PETSc version 3.17.4
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# PCTelescopeSetUseCoarseDM

Set a flag to query the DM attached to the PC if it also has a coarse DM

### Synopsis

    #include "petscksp.h"
    #include "petscdm.h"
    PetscErrorCode PCTelescopeSetUseCoarseDM(PC pc,PetscBool v)

Not Collective

### Input Parameter

| pc | - the preconditioner context

### Output Parameter

| v | - Use PETSC_FALSE to ignore any coarse DM

### Notes

When you have specified to use a coarse DM, the communicator used to create the sub-KSP within PCTelescope will be that of the coarse DM. Hence the flags -pc_telescope_reduction_factor and -pc_telescope_subcomm_type will no longer have any meaning. It is required that the communicator associated with the parent (fine) and the coarse DM are of different sizes. An error will occur of the size of the communicator associated with the coarse DM is the same as that of the parent DM. Furthermore, it is required that the communicator on the coarse DM is a sub-communicator of the parent. This will be checked at the time the preconditioner is setup and an error will occur if the coarse DM does not define a sub-communicator of that used by the parent DM.

The particular Telescope setup invoked when using a coarse DM is agnostic with respect to the type of the DM used (e.g. it supports DMSHELL, DMPLEX, etc).

Support is currently only provided for the case when you are using KSPSetComputeOperators()

The user is required to compose a function with the parent DM to facilitate the transfer of fields (Vec) between the different decompositions defined by the fine and coarse DMs. In the user code, this is achieved via

       {
         DM dm_fine;
         PetscObjectCompose((PetscObject)dm_fine,"PCTelescopeFieldScatter",your_field_scatter_method);
       }

The signature of the user provided field scatter method is

       PetscErrorCode your_field_scatter_method(DM dm_fine,Vec x_fine,ScatterMode mode,DM dm_coarse,Vec x_coarse);

The user must provide support for both mode = SCATTER_FORWARD and mode = SCATTER_REVERSE. SCATTER_FORWARD implies the direction of transfer is from the parent (fine) DM to the coarse DM.

Optionally, the user may also compose a function with the parent DM to facilitate the transfer of state variables between the fine and coarse DMs. In the context of a finite element discretization, an example state variable might be values associated with quadrature points within each element. A user provided state scatter method is composed via

       {
         DM dm_fine;
         PetscObjectCompose((PetscObject)dm_fine,"PCTelescopeStateScatter",your_state_scatter_method);
       }

The signature of the user provided state scatter method is

       PetscErrorCode your_state_scatter_method(DM dm_fine,ScatterMode mode,DM dm_coarse);

SCATTER_FORWARD implies the direction of transfer is from the fine DM to the coarse DM. The user is only required to support mode = SCATTER_FORWARD. No assumption is made about the data type of the state variables. These must be managed by the user and must be accessible from the DM.

Care must be taken in defining the user context passed to KSPSetComputeOperators() which is to be associated with the sub-KSP residing within PCTelescope. In general, PCTelescope assumes that the context on the fine and coarse DM used with KSPSetComputeOperators() should be "similar" in type or origin. Specifically the following rules are used to infer what context on the sub-KSP should be.

First the contexts from the KSP and the fine and coarse DMs are retrieved. Note that the special case of a DMSHELL context is queried.

       DMKSPGetComputeOperators(dm_fine,&dmfine_kspfunc,&dmfine_kspctx);
       DMGetApplicationContext(dm_fine,&dmfine_appctx);
       DMShellGetContext(dm_fine,&dmfine_shellctx);

       DMGetApplicationContext(dm_coarse,&dmcoarse_appctx);
       DMShellGetContext(dm_coarse,&dmcoarse_shellctx);

### The following rules are then enforced

### 1. If dmfine_kspctx = NULL, then we provide a NULL pointer as the context for the sub-KSP

KSPSetComputeOperators(sub_ksp,dmfine_kspfunc,NULL);

2. If dmfine_kspctx != NULL and dmfine_kspctx == dmfine_appctx,

### check that dmcoarse_appctx is also non-NULL. If this is true, then

KSPSetComputeOperators(sub_ksp,dmfine_kspfunc,dmcoarse_appctx);

3. If dmfine_kspctx != NULL and dmfine_kspctx == dmfine_shellctx,

### check that dmcoarse_shellctx is also non-NULL. If this is true, then

KSPSetComputeOperators(sub_ksp,dmfine_kspfunc,dmcoarse_shellctx);

If neither of the above three tests passed, then PCTelescope cannot safely determine what context should be provided to KSPSetComputeOperators() for use with the sub-KSP. In this case, an additional mechanism is provided via a composed function which will return the actual context to be used. To use this feature you must compose the "getter" function with the coarse DM, e.g.

       {
         DM dm_coarse;
         PetscObjectCompose((PetscObject)dm_coarse,"PCTelescopeGetCoarseDMKSPContext",your_coarse_context_getter);
       }

The signature of the user provided method is

       PetscErrorCode your_coarse_context_getter(DM dm_coarse,void **your_kspcontext);

### Level

advanced

### Location

src/ksp/pc/impls/telescope/telescope.c

### Examples

src/ksp/ksp/tutorials/ex73.c.html

### Implementations

PCTelescopeSetUseCoarseDM_Telescope in src/ksp/pc/impls/telescope/telescope.c
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