Date: 2025-03-28T21:10:52+00:00

Actual source code: itcreate.c

      1: /*
      2:      The basic KSP routines, Create, View etc. are here.
      3: */
      4: #include <petsc/private/kspimpl.h>

      6: /* Logging support */
      7: PetscClassId  KSP_CLASSID;
      8: PetscClassId  DMKSP_CLASSID;
      9: PetscClassId  KSPGUESS_CLASSID;
     10: PetscLogEvent KSP_GMRESOrthogonalization, KSP_SetUp, KSP_Solve, KSP_SolveTranspose, KSP_MatSolve, KSP_MatSolveTranspose;

     12: /*
     13:    Contains the list of registered KSP routines
     14: */
     15: PetscFunctionList KSPList              = NULL;
     16: PetscBool         KSPRegisterAllCalled = PETSC_FALSE;

     18: /*
     19:    Contains the list of registered KSP monitors
     20: */
     21: PetscFunctionList KSPMonitorList              = NULL;
     22: PetscFunctionList KSPMonitorCreateList        = NULL;
     23: PetscFunctionList KSPMonitorDestroyList       = NULL;
     24: PetscBool         KSPMonitorRegisterAllCalled = PETSC_FALSE;

     26: /*@
     27:   KSPLoad - Loads a `KSP` that has been stored in a `PETSCVIEWERBINARY`  with `KSPView()`.

     29:   Collective

     31:   Input Parameters:
     32: + newdm  - the newly loaded `KSP`, this needs to have been created with `KSPCreate()` or
     33:            some related function before a call to `KSPLoad()`.
     34: - viewer - binary file viewer, obtained from `PetscViewerBinaryOpen()`

     36:   Level: intermediate

     38:   Note:
     39:   The type is determined by the data in the file, any type set into the `KSP` before this call is ignored.

     41: .seealso: [](ch_ksp), `KSP`, `PetscViewerBinaryOpen()`, `KSPView()`, `MatLoad()`, `VecLoad()`
     42: @*/
     43: PetscErrorCode KSPLoad(KSP newdm, PetscViewer viewer)
     44: {
     45:   PetscBool isbinary;
     46:   PetscInt  classid;
     47:   char      type[256];
     48:   PC        pc;

     50:   PetscFunctionBegin;
     53:   PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERBINARY, &isbinary));
     54:   PetscCheck(isbinary, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONG, "Invalid viewer; open viewer with PetscViewerBinaryOpen()");

     56:   PetscCall(PetscViewerBinaryRead(viewer, &classid, 1, NULL, PETSC_INT));
     57:   PetscCheck(classid == KSP_FILE_CLASSID, PetscObjectComm((PetscObject)newdm), PETSC_ERR_ARG_WRONG, "Not KSP next in file");
     58:   PetscCall(PetscViewerBinaryRead(viewer, type, 256, NULL, PETSC_CHAR));
     59:   PetscCall(KSPSetType(newdm, type));
     60:   PetscTryTypeMethod(newdm, load, viewer);
     61:   PetscCall(KSPGetPC(newdm, &pc));
     62:   PetscCall(PCLoad(pc, viewer));
     63:   PetscFunctionReturn(PETSC_SUCCESS);
     64: }

     66: #include <petscdraw.h>
     67: #if defined(PETSC_HAVE_SAWS)
     68: #include <petscviewersaws.h>
     69: #endif
     70: /*@
     71:   KSPView - Prints the various parameters currently set in the `KSP` object. For example, the convergence tolerances and `KSPType`.
     72:   Also views the `PC` and `Mat` contained by the `KSP` with `PCView()` and `MatView()`.

     74:   Collective

     76:   Input Parameters:
     77: + ksp    - the Krylov space context
     78: - viewer - visualization context

     80:   Options Database Key:
     81: . -ksp_view - print the `KSP` data structure at the end of each `KSPSolve()` call

     83:   Level: beginner

     85:   Notes:
     86:   The available visualization contexts include
     87: +     `PETSC_VIEWER_STDOUT_SELF` - standard output (default)
     88: -     `PETSC_VIEWER_STDOUT_WORLD` - synchronized standard
     89:   output where only the first processor opens
     90:   the file.  All other processors send their
     91:   data to the first processor to print.

     93:   The available formats include
     94: +     `PETSC_VIEWER_DEFAULT` - standard output (default)
     95: -     `PETSC_VIEWER_ASCII_INFO_DETAIL` - more verbose output for PCBJACOBI and PCASM

     97:   The user can open an alternative visualization context with
     98:   `PetscViewerASCIIOpen()` - output to a specified file.

    100:   Use `KSPViewFromOptions()` to allow the user to select many different `PetscViewerType` and formats from the options database.

    102:   In the debugger you can do call `KSPView(ksp,0)` to display the `KSP`. (The same holds for any PETSc object viewer).

    104: .seealso: [](ch_ksp), `KSP`, `PetscViewer`, `PCView()`, `PetscViewerASCIIOpen()`, `KSPViewFromOptions()`
    105: @*/
    106: PetscErrorCode KSPView(KSP ksp, PetscViewer viewer)
    107: {
    108:   PetscBool iascii, isbinary, isdraw, isstring;
    109: #if defined(PETSC_HAVE_SAWS)
    110:   PetscBool issaws;
    111: #endif

    113:   PetscFunctionBegin;
    115:   if (!viewer) PetscCall(PetscViewerASCIIGetStdout(PetscObjectComm((PetscObject)ksp), &viewer));
    117:   PetscCheckSameComm(ksp, 1, viewer, 2);

