[petsc-dev] Nonblocking consensus and alternative "freespace" interface

[Jed Brown]

I pushed PetscCommBuildTwoSided() which is a scalable interface for setting
up two-sided communication given one-sided information.

https://bitbucket.org/petsc/petsc-dev/commits/721cf8e8a07d6de2abc9b4f621f09a70e0d48012#Lsrc/sys/utils/mpits.cT257

The default interface uses the O(size) MPI_Reduce, but if you have an MPI-3
implementation (providing MPI_Ibarrier), it uses the nonblocking consensus
algorithm from

http://www.unixer.de/publications/img/hoefler-dsde-protocols.pdf

You can revert to the old implementation using -build_twosided ALLREDUCE
(versus IBARRIER). I'd like to eventually update most uses of
PetscGatherNumberOfMessages() and PetscGatherMessageLengths() to use this
interface, and to use it in new code whenever possible.

In the implementation, I wrote a utility similar to PetscFreeSpace, but
with what I think is a safer and more generic interface. It's currently
called SegArray, but I'd be happy to change the name (maybe StreamBuffer).
It's essentially used to buffer a chunked stream (with low-latency
incremental allocation) and later return it as a contiguous array. I'd
appreciate comments on the API before moving it to include/petsc-private.

https://bitbucket.org/petsc/petsc-dev/commits/721cf8e8a07d6de2abc9b4f621f09a70e0d48012#Lsrc/sys/utils/mpits.cT42
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