Hi This comment is about example 6.36 of MPI 4.1 document (on page 303) which concerns the usage of collective operations with overlapping communicators. I have already posted my comment as a question on stackoverflow.com, so I won't repeat it here. Here is the link to the post: https://stackoverflow.com/questions/77844104/overlapping-communicators-and-c... The link to the standard: https://www.mpi-forum.org/docs/mpi-4.1/mpi41-report.pdf Kind regards, Arham