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<A NAME="SAMappingSetSizes"><H1>SAMappingSetSizes</H1></A>
Sets the local and global sizes for the domain and range, and checks to determine compatibility
<H3><FONT COLOR="#CC3333">Synopsis</FONT></H3>
<PRE>
#include "petscsa.h"
PetscErrorCode SAMappingSetSizes(SAMapping map, PetscInt m, PetscInt n, PetscInt M, PetscInt N)
</PRE>
Collective on SAMapping
<P>
<H3><FONT COLOR="#CC3333">Input Parameters</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>map </B></TD><TD>- the mapping
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>m </B></TD><TD>- number of local domain indices (or <A HREF="../Sys/PETSC_DETERMINE.html#PETSC_DETERMINE">PETSC_DETERMINE</A>)
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>n </B></TD><TD>- number of local range columns (or <A HREF="../Sys/PETSC_DETERMINE.html#PETSC_DETERMINE">PETSC_DETERMINE</A>)
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>M </B></TD><TD>- number of global domain indices (or <A HREF="../Sys/PETSC_DETERMINE.html#PETSC_DETERMINE">PETSC_DETERMINE</A> or <A HREF="../Sys/PETSC_IGNORE.html#PETSC_IGNORE">PETSC_IGNORE</A>)
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>N </B></TD><TD>- number of global range indices (or <A HREF="../Sys/PETSC_DETERMINE.html#PETSC_DETERMINE">PETSC_DETERMINE</A> or <A HREF="../Sys/PETSC_IGNORE.html#PETSC_IGNORE">PETSC_IGNORE</A>)
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Notes</FONT></H3>
The sizes specify (i) what the domain and range for the graph underlying SAMapping is,
and (b) what the parallel layout of the domain and range are. The local and global sizes
m and M (and n and N) are not independent. Two situations are possible the domain
<H3><FONT COLOR="#CC3333">(and,analogously, for the range)</FONT></H3>
<P>
(P) Parallel layout for the domain demands that the local values of m on all ranks of
the communicator add up to M (see more at <A HREF="../Mat/MatSetSizes.html#MatSetSizes">MatSetSizes</A>, for example). Thus, m and M must
be specified in this compatible way. One way to ensure this is to specify m and leave
M as <A HREF="../Sys/PETSC_DETERMINE.html#PETSC_DETERMINE">PETSC_DETERMINE</A> -- then M is computed by summing the local m across the ranks.
The other option is to specify M (the same on all ranks, which will be checked) and
leave m as <A HREF="../Sys/PETSC_DETERMINE.html#PETSC_DETERMINE">PETSC_DETERMINE</A>. In this case the local m is determined by dividing M as
equally as possible among the ranks (m might end up being 0). If both m and M are specified,
their compatibility is verified by summing the m across the ranks. If m or M are <A HREF="../Sys/PETSC_DETERMINE.html#PETSC_DETERMINE">PETSC_DETERMINE</A>
on one rank, they must be <A HREF="../Sys/PETSC_DETERMINE.html#PETSC_DETERMINE">PETSC_DETERMINE</A> on all of the ranks, or the program might hang.
Finally, both m and M cannot be <A HREF="../Sys/PETSC_DETERMINE.html#PETSC_DETERMINE">PETSC_DETERMINE</A> at once.
<P>
In any case, domain indices can have any value 0 <= i < M on every rank (with the same M).
However, domain indices are split up among the ranks: each rank will "own" m indices with
the indices owned by rank 0 numbered [0,m), followed by the indices on rank 1, and so on.
<P>
(S) Sequential layout for the domain makes it essentially into a disjoint union of local
domains of local size m. This is signalled by specifying M as <A HREF="../Sys/PETSC_IGNORE.html#PETSC_IGNORE">PETSC_IGNORE</A>.
<P>
In this case, domain indices can have any value 0 <= i < m on every rank (with its own m).
<P>
<H3><FONT COLOR="#CC3333">Assembly/Mapping</FONT></H3>
Whether the domain is laid out in parallel (P) or not (S), determines the behavior of SAMapping
during assembly. In case (P), the edges of the underlying graph are migrated to the rank that
owns the corresponding domain indices. SAMapping can map indices lying in its local range,
which is a subset of its local domain. This means that due to parallel assembly edges inserted
by different ranks might be used during the mapping. This is completely analogous to matrix
assembly.
<P>
When the domain is not laid out in parallel, no migration takes place and the mapping of indices
is done purely locally.
<P>
<P>
<P>
<H3><FONT COLOR="#CC3333">Support/Image</FONT></H3>
Observe that the support and image of the graph may be strictly smaller than its domain and range,
if no edges from some domain points (or to some range points) are added to SAMapping.
<P>
<H3><FONT COLOR="#CC3333">Operator</FONT></H3>
Observe also that the linear operator defined by SAMapping will behave essentially as a <A HREF="../Vec/VecScatter.html#VecScatter">VecScatter</A>
(i) between MPI vectors with sizes (m,M) and (n,N), if both the domain and the range are (P),
(ii) between an MPI <A HREF="../Vec/Vec.html#Vec">Vec</A> with size (m,M) and a collection of SEQ <A HREF="../Vec/Vecs.html#Vecs">Vecs</A> (one per rank) of local size (n),
if the domain is (P) and the range is (S),
(iii) between a collection of SEQ <A HREF="../Vec/Vecs.html#Vecs">Vecs</A> (one per rank) of local size (m) and an MPI <A HREF="../Vec/Vec.html#Vec">Vec</A> of size (n,N),
if the domain is (S) and the range is (P),
(iv) between collections of SEQ <A HREF="../Vec/Vecs.html#Vecs">Vecs</A> (one per rank) or local sizes (m) and (n), if both the domain
and the range are (S).
<P>
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
SAMappingGetSizes(), SAMappingGetSupportSize(), SAMappingGetImageSize(), SAMappingMapIndicesLocal()
<BR><P><B><P><B><FONT COLOR="#CC3333">Level:</FONT></B>beginner
<BR><FONT COLOR="#CC3333">Location:</FONT></B><A HREF="../../../src/dm/sa/interface/sa.c.html#SAMappingSetSizes">src/dm/sa/interface/sa.c</A>
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