petsc-3.7.7 2017-09-25

PetscCommBuildTwoSided

discovers communicating ranks given one-sided information, moving constant-sized data in the process (often message lengths)

Synopsis

#include "petscsys.h"  
PetscErrorCode PetscCommBuildTwoSided(MPI_Comm comm,PetscMPIInt count,MPI_Datatype dtype,PetscMPIInt nto,const PetscMPIInt *toranks,const void *todata,PetscMPIInt *nfrom,PetscMPIInt **fromranks,void *fromdata)

Collective on MPI_Comm

Input Arguments

	comm	- communicator
	count	- number of entries to send/receive (must match on all ranks)
	dtype	- datatype to send/receive from each rank (must match on all ranks)
	nto	- number of ranks to send data to
	toranks	- ranks to send to (array of length nto)
	todata	- data to send to each rank (packed)

Output Arguments

	nfrom	- number of ranks receiving messages from
	fromranks	- ranks receiving messages from (length nfrom; caller should PetscFree())
	fromdata	- packed data from each rank, each with count entries of type dtype (length nfrom, caller responsible for PetscFree())

Options Database Keys

-build_twosided <allreduce|ibarrier|redscatter> -algorithm to set up two-sided communication

Notes

This memory-scalable interface is an alternative to calling PetscGatherNumberOfMessages() and PetscGatherMessageLengths(), possibly with a subsequent round of communication to send other constant-size data.

Basic data types as well as contiguous types are supported, but non-contiguous (e.g., strided) types are not.

References

1. -Hoefler, Siebert and Lumsdaine, The MPI_Ibarrier implementation uses the algorithm in Scalable communication protocols for dynamic sparse data exchange, 2010.