1 #include <petsc/private/linesearchimpl.h> /*I "petscsnes.h" I*/ 2 #include <petsc/private/snesimpl.h> 3 4 typedef struct { 5 PetscReal alpha; /* sufficient decrease parameter */ 6 } SNESLineSearch_BT; 7 8 /*@ 9 SNESLineSearchBTSetAlpha - Sets the descent parameter, `alpha`, in the `SNESLINESEARCHBT` `SNESLineSearch` variant. 10 11 Input Parameters: 12 + linesearch - linesearch context 13 - alpha - The descent parameter 14 15 Level: intermediate 16 17 .seealso: [](ch_snes), `SNESLineSearch`, `SNESLineSearchSetLambda()`, `SNESLineSearchGetTolerances()`, `SNESLINESEARCHBT`, `SNESLineSearchBTGetAlpha()` 18 @*/ 19 PetscErrorCode SNESLineSearchBTSetAlpha(SNESLineSearch linesearch, PetscReal alpha) 20 { 21 SNESLineSearch_BT *bt = (SNESLineSearch_BT *)linesearch->data; 22 23 PetscFunctionBegin; 24 PetscValidHeaderSpecific(linesearch, SNESLINESEARCH_CLASSID, 1); 25 bt->alpha = alpha; 26 PetscFunctionReturn(PETSC_SUCCESS); 27 } 28 29 /*@ 30 SNESLineSearchBTGetAlpha - Gets the descent parameter, `alpha`, in the `SNESLINESEARCHBT` variant that was set with `SNESLineSearchBTSetAlpha()` 31 32 Input Parameter: 33 . linesearch - linesearch context 34 35 Output Parameter: 36 . alpha - The descent parameter 37 38 Level: intermediate 39 40 .seealso: [](ch_snes), `SNESLineSearch`, `SNESLineSearchGetLambda()`, `SNESLineSearchGetTolerances()`, `SNESLINESEARCHBT`, `SNESLineSearchBTSetAlpha()` 41 @*/ 42 PetscErrorCode SNESLineSearchBTGetAlpha(SNESLineSearch linesearch, PetscReal *alpha) 43 { 44 SNESLineSearch_BT *bt = (SNESLineSearch_BT *)linesearch->data; 45 46 PetscFunctionBegin; 47 PetscValidHeaderSpecific(linesearch, SNESLINESEARCH_CLASSID, 1); 48 *alpha = bt->alpha; 49 PetscFunctionReturn(PETSC_SUCCESS); 50 } 51 52 static PetscErrorCode SNESLineSearchApply_BT(SNESLineSearch linesearch) 53 { 54 SNESLineSearch_BT *bt = (SNESLineSearch_BT *)linesearch->data; 55 PetscBool changed_y, changed_w; 56 Vec X, F, Y, W, G; 57 SNES snes; 58 PetscReal fnorm, xnorm, ynorm, gnorm; 59 PetscReal lambda, lambdatemp, lambdaprev, minlambda, maxstep, initslope, alpha, stol; 60 PetscReal t1, t2, a, b, d; 61 PetscReal f; 62 PetscReal g, gprev; 63 PetscViewer monitor; 64 PetscInt max_its, count; 65 Mat jac; 66 SNESObjectiveFn *objective; 67 const char *const ordStr[] = {"Linear", "Quadratic", "Cubic"}; 68 69 PetscFunctionBegin; 70 PetscCall(SNESLineSearchGetVecs(linesearch, &X, &F, &Y, &W, &G)); 71 PetscCall(SNESLineSearchGetNorms(linesearch, NULL, &fnorm, NULL)); 72 PetscCall(SNESLineSearchGetLambda(linesearch, &lambda)); 73 PetscCall(SNESLineSearchGetSNES(linesearch, &snes)); 74 PetscCall(SNESLineSearchGetDefaultMonitor(linesearch, &monitor)); 75 PetscCall(SNESLineSearchGetTolerances(linesearch, &minlambda, &maxstep, NULL, NULL, NULL, &max_its)); 76 PetscCall(SNESGetTolerances(snes, NULL, NULL, &stol, NULL, NULL)); 77 PetscCall(SNESGetObjective(snes, &objective, NULL)); 78 alpha = bt->alpha; 79 80 PetscCall(SNESGetJacobian(snes, &jac, NULL, NULL, NULL)); 81 PetscCheck(jac || objective, PetscObjectComm((PetscObject)linesearch), PETSC_ERR_USER, "SNESLineSearchBT requires a Jacobian matrix"); 82 83 PetscCall(SNESLineSearchPreCheck(linesearch, X, Y, &changed_y)); 84 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_SUCCEEDED)); 85 86 