Tpetra::Distributor: Don't fill indicesFrom_
Created by: mhoemmen
@trilinos/tpetra @jjellio @kddevin @vbrunini @crtrott
Tpetra::Distributor would always fill indicesFrom_ with 0, 1, ..., totalReceiveLength_-1. This was not only a waste of time, it would unnecessarily make all reverse-mode communication take the slow path. (Reversing a Distributor swaps indicesTo_ and indicesFrom_. A nonempty indicesTo_ tells Distributor to take the slow path. If indicesFrom_ is just 0, 1, ..., totalReceiveLength_-1, then Distributor does not need to take the slow path.)
Note that Epetra_MpiDistributor comments out the code that fills indices_from_, and claims that doing so fixes reverse communication.
@vbrunini found that applying my patch for fixing this made Aria's post-solve communication of the solution vector of the ViewFactor system a lot faster on 2 GPUs.
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