PETSc version 3.16.5
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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 Keys

| -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.

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.

### See Also

PetscGatherNumberOfMessages(), PetscGatherMessageLengths()

### Level

developer

### Location

src/sys/utils/mpits.c
Index of all Sys routines
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Index of all manual pages