[petsc-users] Additiional DoF per cell

Fri May 8 04:08:40 CDT 2026

On the plus side, using the flags you provided in the previous email

-dm_plex_simplex 0 -mech_petscspace_degree 1 -pres_petscspace_degree 1 -pres_petscspace_poly_tensor 0 -pres_petscdualspace_lagrange_continuity 0

the output of -ds_view is identical to yours, so setting up the fields should be correct.
Thanks. I forgot about the flag "-[]_petscdualspace_lagrange_continuity 0" so as to have a discontinuous field.

However, the closure size was still wrong.

But, I figured out the reason: the manual set up of the section.
For the argument "numDof" in DMPlexCreateSection(), I was computing the number of DoF with PetscSectionGetDof() (which was fine until now, since there was a single field), instead of PetscSectionGetFieldDof().
So as you previously suggested

> [...] It sounds like somehow you have a copy of your fields.

the number of DoF per field were indeed "doubled" with the sum of both fields.

Thanks,
Noam

On Wednesday, May 6th, 2026 at 2:06 PM, Matthew Knepley <knepley at gmail.com> wrote:

> Okay, this is a problem with specifying the space I believe. You want a discontinuous space for pressure.
>
> Here is what I get
>
> Discrete System with 2 fields
> cell total dim 11 total comp 3
> Field Q1 FEM 2 components (implicit) (Nq 4 Nqc 1) 1-jet
> PetscFE Object: Q1 (mech_) 1 MPI process
> type: vector
> Vector Finite Element in 2 dimensions with 2 components
> PetscSpace Object: Q1 (mech_) 1 MPI process
> type: sum
> Space in 2 variables with 2 components, size 8
> Sum space of 2 concatenated subspaces (all identical)
> PetscSpace Object: Q1 (mech_sumcomp_) 1 MPI process
> type: tensor
> Space in 2 variables with 1 components, size 4
> Tensor space of 2 subspaces (all identical)
> PetscSpace Object: sum component tensor component (mech_sumcomp_tensorcomp_) 1 MPI process
> type: poly
> Space in 1 variables with 1 components, size 2
> Polynomial space of degree 1
> PetscDualSpace Object: Q1 (mech_) 1 MPI process
> type: sum
> Dual space with 2 components, size 8
> Sum dual space of 2 concatenated subspaces (all identical)
> PetscDualSpace Object: Q1 1 MPI process
> type: lagrange
> Dual space with 1 components, size 4
> Continuous tensor Lagrange dual space
> Quadrature on a quadrilateral of order 3 on 4 points (dim 2)
> Field Q1 FEM 1 component (implicit) (Nq 4 Nqc 1) 1-jet
> PetscFE Object: Q1 (pres_) 1 MPI process
> type: basic
> Basic Finite Element in 2 dimensions with 1 components
> PetscSpace Object: Q1 (pres_) 1 MPI process
> type: poly
> Space in 2 variables with 1 components, size 3
> Polynomial space of degree 1
> PetscDualSpace Object: Q1 (pres_) 1 MPI process
> type: lagrange
> Dual space with 1 components, size 3
> Discontinuous Lagrange dual space
> Quadrature on a quadrilateral of order 3 on 4 points (dim 2)
> Weak Form System with 2 fields
>
> I have attached the code I used.
>
> Thanks,
>
> Matt
>
> On Wed, May 6, 2026 at 9:32 AM Noam T. <dontbugthedevs at proton.me> wrote:
>
>>>> Field p FEM 3 components (implicit) (Nq 4 Nqc 1) 1-jet
>>>
>>> 1. Why does pressure have 3 components in 2D?
>>
>> I used nc = 3 when calling PetscFECreateDefault() for the pressure field. Confusion between wanting 3 addditional DoF and components.
>>
>> With nc = 1 the closure size is 18 instead.
>>
>>> 2. Did you look at the incompressible example SNES ex69? It sets up this exact element.
>>
>> I changed some of my code following the example and got rid of some errors. But I'll have a more thorough look again.
>>