    119:   PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &iascii));
    120:   PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERBINARY, &isbinary));
    121:   PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERDRAW, &isdraw));
    122:   PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERSTRING, &isstring));
    123: #if defined(PETSC_HAVE_SAWS)
    124:   PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERSAWS, &issaws));
    125: #endif
    126:   if (iascii) {
    127:     PetscCall(PetscObjectPrintClassNamePrefixType((PetscObject)ksp, viewer));
    128:     PetscCall(PetscViewerASCIIPushTab(viewer));
    129:     PetscTryTypeMethod(ksp, view, viewer);
    130:     PetscCall(PetscViewerASCIIPopTab(viewer));
    131:     if (ksp->guess_zero) {
    132:       PetscCall(PetscViewerASCIIPrintf(viewer, "  maximum iterations=%" PetscInt_FMT ", initial guess is zero\n", ksp->max_it));
    133:     } else {
    134:       PetscCall(PetscViewerASCIIPrintf(viewer, "  maximum iterations=%" PetscInt_FMT ", nonzero initial guess\n", ksp->max_it));
    135:     }
    136:     if (ksp->min_it) PetscCall(PetscViewerASCIIPrintf(viewer, "  minimum iterations=%" PetscInt_FMT "\n", ksp->min_it));
    137:     if (ksp->guess_knoll) PetscCall(PetscViewerASCIIPrintf(viewer, "  using preconditioner applied to right-hand side for initial guess\n"));
    138:     PetscCall(PetscViewerASCIIPrintf(viewer, "  tolerances: relative=%g, absolute=%g, divergence=%g\n", (double)ksp->rtol, (double)ksp->abstol, (double)ksp->divtol));
    139:     if (ksp->pc_side == PC_RIGHT) {
    140:       PetscCall(PetscViewerASCIIPrintf(viewer, "  right preconditioning\n"));
    141:     } else if (ksp->pc_side == PC_SYMMETRIC) {
    142:       PetscCall(PetscViewerASCIIPrintf(viewer, "  symmetric preconditioning\n"));
    143:     } else {
    144:       PetscCall(PetscViewerASCIIPrintf(viewer, "  left preconditioning\n"));
    145:     }
    146:     if (ksp->guess) {
    147:       PetscCall(PetscViewerASCIIPushTab(viewer));
    148:       PetscCall(KSPGuessView(ksp->guess, viewer));
    149:       PetscCall(PetscViewerASCIIPopTab(viewer));
    150:     }
    151:     if (ksp->dscale) PetscCall(PetscViewerASCIIPrintf(viewer, "  diagonally scaled system\n"));
    152:     PetscCall(PetscViewerASCIIPrintf(viewer, "  using %s norm type for convergence test\n", KSPNormTypes[ksp->normtype]));
    153:   } else if (isbinary) {
    154:     PetscInt    classid = KSP_FILE_CLASSID;
    155:     MPI_Comm    comm;
    156:     PetscMPIInt rank;
    157:     char        type[256];

    159:     PetscCall(PetscObjectGetComm((PetscObject)ksp, &comm));
    160:     PetscCallMPI(MPI_Comm_rank(comm, &rank));
    161:     if (rank == 0) {
    162:       PetscCall(PetscViewerBinaryWrite(viewer, &classid, 1, PETSC_INT));
    163:       PetscCall(PetscStrncpy(type, ((PetscObject)ksp)->type_name, 256));
    164:       PetscCall(PetscViewerBinaryWrite(viewer, type, 256, PETSC_CHAR));
    165:     }
    166:     PetscTryTypeMethod(ksp, view, viewer);
    167:   } else if (isstring) {
    168:     const char *type;
    169:     PetscCall(KSPGetType(ksp, &type));
    170:     PetscCall(PetscViewerStringSPrintf(viewer, " KSPType: %-7.7s", type));
    171:     PetscTryTypeMethod(ksp, view, viewer);
    172:   } else if (isdraw) {
    173:     PetscDraw draw;
    174:     char      str[36];
    175:     PetscReal x, y, bottom, h;
    176:     PetscBool flg;

    178:     PetscCall(PetscViewerDrawGetDraw(viewer, 0, &draw));
    179:     PetscCall(PetscDrawGetCurrentPoint(draw, &x, &y));
    180:     PetscCall(PetscObjectTypeCompare((PetscObject)ksp, KSPPREONLY, &flg));
    181:     if (!flg) {
    182:       PetscCall(PetscStrncpy(str, "KSP: ", sizeof(str)));
    183:       PetscCall(PetscStrlcat(str, ((PetscObject)ksp)->type_name, sizeof(str)));
    184:       PetscCall(PetscDrawStringBoxed(draw, x, y, PETSC_DRAW_RED, PETSC_DRAW_BLACK, str, NULL, &h));
    185:       bottom = y - h;
    186:     } else {
    187:       bottom = y;
    188:     }
    189:     PetscCall(PetscDrawPushCurrentPoint(draw, x, bottom));
    190: #if defined(PETSC_HAVE_SAWS)
    191:   } else if (issaws) {
    192:     PetscMPIInt rank;
    193:     const char *name;

    195:     PetscCall(PetscObjectGetName((PetscObject)ksp, &name));
    196:     PetscCallMPI(MPI_Comm_rank(PETSC_COMM_WORLD, &rank));
    197:     if (!((PetscObject)ksp)->amsmem && rank == 0) {
    198:       char dir[1024];

    200:       PetscCall(PetscObjectViewSAWs((PetscObject)ksp, viewer));
    201:       PetscCall(PetscSNPrintf(dir, 1024, "/PETSc/Objects/%s/its", name));
    202:       PetscCallSAWs(SAWs_Register, (dir, &ksp->its, 1, SAWs_READ, SAWs_INT));
    203:       if (!ksp->res_hist) PetscCall(KSPSetResidualHistory(ksp, NULL, PETSC_DECIDE, PETSC_TRUE));
    204:       PetscCall(PetscSNPrintf(dir, 1024, "/PETSc/Objects/%s/res_hist", name));
    205:       PetscCallSAWs(SAWs_Register, (dir, ksp->res_hist, 10, SAWs_READ, SAWs_DOUBLE));
    206:     }
    207: #endif
    208:   } else PetscTryTypeMethod(ksp, view, viewer);
    209:   if (ksp->pc) PetscCall(PCView(ksp->pc, viewer));
    210:   if (isdraw) {
    211:     PetscDraw draw;
    212:     PetscCall(PetscViewerDrawGetDraw(viewer, 0, &draw));
    213:     PetscCall(PetscDrawPopCurrentPoint(draw));
    214:   }
    215:   PetscFunctionReturn(PETSC_SUCCESS);
    216: }

    218: /*@
    219:   KSPViewFromOptions - View (print) a `KSP` object based on values in the options database. Also views the `PC` and `Mat` contained by the `KSP`
    220:   with `PCView()` and `MatView()`.

