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For the last several months, we have supported three modes of sending the
xcast messages used to release MPI processes from their various stage gates:
1. Direct - message sent directly to each process in a serial fashion
2. Linear - message sent serially to the daemon on each node, which then
"fans" it out to the application procs on that node
3. Binomial - message sent via binomial tree algorithm to the daemon on each
node, which then "fans" it out to the local application procs
To maintain historical consistency, we have defaulted to "direct". However,
this is not the more scalable mode.
We propose to leave all three of these modes in the system, but to change
the default on the OMPI trunk to "linear" so that it will be tested more
thoroughly by the automated test suite.
Please voice any comments and/or objections. Assuming there is agreement, we
will make the switch (solely on the OMPI trunk - this will not impact the
1.2 series) on June 1.