Draft URL: http://meetings.mpi-forum.org/mpi3.0_draft_2.pdf Page 196 5.12 Nonblocking Collective Operations The proposed MPI-3 non-blocking collective operations are designed for better overlapping computation and communication. The matching of these operations is based on the order of the calls instead of using tags. This limits the possibility to use dynamic code/task execution when the order of calls to collective operations should be decided at run-time. When running with multiple threads, our computational framework, Uintah (www.uintah.utah.edu) has to duplicate MPI communicators to ensure collective messages get to the right buffer. The current draft version of the MPI-3 standard states, �The use of tags for collective operations can prevent certain hardware optimizations.� (Page. 108, Line. 14). However, the standard does allow the use of communicators to match collective operations. Does this usage also prevent certain hardware optimizations? Duplicating communicators for message matching is inefficient, as communicators must be reserved before use and also cause additional overhead. We would like to request further consideration in supporting the use of tags in nonblocking collective operations, to avoid the inefficiencies related to communicator duplication. This would not only benefit our computational framework, but other task-based, parallel frameworks that rely on collective operations as we move toward exascale. Qingyu Meng Scientific Computing and Imaging Institute, University of Utah