    222:   Collective

    224:   Input Parameters:
    225: + A    - Krylov solver context
    226: . obj  - Optional object that provides the options prefix used to query the options database
    227: - name - command line option

    229:   Level: intermediate

    231: .seealso: [](ch_ksp), `KSP`, `KSPView()`, `PetscObjectViewFromOptions()`, `KSPCreate()`
    232: @*/
    233: PetscErrorCode KSPViewFromOptions(KSP A, PetscObject obj, const char name[])
    234: {
    235:   PetscFunctionBegin;
    237:   PetscCall(PetscObjectViewFromOptions((PetscObject)A, obj, name));
    238:   PetscFunctionReturn(PETSC_SUCCESS);
    239: }

    241: /*@
    242:   KSPSetNormType - Sets the type of residual norm that is used for convergence testing in `KSPSolve()` for the given `KSP` context

    244:   Logically Collective

    246:   Input Parameters:
    247: + ksp      - Krylov solver context
    248: - normtype - one of
    249: .vb
    250:    KSP_NORM_NONE             - skips computing the norm, this should generally only be used if you are using
    251:                                the Krylov method as a smoother with a fixed small number of iterations.
    252:                                Implicitly sets `KSPConvergedSkip()` as the `KSP` convergence test.
    253:                                Note that certain algorithms such as `KSPGMRES` ALWAYS require the norm calculation,
    254:                                for these methods the norms are still computed, they are just not used in
    255:                                the convergence test.
    256:    KSP_NORM_PRECONDITIONED   - the default for left-preconditioned solves, uses the l2 norm
    257:                                of the preconditioned residual  P^{-1}(b - A x).
    258:    KSP_NORM_UNPRECONDITIONED - uses the l2 norm of the true $b - Ax$ residual.
    259:    KSP_NORM_NATURAL          - supported by `KSPCG`, `KSPCR`, `KSPCGNE`, `KSPCGS`
    260: .ve

    262:   Options Database Key:
    263: . -ksp_norm_type <none,preconditioned,unpreconditioned,natural> - set `KSP` norm type

    265:   Level: advanced

    267:   Notes:
    268:   The norm is always of the equations residual $\| b - A x^n \|$  (or an approximation to that norm), they are never a norm of the error in the equation.

    270:   Not all combinations of preconditioner side (see `KSPSetPCSide()`) and norm types are supported by all Krylov methods.
    271:   If only one is set, PETSc tries to automatically change the other to find a compatible pair.  If no such combination
    272:   is supported, PETSc will generate an error.

    274:   Developer Note:
    275:   Supported combinations of norm and preconditioner side are set using `KSPSetSupportedNorm()` for each `KSPType`.

    277: .seealso: [](ch_ksp), `KSPSetUp()`, `KSPSolve()`, `KSPDestroy()`, `KSPConvergedSkip()`, `KSPSetCheckNormIteration()`, `KSPSetPCSide()`, `KSPGetPCSide()`, `KSPNormType`
    278: @*/
    279: PetscErrorCode KSPSetNormType(KSP ksp, KSPNormType normtype)
    280: {
    281:   PetscFunctionBegin;
    284:   ksp->normtype = ksp->normtype_set = normtype;
    285:   PetscFunctionReturn(PETSC_SUCCESS);
    286: }

    288: /*@
    289:   KSPSetCheckNormIteration - Sets the first iteration at which the norm of the residual will be
    290:   computed and used in the convergence test of `KSPSolve()` for the given `KSP` context

    292:   Logically Collective

    294:   Input Parameters:
    295: + ksp - Krylov solver context
    296: - it  - use -1 to check at all iterations

    298:   Level: advanced

    300:   Notes:
    301:   Currently only works with `KSPCG`, `KSPBCGS` and `KSPIBCGS`

    303:   Use `KSPSetNormType`(ksp,`KSP_NORM_NONE`) to never check the norm

    305:   On steps where the norm is not computed, the previous norm is still in the variable, so if you run with, for example,
    306:   `-ksp_monitor` the residual norm will appear to be unchanged for several iterations (though it is not really unchanged).

    308: .seealso: [](ch_ksp), `KSP`, `KSPSetUp()`, `KSPSolve()`, `KSPDestroy()`, `KSPConvergedSkip()`, `KSPSetNormType()`, `KSPSetLagNorm()`
    309: @*/
    310: PetscErrorCode KSPSetCheckNormIteration(KSP ksp, PetscInt it)
    311: {
    312:   PetscFunctionBegin;
    315:   ksp->chknorm = it;
    316:   PetscFunctionReturn(PETSC_SUCCESS);
    317: }

    319: /*@
    320:   KSPSetLagNorm - Lags the residual norm calculation so that it is computed as part of the `MPI_Allreduce()` used for
    321:   computing the inner products needed for the next iteration.

    323:   Logically Collective

    325:   Input Parameters:
    326: + ksp - Krylov solver context
    327: - flg - `PETSC_TRUE` or `PETSC_FALSE`

    329:   Options Database Key:
    330: . -ksp_lag_norm - lag the calculated residual norm

    332:   Level: advanced

    334:   Notes:
    335:   Currently only works with `KSPIBCGS`.

    337:   This can reduce communication costs at the expense of doing
    338:   one additional iteration because the norm used in the convergence test of `KSPSolve()` is one iteration behind the actual
    339:   current residual norm (which has not yet been computed due to the lag).

    341:   Use `KSPSetNormType`(ksp,`KSP_NORM_NONE`) to never check the norm

    343:   If you lag the norm and run with, for example, `-ksp_monitor`, the residual norm reported will be the lagged one.

    345:   `KSPSetCheckNormIteration()` is an alternative way of avoiding the expense of computing the residual norm at each iteration.

