If the application code has to take full responsibility for the scheduling, and how it is constructed given what is available in the MPI port for a particular machine, then portability will suffer. This service would of course be compatible with and interract with the real-time OS that is scheduling processes at the nodes. We discussed some of the different hardware features that might support real-time, e.g. priority levels, preemption, packetized messages, multiple routes, virtual channels, guaranteed bandwidth allocation.
Portability is the issue.
Most DoD systems are time-driven. In this approach, the application communicates to the middleware the timing requirements, and the middleware ensures that these requirements are met. The requirements imposed on MPI to make this approach work are unclear. Alternatively, MPI may be required to perform all communication according to a global schedule provided by the middleware. How such a schedule would work with the existing definition of MPI is not clear.