>>> 3. Please send me that code that sets up your DM. It sounds like somehow you have a copy of your fields.
>>>
>>> Thanks,
>>
>> For reference, -ds_view with only one (displacements) fields shows:
>>
>> ---
>> Discrete System with 1 fields
>> cell total dim 8 total comp 2
>> Field u FEM 2 components (implicit) (Nq 4 Nqc 1) 1-jet
>> PetscFE Object: u (disp_) 1 MPI process
>> type: vector
>> Vector Finite Element in 2 dimensions with 2 components
>> PetscSpace Object: Q1 (disp_) 1 MPI process
>> type: sum
>> Space in 2 variables with 2 components, size 8
>> Sum space of 2 concatenated subspaces (all identical)
>> PetscSpace Object: Q1 (disp_sumcomp_) 1 MPI process
>> type: tensor
>> Space in 2 variables with 1 components, size 4
>> Tensor space of 2 subspaces (all identical)
>> PetscSpace Object: sum component tensor component (disp_sumcomp_tensorcomp_) 1 MPI process
>> type: poly
>> Space in 1 variables with 1 components, size 2
>> Polynomial space of degree 1
>> PetscDualSpace Object: Q1 (disp_) 1 MPI process
>> type: sum
>> Dual space with 2 components, size 8
>> Sum dual space of 2 concatenated subspaces (all identical)
>> PetscDualSpace Object: Q1 1 MPI process
>> type: lagrange
>> Dual space with 1 components, size 4
>> Continuous tensor Lagrange dual space
>> Quadrature on a quadrilateral of order 3 on 4 points (dim 2)
>> Weak Form System with 1 fields---
>>
>> and -dm_view
>>
>> ---
>> DM Object: 1 MPI process
>> type: plex
>> DM_0x55c117db1b00_2 in 2 dimensions:
>> Number of 0-cells per rank: 4
>> Number of 1-cells per rank: 4
>> Number of 2-cells per rank: 1
>> Labels:
>> celltype: 3 strata with value/size (0 (4), 1 (4), 4 (1))
>> depth: 3 strata with value/size (0 (4), 1 (4), 2 (1))
>> Cell Sets: 1 strata with value/size (1 (1))
>> surf: 1 strata with value/size (1 (1))
>> Face Sets: 2 strata with value/size (2 (1), 3 (1))
>> edge_right: 1 strata with value/size (3 (1))
>> edge_left: 1 strata with value/size (2 (1))
>> Vertex Sets: 4 strata with value/size (4 (1), 5 (1), 6 (1), 7 (1))
>> P1: 1 strata with value/size (4 (1))
>> P2: 1 strata with value/size (5 (1))
>> P3: 1 strata with value/size (6 (1))
>> P4: 1 strata with value/size (7 (1))
>> Field u:
>> adjacency FEM
>> /// With the second field, the two lines below are added
>> Field p:
>> adjacency FEM
>> ---
>>
>> DM set up
>>
>> DM :: dm_mesh
>> PetscFE :: u_FE, p_FE
>> PetscInt :: dim, dim_u_FE, tdim
>> PetscDS :: dm_ds
>>
>> DMCreate(PETSC_COMM_WORLD, dm_mesh)
>> DMSetType(dm_mesh, DMPLEX)
>> DMGetDimension(dm_mesh, dim) // dim = 2
>> DMSetFromOptions(dm_mesh) // this was called twice by mistake, now removed; no changes that I can see
>>
>> -- using flags
>>
>> -dm_plex_filename QUAD1.msh
>> -dm_plex_interpolate 1
>> -dm_plex_gmsh_use_generic
>> -dm_plex_gmsh_use_regions
>> -dm_plex_gmsh_multiple_tags
>> -dm_plex_gmsh_mark_vertices
>>
>> -- some operations with labels
>>
>> // u field
>> PetscFECreateDefault(PETSC_COMM_SELF, dim, dim, PETSC_FALSE, "mech_", PETSC_DETERMINE, u_FE)
>> PetscFEGetDimension(u_FE, dim_u_FE) // dim_u_FE = 8
>> DMAddField(dm_mesh, NULL, (PetscObject)u_FE)
>>
>> // p field
>> PetscFECreateDefault(PETSC_COMM_SELF, dim , 1, PETSC_FALSE, "pres_", PETSC_DETERMINE, p_FE)
>> PetscFEGetDimension(p_FE, dim_p_FE) // dim_p_FE = 1
>> DMAddField(dm_mesh, NULL, (PetscObject)p_FE)
>>
>> DMCreateDS(dm_mesh)
>> DMGetDS(dm_mesh, dm_DS)
>> PetscDSGetTotalDimension(dm_DS, tdim) // tdim
>>
>> with flags
>>
>> -disp_petscdualspace_lagrange_node_type equispaced
>> -disp_petscdualspace_lagrange_node_endpoints 1
>>
>> I believe these are all the DM related calls.
>>
>> Thanks,
>> Noam
>>