    347: .seealso: [](ch_ksp), `KSPSetUp()`, `KSPSolve()`, `KSPDestroy()`, `KSPConvergedSkip()`, `KSPSetNormType()`, `KSPSetCheckNormIteration()`
    348: @*/
    349: PetscErrorCode KSPSetLagNorm(KSP ksp, PetscBool flg)
    350: {
    351:   PetscFunctionBegin;
    354:   ksp->lagnorm = flg;
    355:   PetscFunctionReturn(PETSC_SUCCESS);
    356: }

    358: /*@
    359:   KSPSetSupportedNorm - Sets a norm and preconditioner side supported by a `KSPType`

    361:   Logically Collective

    363:   Input Parameters:
    364: + ksp      - Krylov method
    365: . normtype - supported norm type of the type `KSPNormType`
    366: . pcside   - preconditioner side, of the type `PCSide` that can be used with this `KSPNormType`
    367: - priority - positive integer preference for this combination; larger values have higher priority

    369:   Level: developer

    371:   Notes:
    372:   This function should be called from the implementation files `KSPCreate_XXX()` to declare
    373:   which norms and preconditioner sides are supported. Users should not call this
    374:   function.

    376:   This function can be called multiple times for each combination of `KSPNormType` and `PCSide`
    377:   the `KSPType` supports

    379: .seealso: [](ch_ksp), `KSP`, `KSPNormType`, `PCSide`, `KSPSetNormType()`, `KSPSetPCSide()`
    380: @*/
    381: PetscErrorCode KSPSetSupportedNorm(KSP ksp, KSPNormType normtype, PCSide pcside, PetscInt priority)
    382: {
    383:   PetscFunctionBegin;
    385:   ksp->normsupporttable[normtype][pcside] = priority;
    386:   PetscFunctionReturn(PETSC_SUCCESS);
    387: }

    389: static PetscErrorCode KSPNormSupportTableReset_Private(KSP ksp)
    390: {
    391:   PetscFunctionBegin;
    392:   PetscCall(PetscMemzero(ksp->normsupporttable, sizeof(ksp->normsupporttable)));
    393:   ksp->pc_side  = ksp->pc_side_set;
    394:   ksp->normtype = ksp->normtype_set;
    395:   PetscFunctionReturn(PETSC_SUCCESS);
    396: }

    398: PetscErrorCode KSPSetUpNorms_Private(KSP ksp, PetscBool errorifnotsupported, KSPNormType *normtype, PCSide *pcside)
    399: {
    400:   PetscInt i, j, best, ibest = 0, jbest = 0;

    402:   PetscFunctionBegin;
    403:   best = 0;
    404:   for (i = 0; i < KSP_NORM_MAX; i++) {
    405:     for (j = 0; j < PC_SIDE_MAX; j++) {
    406:       if ((ksp->normtype == KSP_NORM_DEFAULT || ksp->normtype == i) && (ksp->pc_side == PC_SIDE_DEFAULT || ksp->pc_side == j) && (ksp->normsupporttable[i][j] > best)) {
    407:         best  = ksp->normsupporttable[i][j];
    408:         ibest = i;
    409:         jbest = j;
    410:       }
    411:     }
    412:   }
    413:   if (best < 1 && errorifnotsupported) {
    414:     PetscCheck(ksp->normtype != KSP_NORM_DEFAULT || ksp->pc_side != PC_SIDE_DEFAULT, PetscObjectComm((PetscObject)ksp), PETSC_ERR_PLIB, "The %s KSP implementation did not call KSPSetSupportedNorm()", ((PetscObject)ksp)->type_name);
    415:     PetscCheck(ksp->normtype != KSP_NORM_DEFAULT, PetscObjectComm((PetscObject)ksp), PETSC_ERR_SUP, "KSP %s does not support preconditioner side %s", ((PetscObject)ksp)->type_name, PCSides[ksp->pc_side]);
    416:     PetscCheck(ksp->pc_side != PC_SIDE_DEFAULT, PetscObjectComm((PetscObject)ksp), PETSC_ERR_SUP, "KSP %s does not support norm type %s", ((PetscObject)ksp)->type_name, KSPNormTypes[ksp->normtype]);
    417:     SETERRQ(PetscObjectComm((PetscObject)ksp), PETSC_ERR_SUP, "KSP %s does not support norm type %s with preconditioner side %s", ((PetscObject)ksp)->type_name, KSPNormTypes[ksp->normtype], PCSides[ksp->pc_side]);
    418:   }
    419:   if (normtype) *normtype = (KSPNormType)ibest;
    420:   if (pcside) *pcside = (PCSide)jbest;
    421:   PetscFunctionReturn(PETSC_SUCCESS);
    422: }

    424: /*@
    425:   KSPGetNormType - Gets the `KSPNormType` that is used for convergence testing during `KSPSolve()` for this `KSP` context

    427:   Not Collective

    429:   Input Parameter:
    430: . ksp - Krylov solver context

    432:   Output Parameter:
    433: . normtype - the `KSPNormType` that is used for convergence testing

    435:   Level: advanced

    437: .seealso: [](ch_ksp), `KSPNormType`, `KSPSetNormType()`, `KSPConvergedSkip()`
    438: @*/
    439: PetscErrorCode KSPGetNormType(KSP ksp, KSPNormType *normtype)
    440: {
    441:   PetscFunctionBegin;
    443:   PetscAssertPointer(normtype, 2);
    444:   PetscCall(KSPSetUpNorms_Private(ksp, PETSC_TRUE, &ksp->normtype, &ksp->pc_side));
    445:   *normtype = ksp->normtype;
    446:   PetscFunctionReturn(PETSC_SUCCESS);
    447: }

    449: #if defined(PETSC_HAVE_SAWS)
    450: #include <petscviewersaws.h>
    451: #endif

    453: /*@
    454:   KSPSetOperators - Sets the matrix associated with the linear system
    455:   and a (possibly) different one from which the preconditioner will be built into the `KSP` context. The matrix will then be used during `KSPSolve()`

    457:   Collective

    459:   Input Parameters:
    460: + ksp  - the `KSP` context
    461: . Amat - the matrix that defines the linear system
    462: - Pmat - the matrix to be used in constructing the preconditioner, usually the same as `Amat`.

