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<div id="version" align=right><b>petsc-3.14.5 2021-03-03</b></div>
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<A NAME="PetscCommBuildTwoSided"><H1>PetscCommBuildTwoSided</H1></A>
discovers communicating ranks given one-sided information, moving constant-sized data in the process (often message lengths)
<H3><FONT COLOR="#CC3333">Synopsis</FONT></H3>
<PRE>
#include "petscsys.h"
<A HREF="../Sys/PetscErrorCode.html#PetscErrorCode">PetscErrorCode</A> <A HREF="../Sys/PetscCommBuildTwoSided.html#PetscCommBuildTwoSided">PetscCommBuildTwoSided</A>(<A HREF="../Sys/MPI_Comm.html#MPI_Comm">MPI_Comm</A> comm,<A HREF="../Sys/PetscMPIInt.html#PetscMPIInt">PetscMPIInt</A> count,MPI_Datatype dtype,<A HREF="../Sys/PetscMPIInt.html#PetscMPIInt">PetscMPIInt</A> nto,const <A HREF="../Sys/PetscMPIInt.html#PetscMPIInt">PetscMPIInt</A> *toranks,const void *todata,<A HREF="../Sys/PetscMPIInt.html#PetscMPIInt">PetscMPIInt</A> *nfrom,<A HREF="../Sys/PetscMPIInt.html#PetscMPIInt">PetscMPIInt</A> **fromranks,void *fromdata)
</PRE>
Collective
<P>
<H3><FONT COLOR="#CC3333">Input Arguments</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>comm </B></TD><TD>- communicator
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>count </B></TD><TD>- number of entries to send/receive (must match on all ranks)
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>dtype </B></TD><TD>- datatype to send/receive from each rank (must match on all ranks)
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>nto </B></TD><TD>- number of ranks to send data to
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>toranks </B></TD><TD>- ranks to send to (array of length nto)
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>todata </B></TD><TD>- data to send to each rank (packed)
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Output Arguments</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>nfrom </B></TD><TD>- number of ranks receiving messages from
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>fromranks </B></TD><TD>- ranks receiving messages from (length nfrom; caller should <A HREF="../Sys/PetscFree.html#PetscFree">PetscFree</A>())
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>fromdata </B></TD><TD>- packed data from each rank, each with count entries of type dtype (length nfrom, caller responsible for <A HREF="../Sys/PetscFree.html#PetscFree">PetscFree</A>())
</TD></TR></TABLE>
<P>
<P>
<H3><FONT COLOR="#CC3333">Options Database Keys</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-build_twosided <allreduce|ibarrier|redscatter> </B></TD><TD>- algorithm to set up two-sided communication. Default is allreduce for communicators with <= 1024 ranks, otherwise ibarrier.
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Notes</FONT></H3>
This memory-scalable interface is an alternative to calling <A HREF="../Sys/PetscGatherNumberOfMessages.html#PetscGatherNumberOfMessages">PetscGatherNumberOfMessages</A>() and
<A HREF="../Sys/PetscGatherMessageLengths.html#PetscGatherMessageLengths">PetscGatherMessageLengths</A>(), possibly with a subsequent round of communication to send other constant-size data.
<P>
Basic data types as well as contiguous types are supported, but non-contiguous (e.g., strided) types are not.
<P>
<H3><FONT COLOR="#CC3333">References</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>1. </B></TD><TD>- Hoefler, Siebert and Lumsdaine, The MPI_Ibarrier implementation uses the algorithm in
Scalable communication protocols for dynamic sparse data exchange, 2010.
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
<A HREF="../Sys/PetscGatherNumberOfMessages.html#PetscGatherNumberOfMessages">PetscGatherNumberOfMessages</A>(), <A HREF="../Sys/PetscGatherMessageLengths.html#PetscGatherMessageLengths">PetscGatherMessageLengths</A>()
<BR><P><B></B><H3><FONT COLOR="#CC3333">Level</FONT></H3>developer<BR>
<H3><FONT COLOR="#CC3333">Location</FONT></H3>
</B><A HREF="../../../src/sys/utils/mpits.c.html#PetscCommBuildTwoSided">src/sys/utils/mpits.c</A>
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