.
  • Use topologies. In that case, use the appropriate constructor e.g. MPI_Cart_create.

There may be some overhead in creating excess communicators. Further you may lose specific optimizations for MPI_COMM_WORLD. That shouldn't stop you from using communicators when it makes sense.

Upvotes: 2

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