    464:   Level: beginner

    466:   Notes:
    467:   If you know the operator `Amat` has a null space you can use `MatSetNullSpace()` and `MatSetTransposeNullSpace()` to supply the null
    468:   space to `Amat` and the `KSP` solvers will automatically use that null space as needed during the solution process.

    470:   All future calls to `KSPSetOperators()` must use the same size matrices, unless `KSPReset()` is called!

    472:   Passing a `NULL` for `Amat` or `Pmat` removes the matrix that is currently being used from the `KSP` context.

    474:   If you wish to replace either `Amat` or `Pmat` but leave the other one untouched then
    475:   first call `KSPGetOperators()` to get the one you wish to keep, call `PetscObjectReference()`
    476:   on it and then pass it back in your call to `KSPSetOperators()`.

    478:   Developer Notes:
    479:   If the operators have NOT been set with `KSPSetOperators()` then the operators
    480:   are created in the `PC` and returned to the user. In this case, if both operators
    481:   mat and pmat are requested, two DIFFERENT operators will be returned. If
    482:   only one is requested both operators in the `PC` will be the same (i.e. as
    483:   if one had called `KSPSetOperators()` with the same argument for both `Mat`s).
    484:   The user must set the sizes of the returned matrices and their type etc just
    485:   as if the user created them with `MatCreate()`. For example,

    487: .vb
    488:          KSPGetOperators(ksp/pc,&mat,NULL); is equivalent to
    489:            set size, type, etc of mat

    491:          MatCreate(comm,&mat);
    492:          KSP/PCSetOperators(ksp/pc,mat,mat);
    493:          PetscObjectDereference((PetscObject)mat);
    494:            set size, type, etc of mat

    496:      and

    498:          KSP/PCGetOperators(ksp/pc,&mat,&pmat); is equivalent to
    499:            set size, type, etc of mat and pmat

    501:          MatCreate(comm,&mat);
    502:          MatCreate(comm,&pmat);
    503:          KSP/PCSetOperators(ksp/pc,mat,pmat);
    504:          PetscObjectDereference((PetscObject)mat);
    505:          PetscObjectDereference((PetscObject)pmat);
    506:            set size, type, etc of mat and pmat
    507: .ve

    509:   The rationale for this support is so that when creating a `TS`, `SNES`, or `KSP` the hierarchy
    510:   of underlying objects (i.e. `SNES`, `KSP`, `PC`, `Mat`) and their lifespans can be completely
    511:   managed by the top most level object (i.e. the `TS`, `SNES`, or `KSP`). Another way to look
    512:   at this is when you create a `SNES` you do not NEED to create a `KSP` and attach it to
    513:   the `SNES` object (the `SNES` object manages it for you). Similarly when you create a `KSP`
    514:   you do not need to attach a `PC` to it (the `KSP` object manages the `PC` object for you).
    515:   Thus, why should YOU have to create the `Mat` and attach it to the `SNES`/`KSP`/`PC`, when
    516:   it can be created for you?

    518: .seealso: [](ch_ksp), `KSP`, `Mat`, `KSPSolve()`, `KSPGetPC()`, `PCGetOperators()`, `PCSetOperators()`, `KSPGetOperators()`, `KSPSetComputeOperators()`, `KSPSetComputeInitialGuess()`, `KSPSetComputeRHS()`
    519: @*/
    520: PetscErrorCode KSPSetOperators(KSP ksp, Mat Amat, Mat Pmat)
    521: {
    522:   PetscFunctionBegin;
    526:   if (Amat) PetscCheckSameComm(ksp, 1, Amat, 2);
    527:   if (Pmat) PetscCheckSameComm(ksp, 1, Pmat, 3);
    528:   if (!ksp->pc) PetscCall(KSPGetPC(ksp, &ksp->pc));
    529:   PetscCall(PCSetOperators(ksp->pc, Amat, Pmat));
    530:   if (ksp->setupstage == KSP_SETUP_NEWRHS) ksp->setupstage = KSP_SETUP_NEWMATRIX; /* so that next solve call will call PCSetUp() on new matrix */
    531:   PetscFunctionReturn(PETSC_SUCCESS);
    532: }

    534: /*@
    535:   KSPGetOperators - Gets the matrix associated with the linear system
    536:   and a (possibly) different one used to construct the preconditioner from the `KSP` context

    538:   Collective

    540:   Input Parameter:
    541: . ksp - the `KSP` context

    543:   Output Parameters:
    544: + Amat - the matrix that defines the linear system
    545: - Pmat - the matrix to be used in constructing the preconditioner, usually the same as `Amat`.

    547:   Level: intermediate

    549:   Notes:
    550:   If `KSPSetOperators()` has not been called then the `KSP` object will attempt to automatically create the matrix `Amat` and return it

    552:   Use `KSPGetOperatorsSet()` to determine if matrices have been provided.

    554:   DOES NOT increase the reference counts of the matrix, so you should NOT destroy them.

    556: .seealso: [](ch_ksp), `KSP`, `KSPSolve()`, `KSPGetPC()`, `PCSetOperators()`, `KSPSetOperators()`, `KSPGetOperatorsSet()`
    557: @*/
    558: PetscErrorCode KSPGetOperators(KSP ksp, Mat *Amat, Mat *Pmat)
    559: {
    560:   PetscFunctionBegin;
    562:   if (!ksp->pc) PetscCall(KSPGetPC(ksp, &ksp->pc));
    563:   PetscCall(PCGetOperators(ksp->pc, Amat, Pmat));
    564:   PetscFunctionReturn(PETSC_SUCCESS);
    565: }

    567: /*@
    568:   KSPGetOperatorsSet - Determines if the matrix associated with the linear system and
    569:   possibly a different one from which the preconditioner will be built have been set in the `KSP` with `KSPSetOperators()`

    571:   Not Collective, though the results on all processes will be the same

    573:   Input Parameter:
    574: . ksp - the `KSP` context

    576:   Output Parameters:
    577: + mat  - the matrix associated with the linear system was set
    578: - pmat - matrix from which the preconditioner will be built, usually the same as `mat` was set

    580:   Level: intermediate

    582:   Note:
    583:   This routine exists because if you call `KSPGetOperators()` on a `KSP` that does not yet have operators they are
    584:   automatically created in the call.

