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<div id="version" align=right><b>petsc-3.14.5 2021-03-03</b></div>
<div id="bugreport" align=right><a href="mailto:petsc-maint@mcs.anl.gov?subject=Typo or Error in Documentation &body=Please describe the typo or error in the documentation: petsc-3.14.5 v3.14.5 docs/manualpages/IS/ISAllGather.html "><small>Report Typos and Errors</small></a></div>
<A NAME="ISAllGather"><H1>ISAllGather</H1></A>
Given an index set (<A HREF="../IS/IS.html#IS">IS</A>) on each processor, generates a large index set (same on each processor) by concatenating together each processors index set.
<H3><FONT COLOR="#CC3333">Synopsis</FONT></H3>
<PRE>
#include "petscis.h"
<A HREF="../Sys/PetscErrorCode.html#PetscErrorCode">PetscErrorCode</A> <A HREF="../IS/ISAllGather.html#ISAllGather">ISAllGather</A>(<A HREF="../IS/IS.html#IS">IS</A> is,<A HREF="../IS/IS.html#IS">IS</A> *isout)
</PRE>
Collective on <A HREF="../IS/IS.html#IS">IS</A>
<P>
<H3><FONT COLOR="#CC3333">Input Parameter</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>is </B></TD><TD>- the distributed index set
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Output Parameter</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>isout </B></TD><TD>- the concatenated index set (same on all processors)
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Notes</FONT></H3>
<A HREF="../IS/ISAllGather.html#ISAllGather">ISAllGather</A>() is clearly not scalable for large index sets.
<P>
The <A HREF="../IS/IS.html#IS">IS</A> created on each processor must be created with a common
communicator (e.g., <A HREF="../Sys/PETSC_COMM_WORLD.html#PETSC_COMM_WORLD">PETSC_COMM_WORLD</A>). If the index sets were created
with <A HREF="../Sys/PETSC_COMM_SELF.html#PETSC_COMM_SELF">PETSC_COMM_SELF</A>, this routine will not work as expected, since
each process will generate its own new <A HREF="../IS/IS.html#IS">IS</A> that consists only of
itself.
<P>
The communicator for this new <A HREF="../IS/IS.html#IS">IS</A> is <A HREF="../Sys/PETSC_COMM_SELF.html#PETSC_COMM_SELF">PETSC_COMM_SELF</A>
<P>
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
<A HREF="../IS/ISCreateGeneral.html#ISCreateGeneral">ISCreateGeneral</A>(), <A HREF="../IS/ISCreateStride.html#ISCreateStride">ISCreateStride</A>(), <A HREF="../IS/ISCreateBlock.html#ISCreateBlock">ISCreateBlock</A>()
<BR><P><B></B><H3><FONT COLOR="#CC3333">Level</FONT></H3>intermediate<BR>
<H3><FONT COLOR="#CC3333">Location</FONT></H3>
</B><A HREF="../../../src/vec/is/is/utils/iscoloring.c.html#ISAllGather">src/vec/is/is/utils/iscoloring.c</A>
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