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# DM Basics

The previous chapters have focused on the core numerical solvers in PETSc. However, numerical solvers without efficient ways (in both human and machine time) of connecting the solvers to the mathematical models and discretizations that people wish to build their simulations on, will not get widely used. Thus PETSc provides a set of abstractions represented by the `DM` object to provide a powerful, comprehensive mechanism for translating the problem specification of a model and its discretization to the language and API of solvers.

Some of the model classes `DM` currently supports are PDEs on structured and staggered grids with finite difference methods (`DMDA` and `DMSTAG` – DMSTAG: Staggered, Structured Grid), PDEs on unstructured grids with finite element and finite volume methods (`DMPLEX` – DMPlex: Unstructured Grids), PDEs on quad and octree-grids (`DMFOREST`), models on networks (graphs) such as the power grid or river networks (`DMNETWORK` – Networks), and particle-in-cell simulations (`DMSWARM`).

In previous chapters, we have demonstrated some simple usage of `DM` to provide the input for the solvers. In this chapter, and those that follow, we will dive deep into the capabilities of `DM`.

It is possible to create a `DM` with

    DM dm;
    DMCreate(MPI_Comm comm, DM *dm);
    DMSetType(DM dm, DMType type);

but more commonly, a `DM` is created with a type-specific constructor; the construction process for each type of `DM` is discussed in the sections on each `DMType`. This chapter focuses on commonalities between all the `DM` so we assume the `DM` already exists and we wish to work with it.

As discussed earlier, a `DM` can construct vectors and matrices appropriate for a model and discretization and provide the mapping between the global and local vector representations.

    DMCreateLocalVector(DM dm,Vec *l);
    DMCreateGlobalVector(DM dm,Vec *g);
    DMGlobalToLocal(dm,g,l,INSERT_VALUES);
    DMLocalToGlobal(dm,l,g,ADD_VALUES);
    DMCreateMatrix(dm,Mat *m);

The matrices produced may support `MatSetValuesLocal()` allowing one to work with the local numbering on each MPI rank. For `DMDA` one can also use `MatSetValuesStencil()` and for `DMSTAG` with `DMStagMatSetValuesStencil()`.

A given `DM` can be refined for certain `DMType`s with `DMRefine()` or coarsened with `DMCoarsen()`. Mappings between `DM`s may be obtained with routines such as `DMCreateInterpolation()`, `DMCreateRestriction()` and `DMCreateInjection()`.

One attaches a `DM` to a solver object with

    KSPSetDM(KSP ksp,DM dm);
    SNESSetDM(SNES snes,DM dm);
    TSSetDM(TS ts,DM dm);

Once the `DM` is attached, the solver can utilize it to create and process much of the data that the solver needs to set up and implement its solve. For example, with `PCMG` simply providing a `DM` can allow it to create all the data structures needed to run geometric multigrid on your problem.

SNES Tutorial ex19 demonstrates how this may be done with `DMDA`.