    586: .seealso: [](ch_ksp), `KSP`, `PCSetOperators()`, `KSPGetOperators()`, `KSPSetOperators()`, `PCGetOperators()`, `PCGetOperatorsSet()`
    587: @*/
    588: PetscErrorCode KSPGetOperatorsSet(KSP ksp, PetscBool *mat, PetscBool *pmat)
    589: {
    590:   PetscFunctionBegin;
    592:   if (!ksp->pc) PetscCall(KSPGetPC(ksp, &ksp->pc));
    593:   PetscCall(PCGetOperatorsSet(ksp->pc, mat, pmat));
    594:   PetscFunctionReturn(PETSC_SUCCESS);
    595: }

    597: /*@C
    598:   KSPSetPreSolve - Sets a function that is called at the beginning of each `KSPSolve()`. Used in conjunction with `KSPSetPostSolve()`.

    600:   Logically Collective

    602:   Input Parameters:
    603: + ksp      - the solver object
    604: . presolve - the function to call before the solve
    605: - ctx      - an optional context needed by the function

    607:   Calling sequence of `presolve`:
    608: + ksp - the `KSP` context
    609: . rhs - the right-hand side vector
    610: . x   - the solution vector
    611: - ctx - optional user-provided context

    613:   Level: developer

    615:   Notes:
    616:   The function provided here `presolve` is used to modify the right hand side, and possibly the matrix, of the linear system to be solved.
    617:   The function provided with `KSPSetPostSolve()` then modifies the resulting solution of that linear system to obtain the correct solution
    618:   to the initial linear system.

    620:   The functions `PCPreSolve()` and `PCPostSolve()` provide a similar functionality and are used, for example with `PCEISENSTAT`.

    622: .seealso: [](ch_ksp), `KSPSetUp()`, `KSPSolve()`, `KSPDestroy()`, `KSP`, `KSPSetPostSolve()`, `PCEISENSTAT`, `PCPreSolve()`, `PCPostSolve()`
    623: @*/
    624: PetscErrorCode KSPSetPreSolve(KSP ksp, PetscErrorCode (*presolve)(KSP ksp, Vec rhs, Vec x, void *ctx), void *ctx)
    625: {
    626:   PetscFunctionBegin;
    628:   ksp->presolve = presolve;
    629:   ksp->prectx   = ctx;
    630:   PetscFunctionReturn(PETSC_SUCCESS);
    631: }

    633: /*@C
    634:   KSPSetPostSolve - Sets a function that is called at the end of each `KSPSolve()` (whether it converges or not). Used in conjunction with `KSPSetPreSolve()`.

    636:   Logically Collective

    638:   Input Parameters:
    639: + ksp       - the solver object
    640: . postsolve - the function to call after the solve
    641: - ctx       - an optional context needed by the function

    643:   Calling sequence of `postsolve`:
    644: + ksp - the `KSP` context
    645: . rhs - the right-hand side vector
    646: . x   - the solution vector
    647: - ctx - optional user-provided context

    649:   Level: developer

    651: .seealso: [](ch_ksp), `KSPSetUp()`, `KSPSolve()`, `KSPDestroy()`, `KSP`, `KSPSetPreSolve()`, `PCEISENSTAT`
    652: @*/
    653: PetscErrorCode KSPSetPostSolve(KSP ksp, PetscErrorCode (*postsolve)(KSP ksp, Vec rhs, Vec x, void *ctx), void *ctx)
    654: {
    655:   PetscFunctionBegin;
    657:   ksp->postsolve = postsolve;
    658:   ksp->postctx   = ctx;
    659:   PetscFunctionReturn(PETSC_SUCCESS);
    660: }

    662: /*@
    663:   KSPSetNestLevel - sets the amount of nesting the `KSP` has. That is the number of levels of `KSP` above this `KSP` in a linear solve.

    665:   Collective

    667:   Input Parameters:
    668: + ksp   - the `KSP`
    669: - level - the nest level

    671:   Level: developer

    673:   Note:
    674:   For example, the `KSP` in each block of a `KSPBJACOBI` has a level of 1, while the outer `KSP` has a level of 0.

    676: .seealso: [](ch_ksp), `KSPSetUp()`, `KSPSolve()`, `KSPDestroy()`, `KSP`, `KSPGMRES`, `KSPType`, `KSPGetNestLevel()`, `PCSetKSPNestLevel()`, `PCGetKSPNestLevel()`
    677: @*/
    678: PetscErrorCode KSPSetNestLevel(KSP ksp, PetscInt level)
    679: {
    680:   PetscFunctionBegin;
    683:   ksp->nestlevel = level;
    684:   PetscFunctionReturn(PETSC_SUCCESS);
    685: }

    687: /*@
    688:   KSPGetNestLevel - gets the amount of nesting the `KSP` has

    690:   Not Collective

    692:   Input Parameter:
    693: . ksp - the `KSP`

    695:   Output Parameter:
    696: . level - the nest level

    698:   Level: developer

    700: .seealso: [](ch_ksp), `KSPSetUp()`, `KSPSolve()`, `KSPDestroy()`, `KSP`, `KSPGMRES`, `KSPType`, `KSPSetNestLevel()`, `PCSetKSPNestLevel()`, `PCGetKSPNestLevel()`
    701: @*/
    702: PetscErrorCode KSPGetNestLevel(KSP ksp, PetscInt *level)
    703: {
    704:   PetscFunctionBegin;
    706:   PetscAssertPointer(level, 2);
    707:   *level = ksp->nestlevel;
    708:   PetscFunctionReturn(PETSC_SUCCESS);
    709: }

    711: /*@
    712:   KSPCreate - Creates the `KSP` context. This `KSP` context is used in PETSc to solve linear systems with `KSPSolve()`

    714:   Collective

    716:   Input Parameter:
    717: . comm - MPI communicator

    719:   Output Parameter:
    720: . inksp - location to put the `KSP` context

    722:   Level: beginner

    724:   Note:
    725:   The default `KSPType` is `KSPGMRES` with a restart of 30, using modified Gram-Schmidt orthogonalization. The `KSPType` may be
    726:   changed with `KSPSetType()`

