I sent mail asking for comment and received very little. The current text contains a simple fixed crossover version. More discussion is needed. Here is an excerpt from the mail I sent.
1. Determining the cross over point from short to tree gather/scatters? There was a fair amount of discussion on how to determine this and no consensus was reached. The following alternatives represent the various ideas put forward and are offered up for debate. If I have missed any please post them. I have tried to list them in order of increasing complexity. a. fixed size specified by the standard b. each implementation at startup provides their idea of the size and the crossover is taken to be the min (or max?) c. the user specifies it via an option to mpirun. In this case a subsidiary issue is whether this is a required option or whether we fall back to (a) or (b) when it is not specified. d. at startup time implementations supply local latency/bw estimates and a global latency/bw estimate is obtained from a config file or some other mechanism. The crossover is then calculated from these via some yet to be determined formula.