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# SNESNGS

Either calls the user-provided solution routine provided with SNESSetNGS() or does a finite difference secant approximation using coloring.

### Options Database

| -snes_ngs_sweeps <n> | - Number of sweeps of GS to apply
| -snes_ngs_atol <atol> | - Absolute residual tolerance for GS iteration
| -snes_ngs_rtol <rtol> | - Relative residual tolerance for GS iteration
| -snes_ngs_stol <stol> | - Absolute update tolerance for GS iteration
| -snes_ngs_max_it <maxit> | - Maximum number of sweeps of GS to apply
| -snes_ngs_secant | - Use pointwise secant local Jacobian approximation with coloring instead of user provided Gauss-Seidel routine, this is used by default if no user provided Gauss-Seidel routine is available. Requires either that a DM that can compute a coloring is available or a Jacobian sparse matrix is provided (from which to get the coloring).
| -snes_ngs_secant_h <h> | - Differencing parameter for secant approximation
| -snes_ngs_secant_mat_coloring | - Use the graph coloring of the Jacobian for the secant GS even if a DM is available.
| -snes_norm_schedule <none, always, initialonly, finalonly, initialfinalonly> | - how often the residual norms are computed

### Notes

the Gauss-Seidel smoother is inherited through composition. If a solver has been created with SNESGetNPC(), it will have its parent's Gauss-Seidel routine associated with it.

By default this routine computes the solution norm at each iteration, this can be time consuming, you can turn this off with SNESSetNormSchedule() or -snes_norm_schedule

### References

| * | - Peter R. Brune, Matthew G. Knepley, Barry F. Smith, and Xuemin Tu, "Composing Scalable Nonlinear Algebraic Solvers", SIAM Review, 57(4), 2015

### See Also

SNESCreate(), SNES, SNESSetType(), SNESSetNGS(), SNESType (for list of available types), SNESNGSSetSweeps(), SNESNGSSetTolerances(),
SNESSetNormSchedule()

### Level

advanced

### Location

src/snes/impls/gs/snesgs.c
Index of all SNES routines
Table of Contents for all manual pages
Index of all manual pages