    728: .seealso: [](ch_ksp), `KSPSetUp()`, `KSPSolve()`, `KSPDestroy()`, `KSP`, `KSPGMRES`, `KSPType`, `KSPSetType()`
    729: @*/
    730: PetscErrorCode KSPCreate(MPI_Comm comm, KSP *inksp)
    731: {
    732:   KSP   ksp;
    733:   void *ctx;

    735:   PetscFunctionBegin;
    736:   PetscAssertPointer(inksp, 2);
    737:   PetscCall(KSPInitializePackage());

    739:   PetscCall(PetscHeaderCreate(ksp, KSP_CLASSID, "KSP", "Krylov Method", "KSP", comm, KSPDestroy, KSPView));
    740:   ksp->default_max_it = ksp->max_it = 10000;
    741:   ksp->pc_side = ksp->pc_side_set = PC_SIDE_DEFAULT;

    743:   ksp->default_rtol = ksp->rtol = 1.e-5;
    744:   ksp->default_abstol = ksp->abstol = PetscDefined(USE_REAL_SINGLE) ? 1.e-25 : 1.e-50;
    745:   ksp->default_divtol = ksp->divtol = 1.e4;

    747:   ksp->chknorm  = -1;
    748:   ksp->normtype = ksp->normtype_set = KSP_NORM_DEFAULT;
    749:   ksp->rnorm                        = 0.0;
    750:   ksp->its                          = 0;
    751:   ksp->guess_zero                   = PETSC_TRUE;
    752:   ksp->calc_sings                   = PETSC_FALSE;
    753:   ksp->res_hist                     = NULL;
    754:   ksp->res_hist_alloc               = NULL;
    755:   ksp->res_hist_len                 = 0;
    756:   ksp->res_hist_max                 = 0;
    757:   ksp->res_hist_reset               = PETSC_TRUE;
    758:   ksp->err_hist                     = NULL;
    759:   ksp->err_hist_alloc               = NULL;
    760:   ksp->err_hist_len                 = 0;
    761:   ksp->err_hist_max                 = 0;
    762:   ksp->err_hist_reset               = PETSC_TRUE;
    763:   ksp->numbermonitors               = 0;
    764:   ksp->numberreasonviews            = 0;
    765:   ksp->setfromoptionscalled         = 0;
    766:   ksp->nmax                         = PETSC_DECIDE;

    768:   PetscCall(KSPConvergedDefaultCreate(&ctx));
    769:   PetscCall(KSPSetConvergenceTest(ksp, KSPConvergedDefault, ctx, KSPConvergedDefaultDestroy));
    770:   ksp->ops->buildsolution = KSPBuildSolutionDefault;
    771:   ksp->ops->buildresidual = KSPBuildResidualDefault;

    773:   ksp->vec_sol    = NULL;
    774:   ksp->vec_rhs    = NULL;
    775:   ksp->pc         = NULL;
    776:   ksp->data       = NULL;
    777:   ksp->nwork      = 0;
    778:   ksp->work       = NULL;
    779:   ksp->reason     = KSP_CONVERGED_ITERATING;
    780:   ksp->setupstage = KSP_SETUP_NEW;

    782:   PetscCall(KSPNormSupportTableReset_Private(ksp));

    784:   *inksp = ksp;
    785:   PetscFunctionReturn(PETSC_SUCCESS);
    786: }

    788: /*@
    789:   KSPSetType - Sets the algorithm/method to be used to solve the linear system with the given `KSP`

    791:   Logically Collective

    793:   Input Parameters:
    794: + ksp  - the Krylov space context
    795: - type - a known method

    797:   Options Database Key:
    798: . -ksp_type  <method> - Sets the method; use `-help` for a list  of available methods (for instance, cg or gmres)

    800:   Level: intermediate

    802:   Notes:
    803:   See `KSPType` for available methods (for instance, `KSPCG` or `KSPGMRES`).

    805:   Normally, it is best to use the `KSPSetFromOptions()` command and
    806:   then set the `KSP` type from the options database rather than by using
    807:   this routine.  Using the options database provides the user with
    808:   maximum flexibility in evaluating the many different Krylov methods.
    809:   The `KSPSetType()` routine is provided for those situations where it
    810:   is necessary to set the iterative solver independently of the command
    811:   line or options database.  This might be the case, for example, when
    812:   the choice of iterative solver changes during the execution of the
    813:   program, and the user's application is taking responsibility for
    814:   choosing the appropriate method.  In other words, this routine is
    815:   not for beginners.

    817:   Developer Note:
    818:   `KSPRegister()` is used to add Krylov types to `KSPList` from which they are accessed by `KSPSetType()`.

    820: .seealso: [](ch_ksp), `PCSetType()`, `KSPType`, `KSPRegister()`, `KSPCreate()`, `KSP`
    821: @*/
    822: PetscErrorCode KSPSetType(KSP ksp, KSPType type)
    823: {
    824:   PetscBool match;
    825:   PetscErrorCode (*r)(KSP);

    827:   PetscFunctionBegin;
    829:   PetscAssertPointer(type, 2);

    831:   PetscCall(PetscObjectTypeCompare((PetscObject)ksp, type, &match));
    832:   if (match) PetscFunctionReturn(PETSC_SUCCESS);

    834:   PetscCall(PetscFunctionListFind(KSPList, type, &r));
    835:   PetscCheck(r, PetscObjectComm((PetscObject)ksp), PETSC_ERR_ARG_UNKNOWN_TYPE, "Unable to find requested KSP type %s", type);
    836:   /* Destroy the previous private KSP context */
    837:   PetscTryTypeMethod(ksp, destroy);
    838:   ksp->ops->destroy = NULL;

    840:   /* Reinitialize function pointers in KSPOps structure */
    841:   PetscCall(PetscMemzero(ksp->ops, sizeof(struct _KSPOps)));
    842:   ksp->ops->buildsolution = KSPBuildSolutionDefault;
    843:   ksp->ops->buildresidual = KSPBuildResidualDefault;
    844:   PetscCall(KSPNormSupportTableReset_Private(ksp));
    845:   ksp->converged_neg_curve = PETSC_FALSE; // restore default
    846:   ksp->setupnewmatrix      = PETSC_FALSE; // restore default (setup not called in case of new matrix)
    847:   /* Call the KSPCreate_XXX routine for this particular Krylov solver */
    848:   ksp->setupstage     = KSP_SETUP_NEW;
    849:   ksp->guess_not_read = PETSC_FALSE; // restore default
    850:   PetscCall((*r)(ksp));
    851:   PetscCall(PetscObjectChangeTypeName((PetscObject)ksp, type));
    852:   PetscFunctionReturn(PETSC_SUCCESS);
    853: }

    855: /*@
    856:   KSPGetType - Gets the `KSP` type as a string from the `KSP` object.