PetscCall(VecNormBegin(Y, NORM_2, &ynorm)); 87 PetscCall(VecNormBegin(X, NORM_2, &xnorm)); 88 PetscCall(VecNormEnd(Y, NORM_2, &ynorm)); 89 PetscCall(VecNormEnd(X, NORM_2, &xnorm)); 90 91 if (ynorm == 0.0) { 92 if (monitor) { 93 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 94 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: Initial direction and size is 0\n")); 95 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 96 } 97 PetscCall(VecCopy(X, W)); 98 PetscCall(VecCopy(F, G)); 99 PetscCall(SNESLineSearchSetNorms(linesearch, xnorm, fnorm, ynorm)); 100 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_FAILED_REDUCT)); 101 PetscFunctionReturn(PETSC_SUCCESS); 102 } 103 if (ynorm > maxstep) { /* Step too big, so scale back */ 104 if (monitor) { 105 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 106 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: Scaling step by %14.12e old ynorm %14.12e\n", (double)(maxstep / ynorm), (double)ynorm)); 107 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 108 } 109 PetscCall(VecScale(Y, maxstep / ynorm)); 110 ynorm = maxstep; 111 } 112 113 /* if the SNES has an objective set, use that instead of the function value */ 114 if (objective) { 115 PetscCall(SNESComputeObjective(snes, X, &f)); 116 } else { 117 f = 0.5 * PetscSqr(fnorm); 118 } 119 120 /* compute the initial slope */ 121 if (objective) { 122 /* slope comes from the function (assumed to be the gradient of the objective) */ 123 PetscCall(VecDotRealPart(Y, F, &initslope)); 124 } else { 125 /* slope comes from the normal equations */ 126 PetscCall(MatMult(jac, Y, W)); 127 PetscCall(VecDotRealPart(F, W, &initslope)); 128 if (initslope > 0.0) initslope = -initslope; 129 if (initslope == 0.0) initslope = -1.0; 130 } 131 132 while (PETSC_TRUE) { 133 PetscCall(VecWAXPY(W, -lambda, Y, X)); 134 if (linesearch->ops->viproject) PetscCall((*linesearch->ops->viproject)(snes, W)); 135 if (snes->nfuncs >= snes->max_funcs && snes->max_funcs >= 0) { 136 PetscCall(PetscInfo(snes, "Exceeded maximum function evaluations, while checking full step length!\n")); 137 snes->reason = SNES_DIVERGED_FUNCTION_COUNT; 138 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_FAILED_FUNCTION)); 139 PetscFunctionReturn(PETSC_SUCCESS); 140 } 141 142 if (objective) { 143 PetscCall(SNESComputeObjective(snes, W, &g)); 144 } else { 145 PetscCall((*linesearch->ops->snesfunc)(snes, W, G)); 146 if (linesearch->ops->vinorm) { 147 gnorm = fnorm; 148 PetscCall((*linesearch->ops->vinorm)(snes, G, W, &gnorm)); 149 } else { 150 PetscCall(VecNorm(G, NORM_2, &gnorm)); 151 } 152 g = 0.5 * PetscSqr(gnorm); 153 } 154 PetscCall(SNESLineSearchMonitor(linesearch)); 155 156 if (!PetscIsInfOrNanReal(g)) break; 157 if (monitor) { 158 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 159 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: objective function at lambdas = %g is Inf or Nan, cutting lambda\n", (double)lambda)); 160 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 161 } 162 if (lambda <= minlambda) SNESCheckFunctionNorm(snes, g); 163 lambda *= .5; 164 } 165 166 if (!objective) PetscCall(PetscInfo(snes, "Initial fnorm %14.12e gnorm %14.12e\n", (double)fnorm, (double)gnorm)); 167 if (g <= f + lambda * alpha * initslope) { /* Sufficient reduction or step tolerance convergence */ 168 if (monitor) { 169 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 170 if (!objective) { 171 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: Using full step: fnorm %14.12e gnorm %14.12e\n", (double)fnorm, (double)gnorm)); 172 } else { 173 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: Using full step: old obj %14.12e new obj %14.12e\n", (double)f, (double)g)); 174 } 175 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 176 } 177 } else { 178 if (stol * xnorm > ynorm) { 179 /* Since the full step didn't give sufficient decrease and the step is tiny, exit */ 180 PetscCall(SNESLineSearchSetNorms(linesearch, xnorm, fnorm, ynorm)); 181 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_SUCCEEDED)); 182 if (monitor) { 183 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 184 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: Ended due to ynorm < stol*xnorm (%14.12e < %14.12e).\n", (double)ynorm, (double)(stol * xnorm))); 185 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 186 } 187 PetscFunctionReturn(PETSC_SUCCESS); 188 } 189 /* Here to avoid -Wmaybe-uninitiliazed warnings */ 190 lambdaprev = lambda; 191 gprev = g; 192 if (linesearch->order != SNES_LINESEARCH_ORDER_LINEAR) { 193 /* Fit points with quadratic */ 194 lambdatemp = -initslope * PetscSqr(lambda) / (2.0 * (g - f - lambda * initslope)); 195 lambda = PetscClipInterval(lambdatemp, .1 * lambda, .5 * lambda); 196 197 PetscCall(VecWAXPY(W, -lambda, Y, X)); 198 if (linesearch->ops->viproject) PetscCall((*linesearch->ops->viproject)(snes, W)); 199 if (snes->nfuncs >= snes->max_funcs && snes->max_funcs >= 0) { 200 PetscCall(PetscInfo(snes, "Exceeded maximum function evaluations, while attempting quadratic backtracking! %" PetscInt_FMT " \n", snes->nfuncs)); 201 snes->reason = SNES_DIVERGED_FUNCTION_COUNT; 202 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_FAILED_FUNCTION)); 203 PetscFunctionReturn(PETSC_SUCCESS); 204 } 205 if (objective) { 206 PetscCall(SNESComputeObjective(snes, W, &g)); 207 } else { 208 PetscCall((*linesearch->ops->snesfunc)(snes, W, G)); 209 if (linesearch->ops->vinorm) { 210 gnorm = fnorm; 211 PetscCall((*linesearch->ops->vinorm)(snes, G, W, &gnorm)); 212 } else { 213 PetscCall(VecNorm(G, NORM_2, &gnorm)); 214 } 215 g = 0.5 * PetscSqr(gnorm); 216 } 217 if (PetscIsInfOrNanReal(g)) { 218 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_FAILED_NANORINF)); 219 PetscCall(PetscInfo(snes, "Aborted due to Nan or Inf in function evaluation\n")); 220 PetscFunctionReturn(PETSC_SUCCESS); 221 } 222 if (monitor) { 223 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 224 if (!objective) { 225 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: gnorm after quadratic fit %14.12e\n", (double)gnorm)); 226 } else { 227 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: obj after quadratic fit %14.12e\n", (double)g)); 228 } 229 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 230 } 231 } 232 if (linesearch->order != SNES_LINESEARCH_ORDER_LINEAR && g <= f + lambda * alpha * initslope) { /* sufficient reduction */ 233 if (monitor) { 234 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 235 