[petsc-users] question about MatSetLocalToGlobalMapping

[Pierre Jolivet]

Hong,
1) Is there any hope to get PDIPDM to use a MatNest?
2) Is this fixed https://lists.mcs.anl.gov/pipermail/petsc-dev/2020-September/026398.html ?
I cannot get users to transition away from Ipopt because of these two missing features.

Thanks,
Pierre

> On 20 Apr 2023, at 5:47 PM, Zhang, Hong via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> Karthik,
> We built a KKT matrix in  TaoSNESJacobian_PDIPM() (see petsc/src/tao/constrained/impls/ipm/pdipm.c) which assembles several small matrices into a large KKT matrix in mpiaij format. You could take the same approach to insert P and P^T into your K.
> FYI, I attached our paper.
> Hong
> From: petsc-users <petsc-users-bounces at mcs.anl.gov> on behalf of Matthew Knepley <knepley at gmail.com>
> Sent: Thursday, April 20, 2023 5:37 AM
> To: Karthikeyan Chockalingam - STFC UKRI <karthikeyan.chockalingam at stfc.ac.uk>
> Cc: petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
> Subject: Re: [petsc-users] question about MatSetLocalToGlobalMapping
>  
> On Thu, Apr 20, 2023 at 6:13 AM Karthikeyan Chockalingam - STFC UKRI via petsc-users <petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov>> wrote:
> Hello,
>  
> I created a new thread, thought would it be more appropriate (and is a continuation of my previous post). I want to construct the below K matrix (which is composed of submatrices)
>  
> K = [A P^T
>        P   0]
>  
> Where K is of type MatMPIAIJ. I first constructed the top left [A] using MatSetValues().
>  
> Now, I would like to construct the bottom left [p] and top right [p^T] using MatSetValuesLocal().
>  
> To use  MatSetValuesLocal(),  I first have to create a local-to-global mapping using ISLocalToGlobalMappingCreate. I have created two mapping row_mapping and column_mapping.
> 
> I do not understand why they are not the same map. Maybe I was unclear before. It looks like you have two fields, say phi and lambda, where lambda is a Lagrange multiplier imposing some constraint. Then you get a saddle point like this. You can imagine matrices
> 
>   (phi, phi)        --> A
>   (phi, lambda) --> P^T
>   (lambda, phi) --> P
> 
> So you make a L2G map for the phi field and the lambda field. Oh, you are calling them row and col map, but they are my phi and lambda
> maps. I do not like the row and col names since in P they reverse.
>  
> Q1) At what point should I declare MatSetLocalToGlobalMapping – is it just before I use MatSetValuesLocal()?
> 
> Okay, it is good you are asking this because my thinking was somewhat confused. I think the precise steps are:
> 
>   1) Create the large saddle point matrix K
> 
>     1a) We must call https://petsc.org/main/manualpages/Mat/MatSetLocalToGlobalMapping/ on it. In the simplest case, this just maps
>            the local rows numbers [0, Nrows) to the global rows numbers [rowStart, rowStart + Nrows).
> 
>   2) To form each piece:
> 
>     2a) Extract that block using https://petsc.org/main/manualpages/Mat/MatGetLocalSubMatrix/
> 
>            This gives back a Mat object that you subsequently restore using https://petsc.org/main/manualpages/Mat/MatRestoreLocalSubMatrix/
> 
>      2b) Insert values using https://petsc.org/main/manualpages/Mat/MatSetValuesLocal/
> 
>             The local indices used for insertion here are indices relative to the block itself, and the L2G map for this matrix
>             has been rewritten to insert into that block in the larger matrix. Thus this looks like just calling MatSetValuesLocal()
>             on the smaller matrix block, but inserts correctly into the larger matrix.
> 
> Therefore, the code you write code in 2) could work equally well making the large matrix from 1), or independent smaller matrix blocks.
> 
> Does this make sense?
> 
>   Thanks,
> 
>      Matt
>  
> I will use MatSetLocalToGlobalMapping(K, row_mapping, column_mapping) to build the bottom left [P].
>  
>  
> Q2) Can now I reset the mapping as MatSetLocalToGlobalMapping(K, column_mapping, row_mapping) to build the top right [P^T]? 
>  
>  
> Many thanks!
>  
> Kind regards,
> Karthik.
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> 
> -- 
> 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/>
> <IET Generation Trans   Dist - 2022 - Sundermann - Parallel primal‐dual interior point method for the solution of dynamic.pdf>

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