Thank you for your response. I spend some time understanding how
MatSetValuesLocal and ISLocalToGlobalMappingCreate work.
Q1) Will the matrix K be of type MATMPIAIJ or MATIS?
K = [A P^T
P 0]
Q2) Can I use both MatSetValues() to MatSetValuesLocal() to populate K? Since I
have already used MatSetValues() to construct A.
Q3) What are the advantages of using MatSetValuesLocal()? Is it that I can
construct P directly using local indies and map the entrees to the global index
in K?
Q4) I probably don’t have to construct an independent P matrix?
Best regards,
Karthik.
From: Matthew Knepley <knep...@gmail.com>
Date: Tuesday, 18 April 2023 at 11:08
To: Chockalingam, Karthikeyan (STFC,DL,HC) <karthikeyan.chockalin...@stfc.ac.uk>
Cc: petsc-users@mcs.anl.gov <petsc-users@mcs.anl.gov>
Subject: Re: [petsc-users] Setting up a matrix for Lagrange multiplier
On Tue, Apr 18, 2023 at 5:24 AM Karthikeyan Chockalingam - STFC UKRI via
petsc-users <petsc-users@mcs.anl.gov<mailto:petsc-users@mcs.anl.gov>> wrote:
Hello,
I'm solving a problem using the Lagrange multiplier, the matrix has the form
K = [A P^T
P 0]
I am familiar with constructing K using MATMPIAIJ. However, I would like to
know if had [A], can I augment it with [P], [P^T] and [0] of type MATMPIAIJ?
Likewise for vectors as well.
Can you please point me to the right resource, if it is a common operation in
PETSc?
You can do this at least 2 ways:
1) Assemble you submatrices directly into the larger matrix by constructing
local-to-global maps for the emplacement. so that you do
not change your assembly code, except to change MatSetValues() to
MatSetValuesLocal(). This is usually preferable.
2) Use MATNEST and VecNEST to put pointers to submatrices and subvectors
directly in.
Thanks,
Matt
Many thanks.
Kind regards,
Karthik.
--
What most experimenters take for granted before they begin their experiments is
infinitely more interesting than any results to which their experiments lead.
-- Norbert Wiener
https://www.cse.buffalo.edu/~knepley/<http://www.cse.buffalo.edu/~knepley/>