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: Quadratically determined step, lambda=%18.16e\n", (double)lambda)); 236 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 237 } 238 } else { 239 for (count = 0; count < max_its; count++) { 240 if (lambda <= minlambda) { 241 if (monitor) { 242 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 243 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: unable to find good step length! After %" PetscInt_FMT " tries \n", count)); 244 if (!objective) { 245 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: fnorm=%18.16e, gnorm=%18.16e, ynorm=%18.16e, minlambda=%18.16e, lambda=%18.16e, initial slope=%18.16e\n", (double)fnorm, (double)gnorm, (double)ynorm, (double)minlambda, (double)lambda, (double)initslope)); 246 } else { 247 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: obj(0)=%18.16e, obj=%18.16e, ynorm=%18.16e, minlambda=%18.16e, lambda=%18.16e, initial slope=%18.16e\n", (double)f, (double)g, (double)ynorm, (double)minlambda, (double)lambda, (double)initslope)); 248 } 249 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 250 } 251 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_FAILED_REDUCT)); 252 PetscFunctionReturn(PETSC_SUCCESS); 253 } 254 if (linesearch->order == SNES_LINESEARCH_ORDER_CUBIC) { 255 /* Fit points with cubic */ 256 t1 = g - f - lambda * initslope; 257 t2 = gprev - f - lambdaprev * initslope; 258 a = (t1 / (lambda * lambda) - t2 / (lambdaprev * lambdaprev)) / (lambda - lambdaprev); 259 b = (-lambdaprev * t1 / (lambda * lambda) + lambda * t2 / (lambdaprev * lambdaprev)) / (lambda - lambdaprev); 260 d = b * b - 3 * a * initslope; 261 if (d < 0.0) d = 0.0; 262 if (a == 0.0) lambdatemp = -initslope / (2.0 * b); 263 else lambdatemp = (-b + PetscSqrtReal(d)) / (3.0 * a); 264 } else if (linesearch->order == SNES_LINESEARCH_ORDER_QUADRATIC) { 265 lambdatemp = -initslope * PetscSqr(lambda) / (2.0 * (g - f - lambda * initslope)); 266 } else if (linesearch->order == SNES_LINESEARCH_ORDER_LINEAR) { /* Just backtrack */ 267 lambdatemp = .5 * lambda; 268 } else SETERRQ(PetscObjectComm((PetscObject)linesearch), PETSC_ERR_SUP, "Line search order %" PetscInt_FMT " for type bt", linesearch->order); 269 lambdaprev = lambda; 270 gprev = g; 271 272 lambda = PetscClipInterval(lambdatemp, .1 * lambda, .5 * lambda); 273 PetscCall(VecWAXPY(W, -lambda, Y, X)); 274 if (linesearch->ops->viproject) PetscCall((*linesearch->ops->viproject)(snes, W)); 275 if (snes->nfuncs >= snes->max_funcs && snes->max_funcs >= 0) { 276 PetscCall(PetscInfo(snes, "Exceeded maximum function evaluations, while looking for good step length! %" PetscInt_FMT " \n", count)); 277 if (!objective) PetscCall(PetscInfo(snes, "fnorm=%18.16e, gnorm=%18.16e, ynorm=%18.16e, lambda=%18.16e, initial slope=%18.16e\n", (double)fnorm, (double)gnorm, (double)ynorm, (double)lambda, (double)initslope)); 278 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_FAILED_FUNCTION)); 279 snes->reason = SNES_DIVERGED_FUNCTION_COUNT; 280 PetscFunctionReturn(PETSC_SUCCESS); 281 } 282 if (objective) { 283 PetscCall(SNESComputeObjective(snes, W, &g)); 284 } else { 285 PetscCall((*linesearch->ops->snesfunc)(snes, W, G)); 286 if (linesearch->ops->vinorm) { 287 gnorm = fnorm; 288 