[Pympi-users] Answer to: Is there any way to gather a local variable into a list on the root?

[Julian C. <rjc...@cs...>]

This is just a post to document something Pat emailed to me

1. You call a parallel function and get back a double called FinalVal e.g.
>>> FinalVal = McTermIncrAccCall(dictInputs)

2. FinalVal is a local variable on each CPU. You can view them all with:
>>> mpi.synchronizedWrite( mpi.rank, FinalVal, "\n" )
0 -291.326756623
1 12.493514467
2 -184.882043854
3 -508.240980905
4 -300.15308712
5 -240.614966934
6 -71.3924387702
7 -113.294540179
8 -462.743023119
9 -269.701236158

2. What you really want to do is concatenate them into a list on the root
processor. To do this you use;
>>> allValues = mpi.gather([FinalVal])
allValues is now a list containing all the local values of FinalVal.
"allvalues" only exists on the machine who's mpi.rank == 0.
Don't ask me why you need to address FinalVal as [FinalVal], possibly a one
element list?, but it definitely works.

Pat Miller:
"this is equivalent to mpi.gather([finalVal],0) and there is an allgather
equivalent too."

Julian Cook