>> On Wednesday, May 6th, 2026 at 11:39 AM, Matthew Knepley <knepley at gmail.com> wrote:
>>
>>> On Wed, May 6, 2026 at 6:56 AM Noam T. <dontbugthedevs at proton.me> wrote:
>>>
>>>> Hello,
>>>>
>>>>> That is wrong. Are you sure about 22?
>>>>
>>>> That's what I get, from the argument "csize" in DMPlexGetVecClosure().
>>>>
>>>> Here's the output of PetscDSView:
>>>>
>>>> Discrete System with 2 fields
>>>> cell total dim 11 total comp 5
>>>> Field u FEM 2 components (implicit) (Nq 4 Nqc 1) 1-jet
>>>> PetscFE Object: u (mech_) 1 MPI process
>>>> type: vector
>>>> Vector Finite Element in 2 dimensions with 2 components
>>>> PetscSpace Object: Q1 (mech_) 1 MPI process
>>>> type: sum
>>>> Space in 2 variables with 2 components, size 8
>>>> Sum space of 2 concatenated subspaces (all identical)
>>>> PetscSpace Object: Q1 (mech_sumcomp_) 1 MPI process
>>>> type: tensor
>>>> Space in 2 variables with 1 components, size 4
>>>> Tensor space of 2 subspaces (all identical)
>>>> PetscSpace Object: sum component tensor component (mech_sumcomp_tensorcomp_) 1 MPI process
>>>> type: poly
>>>> Space in 1 variables with 1 components, size 2
>>>> Polynomial space of degree 1
>>>> PetscDualSpace Object: Q1 (mech_) 1 MPI process
>>>> type: sum
>>>> Dual space with 2 components, size 8
>>>> Sum dual space of 2 concatenated subspaces (all identical)
>>>> PetscDualSpace Object: Q1 1 MPI process
>>>> type: lagrange
>>>> Dual space with 1 components, size 4
>>>> Continuous tensor Lagrange dual space
>>>> Quadrature on a quadrilateral of order 3 on 4 points (dim 2)
>>>> Field p FEM 3 components (implicit) (Nq 4 Nqc 1) 1-jet
>>>
>>> 1. Why does pressure have 3 components in 2D?
>>>
>>>> PetscFE Object: p (pres_) 1 MPI process
>>>> type: vector
>>>> Vector Finite Element in 2 dimensions with 3 components
>>>> PetscSpace Object: Q0 (pres_) 1 MPI process
>>>> type: sum
>>>> Space in 2 variables with 3 components, size 3
>>>> Sum space of 3 concatenated subspaces (all identical)
>>>> PetscSpace Object: Q0 (pres_sumcomp_) 1 MPI process
>>>> type: tensor
>>>> Space in 2 variables with 1 components, size 1
>>>> Tensor space of 2 subspaces (all identical)
>>>> PetscSpace Object: sum component tensor component (pres_sumcomp_tensorcomp_) 1 MPI process
>>>> type: poly
>>>> Space in 1 variables with 1 components, size 1
>>>> Polynomial space of degree 0
>>>> PetscDualSpace Object: Q0 (pres_) 1 MPI process
>>>> type: sum
>>>> Dual space with 3 components, size 3
>>>> Sum dual space of 3 concatenated subspaces (all identical)
>>>> PetscDualSpace Object: Q0 1 MPI process
>>>> type: lagrange
>>>> Dual space with 1 components, size 1
>>>> Discontinuous tensor Lagrange dual space
>>>> Quadrature on a quadrilateral of order 3 on 4 points (dim 2)
>>>> Weak Form System with 2 fields
>>>>
>>>> using as the second field (dim = 2, nc = 3, prefix = "pres") and setting the FE object name to "p".
>>>>
>>>> The entry "cell total dim 10" agrees with the output of PetscFEGetTotalDimension(); "comp = 5" I assume is 2 (u) + 3 (p)
>>>>
>>>> However, the closure of a Vec is still 22.
>>>
>>> 2. Did you look at the incompressible example SNES ex69? It sets up this exact element.
>>>
>>> 3. Please send me that code that sets up your DM. It sounds like somehow you have a copy of your fields.
>>>
>>> Thanks,
>>>
>>> Matt
>>>
>>>> On Tuesday, May 5th, 2026 at 1:58 PM, Matthew Knepley <knepley at gmail.com> wrote:
>>>>
>>>>> On Tue, May 5, 2026 at 9:06 AM Noam T. via petsc-users <petsc-users at mcs.anl.gov> wrote:
