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# 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

## Input Parameters

- 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 Parameters

- 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 Key

- -build_twosided <allreduce|ibarrier|redscatter> - algorithm to set up two-sided communication. Default is allreduce for communicators with <= 1024 ranks, otherwise ibarrier.

## 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, see [HSL10].

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

## References

[HSL10]

T. Hoefler, C. Siebert, and A. Lumsdaine. Scalable Communication Protocols for Dynamic Sparse Data Exchange. In Proceedings of the 2010 ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming (PPoPP'10), 159–168. ACM, January 2010.

## See Also

`PetscGatherNumberOfMessages()`, `PetscGatherMessageLengths()`, `PetscCommBuildTwoSidedSetType()`, `PetscCommBuildTwoSidedType`

## Level

developer

## Location

src/sys/utils/mpits.c

Index of all Sys routines
Table of Contents for all manual pages
Index of all manual pages