PetscCall((*linesearch->ops->vinorm)(snes, G, W, &gnorm)); 289 } else { 290 PetscCall(VecNorm(G, NORM_2, &gnorm)); 291 } 292 g = 0.5 * PetscSqr(gnorm); 293 } 294 if (PetscIsInfOrNanReal(g)) { 295 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_FAILED_NANORINF)); 296 PetscCall(PetscInfo(snes, "Aborted due to Nan or Inf in function evaluation\n")); 297 PetscFunctionReturn(PETSC_SUCCESS); 298 } 299 if (g <= f + lambda * alpha * initslope) { /* is reduction enough? */ 300 if (monitor) { 301 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 302 if (!objective) { 303 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: %s step, current gnorm %14.12e lambda=%18.16e\n", ordStr[linesearch->order - 1], (double)gnorm, (double)lambda)); 304 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 305 } else { 306 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: %s step, obj %14.12e lambda=%18.16e\n", ordStr[linesearch->order - 1], (double)g, (double)lambda)); 307 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 308 } 309 } 310 break; 311 } else if (monitor) { 312 PetscCall(PetscViewerASCIIAddTab(monitor, ((PetscObject)linesearch)->tablevel)); 313 if (!objective) { 314 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: %s step no good, shrinking lambda, current gnorm %12.12e lambda=%18.16e\n", ordStr[linesearch->order - 1], (double)gnorm, (double)lambda)); 315 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 316 } else { 317 PetscCall(PetscViewerASCIIPrintf(monitor, " Line search: %s step no good, shrinking lambda, obj %12.12e lambda=%18.16e\n", ordStr[linesearch->order - 1], (double)g, (double)lambda)); 318 PetscCall(PetscViewerASCIISubtractTab(monitor, ((PetscObject)linesearch)->tablevel)); 319 } 320 } 321 } 322 } 323 } 324 325 /* postcheck */ 326 PetscCall(SNESLineSearchSetLambda(linesearch, lambda)); 327 PetscCall(SNESLineSearchPostCheck(linesearch, X, Y, W, &changed_y, &changed_w)); 328 if (changed_y) { 329 if (!changed_w) PetscCall(VecWAXPY(W, -lambda, Y, X)); 330 if (linesearch->ops->viproject) PetscCall((*linesearch->ops->viproject)(snes, W)); 331 } 332 if (changed_y || changed_w || objective) { /* recompute the function norm if the step has changed or the objective isn't the norm */ 333 PetscCall((*linesearch->ops->snesfunc)(snes, W, G)); 334 if (linesearch->ops->vinorm) { 335 gnorm = fnorm; 336 PetscCall((*linesearch->ops->vinorm)(snes, G, W, &gnorm)); 337 } else { 338 PetscCall(VecNorm(G, NORM_2, &gnorm)); 339 } 340 PetscCall(VecNorm(Y, NORM_2, &ynorm)); 341 if (PetscIsInfOrNanReal(gnorm)) { 342 PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_FAILED_NANORINF)); 343 PetscCall(PetscInfo(snes, "Aborted due to Nan or Inf in function evaluation\n")); 344 PetscFunctionReturn(PETSC_SUCCESS); 345 } 346 } 347 348 /* copy the solution over */ 349 PetscCall(VecCopy(W, X)); 350 PetscCall(VecCopy(G, F)); 351 PetscCall(VecNorm(X, NORM_2, &xnorm)); 352 PetscCall(SNESLineSearchSetNorms(linesearch, xnorm, gnorm, ynorm)); 353 PetscFunctionReturn(PETSC_SUCCESS); 354 } 355 356 static PetscErrorCode SNESLineSearchView_BT(SNESLineSearch linesearch, PetscViewer viewer) 357 { 358 PetscBool iascii; 359 SNESLineSearch_BT *bt = (SNESLineSearch_BT *)linesearch->data; 360 361 PetscFunctionBegin; 362 PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &iascii)); 363 if (iascii) { 364 if (linesearch->order == SNES_LINESEARCH_ORDER_CUBIC) { 365 PetscCall(PetscViewerASCIIPrintf(viewer, " interpolation: cubic\n")); 366 } else if (linesearch->order == SNES_LINESEARCH_ORDER_QUADRATIC) { 367 PetscCall(PetscViewerASCIIPrintf(viewer, " interpolation: quadratic\n")); 368 } 369 PetscCall(PetscViewerASCIIPrintf(viewer, " alpha=%e\n", (double)bt->alpha)); 370 } 371 PetscFunctionReturn(PETSC_SUCCESS); 372 } 373 374 static PetscErrorCode SNESLineSearchDestroy_BT(SNESLineSearch linesearch) 375 { 376 PetscFunctionBegin; 377 PetscCall(PetscFree(linesearch->data)); 378 PetscFunctionReturn(PETSC_SUCCESS); 379 } 380 381 static PetscErrorCode SNESLineSearchSetFromOptions_BT(SNESLineSearch linesearch, PetscOptionItems PetscOptionsObject) 382 { 383 SNESLineSearch_BT *bt = (SNESLineSearch_BT *)linesearch->data; 384 385 PetscFunctionBegin; 386 PetscOptionsHeadBegin(PetscOptionsObject, "SNESLineSearch BT options"); 387 PetscCall(PetscOptionsReal("-snes_linesearch_alpha", "Descent tolerance", "SNESLineSearchBT", bt->alpha, &bt->alpha, NULL)); 388 PetscOptionsHeadEnd(); 389 PetscFunctionReturn(PETSC_SUCCESS); 390 } 391 392 /*MC 393 SNESLINESEARCHBT - Backtracking line search {cite}`dennis:83`. 394 395 This line search finds the minimum of a polynomial fitting of the L2 norm of the 396 function or the objective function if it is provided with `SNESSetObjective()`. 397 If this fit does not satisfy the conditions for progress, the interval shrinks 398 and the fit is reattempted at most `max_it` times or until $\lambda$ is below `minlambda`. 399 400 Options Database Keys: 401 + -snes_linesearch_alpha <1e\-4> - slope descent parameter 402 . -snes_linesearch_damping <1.0> - scaling of initial step length on entry to the line search 403 . -snes_linesearch_maxstep <length> - if the length the initial step is larger than this then the 404 step is scaled back to be of this length at the beginning of the line search 405 . -snes_linesearch_max_it <40> - maximum number of shrinking steps 406 . -snes_linesearch_minlambda <1e\-12> - minimum step length allowed 407 - -snes_linesearch_order <1,2,3> - order of the approximation. With order 1, it performs a simple backtracking without any curve fitting 408 409 Level: advanced 410 411 Note: 412 This line search will always produce a step that is less than or equal to, in length, the full step size. 413 414 .seealso: [](ch_snes), `SNESLineSearch`, `SNESLineSearchType`, `SNESLineSearchCreate()`, `SNESLineSearchSetType()` 415 M*/ 416 PETSC_EXTERN PetscErrorCode SNESLineSearchCreate_BT(SNESLineSearch linesearch) 417 { 418 SNESLineSearch_BT *bt; 419 420 PetscFunctionBegin; 421 linesearch->ops->apply = SNESLineSearchApply_BT; 422 linesearch->ops->destroy = SNESLineSearchDestroy_BT; 423 linesearch->ops->setfromoptions = SNESLineSearchSetFromOptions_BT; 424 linesearch->ops->reset = NULL; 425 linesearch->ops->view = SNESLineSearchView_BT; 426 linesearch->ops->setup = NULL; 427 428 PetscCall(PetscNew(&bt)); 429 430 linesearch->data = (void *)bt; 431 linesearch->max_its = 40; 432 linesearch->order = SNES_LINESEARCH_ORDER_CUBIC; 433 bt->alpha = 1e-4; 434 PetscFunctionReturn(PETSC_SUCCESS); 435 } 436