    858:   Not Collective

    860:   Input Parameter:
    861: . ksp - Krylov context

    863:   Output Parameter:
    864: . type - name of the `KSP` method

    866:   Level: intermediate

    868: .seealso: [](ch_ksp), `KSPType`, `KSP`, `KSPSetType()`
    869: @*/
    870: PetscErrorCode KSPGetType(KSP ksp, KSPType *type)
    871: {
    872:   PetscFunctionBegin;
    874:   PetscAssertPointer(type, 2);
    875:   *type = ((PetscObject)ksp)->type_name;
    876:   PetscFunctionReturn(PETSC_SUCCESS);
    877: }

    879: /*@C
    880:   KSPRegister -  Adds a method, `KSPType`, to the Krylov subspace solver package.

    882:   Not Collective, No Fortran Support

    884:   Input Parameters:
    885: + sname    - name of a new user-defined solver
    886: - function - routine to create method

    888:   Level: advanced

    890:   Note:
    891:   `KSPRegister()` may be called multiple times to add several user-defined solvers.

    893:   Example Usage:
    894: .vb
    895:    KSPRegister("my_solver", MySolverCreate);
    896: .ve

    898:   Then, your solver can be chosen with the procedural interface via
    899: .vb
    900:   KSPSetType(ksp, "my_solver")
    901: .ve
    902:   or at runtime via the option `-ksp_type my_solver`

    904: .seealso: [](ch_ksp), `KSP`, `KSPType`, `KSPSetType`, `KSPRegisterAll()`
    905: @*/
    906: PetscErrorCode KSPRegister(const char sname[], PetscErrorCode (*function)(KSP))
    907: {
    908:   PetscFunctionBegin;
    909:   PetscCall(KSPInitializePackage());
    910:   PetscCall(PetscFunctionListAdd(&KSPList, sname, function));
    911:   PetscFunctionReturn(PETSC_SUCCESS);
    912: }

    914: PetscErrorCode KSPMonitorMakeKey_Internal(const char name[], PetscViewerType vtype, PetscViewerFormat format, char key[])
    915: {
    916:   PetscFunctionBegin;
    917:   PetscCall(PetscStrncpy(key, name, PETSC_MAX_PATH_LEN));
    918:   PetscCall(PetscStrlcat(key, ":", PETSC_MAX_PATH_LEN));
    919:   PetscCall(PetscStrlcat(key, vtype, PETSC_MAX_PATH_LEN));
    920:   PetscCall(PetscStrlcat(key, ":", PETSC_MAX_PATH_LEN));
    921:   PetscCall(PetscStrlcat(key, PetscViewerFormats[format], PETSC_MAX_PATH_LEN));
    922:   PetscFunctionReturn(PETSC_SUCCESS);
    923: }

    925: /*@C
    926:   KSPMonitorRegister -  Registers a Krylov subspace solver monitor routine that may be accessed with `KSPMonitorSetFromOptions()`

    928:   Not Collective

    930:   Input Parameters:
    931: + name    - name of a new monitor routine
    932: . vtype   - A `PetscViewerType` for the output
    933: . format  - A `PetscViewerFormat` for the output
    934: . monitor - Monitor routine
    935: . create  - Creation routine, or `NULL`
    936: - destroy - Destruction routine, or `NULL`

    938:   Level: advanced

    940:   Note:
    941:   `KSPMonitorRegister()` may be called multiple times to add several user-defined monitors.

    943:   Example Usage:
    944: .vb
    945:   KSPMonitorRegister("my_monitor", PETSCVIEWERASCII, PETSC_VIEWER_ASCII_INFO_DETAIL, MyMonitor, NULL, NULL);
    946: .ve

    948:   Then, your monitor can be chosen with the procedural interface via
    949: .vb
    950:   KSPMonitorSetFromOptions(ksp, "-ksp_monitor_my_monitor", "my_monitor", NULL)
    951: .ve
    952:   or at runtime via the option `-ksp_monitor_my_monitor`

    954: .seealso: [](ch_ksp), `KSP`, `KSPMonitorSet()`, `KSPMonitorRegisterAll()`, `KSPMonitorSetFromOptions()`
    955: @*/
    956: PetscErrorCode KSPMonitorRegister(const char name[], PetscViewerType vtype, PetscViewerFormat format, PetscErrorCode (*monitor)(KSP, PetscInt, PetscReal, PetscViewerAndFormat *), PetscErrorCode (*create)(PetscViewer, PetscViewerFormat, void *, PetscViewerAndFormat **), PetscErrorCode (*destroy)(PetscViewerAndFormat **))
    957: {
    958:   char key[PETSC_MAX_PATH_LEN];

    960:   PetscFunctionBegin;
    961:   PetscCall(KSPInitializePackage());
    962:   PetscCall(KSPMonitorMakeKey_Internal(name, vtype, format, key));
    963:   PetscCall(PetscFunctionListAdd(&KSPMonitorList, key, monitor));
    964:   if (create) PetscCall(PetscFunctionListAdd(&KSPMonitorCreateList, key, create));
    965:   if (destroy) PetscCall(PetscFunctionListAdd(&KSPMonitorDestroyList, key, destroy));
    966:   PetscFunctionReturn(PETSC_SUCCESS);
    967: }