For large scale systems, we would like to avoid doing a full modex during MPI_Init so that launch will scale a little better. At the moment, our options are somewhat limited as only a few BTLs don't immediately call modex_recv on all procs during startup. However, for those situations where someone can take advantage of it, add the ability to do a "modex on demand" retrieval of data from remote procs when we launch via mpirun.

NOTE: launch performance will be absolutely awful if you do this with BTLs that aren't configured to modex_recv on first message!

Even with "modex on demand", we still have to do a barrier in place of the modex - we simply don't move any data around, which does reduce the time impact. The barrier is required to ensure that the other proc has in fact registered all its BTL info and therefore is prepared to hand over a complete data package. Otherwise, you may not get the info you need. In addition, the shared memory BTL can fail to properly rendezvous as it expects the barrier to be in place.

This behavior will *only* take effect under the following conditions:

1. launched via mpirun

2. #procs is greater than ompi_hostname_cutoff, which defaults to UINT32_MAX

3. mca param rte_orte_direct_modex is set to 1. At the moment, we are having problems getting this param to register properly, so only the first two conditions are in effect. Still, the bottom line is you have to *want* this behavior to get it.

The planned next evolution of this will be to make the direct modex be non-blocking - this will require two fixes:

1. if the remote proc doesn't have the required info, then let it delay its response until it does. This means we need a way for the MPI layer to tell the RTE "I am done entering modex data".

2. adjust the SM rendezvous logic to loop until the required file has been created

Creating a placeholder to bring this over to 1.7.5 when ready.

cmr=v1.7.5:reviewer=hjelmn:subject=Enable direct modex at scale