[petsc-users] Απ: Question about a parallel implementation of PCFIELDSPLIT

Tue Jan 23 07:16:42 CST 2024

Dear Matt,

Thank you for your response. This is an idealized setup where I have only one row/column. Sometimes we might need two or even three constraints based on the application. Thus, I will pursue the user-defined IS.
When I supply the IS using the command PCFieldSplitSetIS, I do not specify anything in the matrix set up right?

Thanks,
Pantelis
________________________________
Από: Matthew Knepley <knepley at gmail.com>
Στάλθηκε: Τρίτη, 23 Ιανουαρίου 2024 2:51 μμ
Προς: Pantelis Moschopoulos <pmoschopoulos at outlook.com>
Κοιν.: Barry Smith <bsmith at petsc.dev>; petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
Θέμα: Re: [petsc-users] Question about a parallel implementation of PCFIELDSPLIT

On Tue, Jan 23, 2024 at 4:23 AM Pantelis Moschopoulos <pmoschopoulos at outlook.com<mailto:pmoschopoulos at outlook.com>> wrote:
Dear Matt and Dear Barry,

I have some follow up questions regarding FieldSplit.
Let's assume that I solve again the 3D Stokes flow but now I have also a global constraint that controls the flow rate at the inlet. Now, the matrix has the same unknowns as before, i.e. ux0,uy0,uz0,p0//ux1,uy1,uz1,p1//..., but the last line (and the last column) corresponds to the contribution of the global constraint equation. I want to incorporate the last line (and last column)  into the local block of velocities (split 0) and the pressure. The problem is how I do that. I have two questions:

  1.  Now, the block size should be 5 in the matrix and vector creation for this problem?

No. Blocksize is only useful when the vector/matrix layout is completely regular, meaning _every_ block looks the same. Here you have a single row to be added in.

  1.  I have to rely entirely on PCFieldSplitSetIS to create the two blocks? Can I augment simply the previously defined block 0 with the last line of the matrix?

If you want to add in a single row, then you have to specify the IS yourself since we cannot generate it from the regular pattern.

However, if you know that you will only ever have a single constraint row (which I assume is fairly dense), then I would suggest instead using MatLRC, which Jose developed for SLEPc. This handles the last row/col as a low-rank correction. One step of Sherman-Morrison-Woobury solves this exactly. It requires a solve for A, for which you can use FieldSplit as normal.

  Thanks,

     Matt

Up to this moment, I use the following commands to create the Field split:
ufields(3) = [0, 1, 2]
pfields(1) = [3]

call PCSetType(pc, PCFIELDSPLIT, ierr)
call PCFieldSplitSetBlockSize(pc, 4,ierr)
 call PCFieldSplitSetFields(pc, "0", 3, ufields, ufields,ierr)
 call PCFieldSplitSetFields(pc, "1", 1, pfields, pfields,ierr)

Thanks,
Pantelis

________________________________
Από: Matthew Knepley <knepley at gmail.com<mailto:knepley at gmail.com>>
Στάλθηκε: Παρασκευή, 19 Ιανουαρίου 2024 11:31 μμ
Προς: Barry Smith <bsmith at petsc.dev<mailto:bsmith at petsc.dev>>
Κοιν.: Pantelis Moschopoulos <pmoschopoulos at outlook.com<mailto:pmoschopoulos at outlook.com>>; petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>>
Θέμα: Re: [petsc-users] Question about a parallel implementation of PCFIELDSPLIT

On Fri, Jan 19, 2024 at 4:25 PM Barry Smith <bsmith at petsc.dev<mailto:bsmith at petsc.dev>> wrote:

   Generally fieldsplit is used on problems that have a natural "split" of the variables into two or more subsets. For example u0,v0,u1,v1,u2,v2,u3,v4 This is often indicated in the vectors and matrices with the "blocksize" argument, 2 in this case. DM also often provides this information.

   When laying out a vector/matrix with a blocksize one must ensure that an equal number of of the subsets appears on each MPI process. So, for example, if the above vector is distributed over 3 MPI processes one could use   u0,v0,u1,v1       u2,v2      u3,v3  but one cannot use u0,v0,u1    v1,u2,v2   u3,v3.  Another way to think about it is that one must split up the vector as indexed by block among the processes. For most multicomponent problems this type of decomposition is very natural in the logic of the code.

This blocking is only convenient, not necessary. You can specify your own field division using PCFieldSplitSetIS().

  Thanks,

     Matt

  Barry

On Jan 19, 2024, at 3:19 AM, Pantelis Moschopoulos <pmoschopoulos at outlook.com<mailto:pmoschopoulos at outlook.com>> wrote:

Dear all,

When I am using PCFIELDSPLIT and pc type "schur" in serial mode everything works fine. When I turn now to parallel, I observe that the number of ranks that I can use must divide the number of N without any remainder, where N is the number of unknowns. Otherwise, an error of the following form emerges: "Local columns of A10 3473 do not equal local rows of A00 3471".

Can I do something to overcome this?

Thanks,
Pantelis

--
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/>

--
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/>
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