>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> I am trying to work with a "mixed" FE discretization, where besides the usual displacements DoF in nodes, there is an additional field (e.g. pressure) that is also part of the system. This additional field has a certain number of additional dof : p0, p1, p2...
>>>>>>
>>>>>> Looking at example 77 ([https://urldefense.us/v3/__https://petsc.org/release/src/snes/tutorials/ex77.c.html*5D(https:/*urldefense.us/v3/__https:/*petsc.org/release/src/snes/tutorials/ex77.c.html__;!!G_uCfscf7eWS!aZNOxZZeoHYqe6lRZV0wHMVVTu3YIEfc1Sr-dca7xiGVfG3enULFD4_g6_fxWIa7A_Cu9LcXU-MuxRj4sOwDRyWqvsdGdrEd$)__;JS8v!!G_uCfscf7eWS!coaEY5gLR5T_HZTbCB5LQ1_YdCo5TIY7difWJI7pNqUH9KO-YkgY51Bt_zimYz1oJJE6ri7CqRCfudTctaK7aEfiphUlkAUc$ ) this seems to be handled with an additional field, added to the DM. I've tried so, but then I am getting from the DM arrays whose size/contents are not what I expected.
>>>>>>
>>>>>> For example, a four-noded Q1 element (PetscFE created with a 2x2 Gauss rule for quadrature), with just one field, a call to DMPlexGetVecClosure() gives me an array with 8 entries (say, the coordinates of the initial mesh): x = x0, y0, .... x3, y3
>>>>>>
>>>>>> Then add the new field:
>>>>>>
>>>>>> PetscFECreateDefault(..., nc = 3, ..., p_FE) /* not sure about the value of nc here */
>>>>>> DMAddField(dm, ..., p_FE)
>>>>>>
>>>>>> The total dimension, from PetscDSGetTotalDimension(), is now 11 (8 + 3). This results in a closure of size 22.
>>>>>
>>>>> That is wrong. Are you sure about 22?
>>>>>
>>>>>> However, what I am looking for is a closure of size 8 + 3 i.e. the original 8 DoF at the nodes, plus exactly 3 DoF (for the cell, so to speak) p0, p1, p2, so that in a system with "block" matrices of the form
>>>>>>
>>>>>> [K_uu, K_up | K_pu, K_pp] { u | p } = RHS
>>>>>>
>>>>>> the unknowns {p} has size 3 per cell. Is this possible? I tried some combinations of dim / nc for the new PetscFE, but when creating the DM section I get errors e.g.
>>>>>
>>>>> This is what you should get. I definitely have examples that do this. For example, here is Q1-P0 (I think that is what you are suggesting) for incompressible Stokes
>>>>>
>>>>> https://urldefense.us/v3/__https://gitlab.com/petsc/petsc/-/blob/main/src/snes/tutorials/ex69.c?ref_type=heads*L3454__;Iw!!G_uCfscf7eWS!coaEY5gLR5T_HZTbCB5LQ1_YdCo5TIY7difWJI7pNqUH9KO-YkgY51Bt_zimYz1oJJE6ri7CqRCfudTctaK7aEfippxKIxkx$ 
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Matt
>>>>>
>>>>>> "point X has a number of DoF not divisible by 2 field components"
>>>>>>
>>>>>> Is this a hint that I should have 3 x dim new DoF, and simply not deal with entries that I don't need? Or I am not setting up the section properly (works with just one field)?
>>>>>>
>>>>>> Thank you,
>>>>>> Noam.
>>>>>
>>>>> --
>>>>>
>>>>> 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://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/*5D(http:/*www.cse.buffalo.edu/*knepley/)__;fiUvfg!!G_uCfscf7eWS!coaEY5gLR5T_HZTbCB5LQ1_YdCo5TIY7difWJI7pNqUH9KO-YkgY51Bt_zimYz1oJJE6ri7CqRCfudTctaK7aEfipo1f9jek$ 
>>>
>>> --
>>>
>>> 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://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/*5D(http:/*www.cse.buffalo.edu/*knepley/)__;fiUvfg!!G_uCfscf7eWS!coaEY5gLR5T_HZTbCB5LQ1_YdCo5TIY7difWJI7pNqUH9KO-YkgY51Bt_zimYz1oJJE6ri7CqRCfudTctaK7aEfipo1f9jek$ 
>
> --
>
> 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://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/*5D(http:/*www.cse.buffalo.edu/*knepley/)__;fiUvfg!!G_uCfscf7eWS!coaEY5gLR5T_HZTbCB5LQ1_YdCo5TIY7difWJI7pNqUH9KO-YkgY51Bt_zimYz1oJJE6ri7CqRCfudTctaK7aEfipo1f9jek$ 
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