[petsc-users] solving stokes-like equation in 3D staggered grid in irregular domain with petscsection

[Jed Brown]

Bishesh Khanal <bisheshkh at gmail.com> writes:

> With options ksp_converged_reason and ksp_monitor_true_residual set only
> for the fieldsplit_1 , I get the result as below. -ksp_view reports that
> the preconditioner for the schur complement is built from A11.
> preconditioned resid norm below looks to be the same and in the order of
> 10e+14 for every iteration. Does this imply the precondtioner is singular?
> Does it mean that the problem is possibly in assembling the A11 block? 

I would suspect that, yes.  The preconditioner is amplifying the
residual, but then it is not converging either, which indicates that the
original system may also be singular, with an inconsistent right hand
side.

> It is the diagonal of this block corresponding to the domain B cells
> that changes when I set c(x) = 1.  Residual norms for fieldsplit_1_
> solve.  0 KSP preconditioned resid norm 2.890735677419e+14 true resid
> norm 1.265820125363e+01 ||r(i)||/||b|| 1.000000000000e+00 1 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.117075725063e+01 ||r(i)||/||b|| 8.824916768826e-01 2 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109990265188e+01 ||r(i)||/||b|| 8.768941518210e-01 3 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109895437193e+01 ||r(i)||/||b|| 8.768192375476e-01 4 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109892185160e+01 ||r(i)||/||b|| 8.768166684358e-01 5 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109889770615e+01 ||r(i)||/||b|| 8.768147609417e-01 6 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109886495655e+01 ||r(i)||/||b|| 8.768121737173e-01 7 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109886096749e+01 ||r(i)||/||b|| 8.768118585813e-01 8 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109885236160e+01 ||r(i)||/||b|| 8.768111787141e-01 9 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109882829958e+01 ||r(i)||/||b|| 8.768092778106e-01 10 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109882703198e+01 ||r(i)||/||b|| 8.768091776699e-01 11 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109882171007e+01 ||r(i)||/||b|| 8.768087572386e-01 12 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109882129003e+01 ||r(i)||/||b|| 8.768087240548e-01 13 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109880708177e+01 ||r(i)||/||b|| 8.768076016004e-01 14 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.109884240385e+01 ||r(i)||/||b|| 8.768103920501e-01 15 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.110342530238e+01 ||r(i)||/||b|| 8.771724417953e-01 16 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.668559859400e+01 ||r(i)||/||b|| 1.318165058342e+00 17 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 2.379573238198e+02 ||r(i)||/||b|| 1.879866807707e+01 18 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 7.475251038198e+02 ||r(i)||/||b|| 5.905460727331e+01 19 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 7.610489296073e+02 ||r(i)||/||b|| 6.012299175517e+01 20 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.162871674016e+03 ||r(i)||/||b|| 9.186705525659e+01 21 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 2.438552126509e+03 ||r(i)||/||b|| 1.926460227364e+02 22 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 2.112154724517e+03 ||r(i)||/||b|| 1.668605738047e+02 23 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 1.867895524623e+03 ||r(i)||/||b|| 1.475640564719e+02 24 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 2.760528221023e+03 ||r(i)||/||b|| 2.180821876435e+02 25 KSP
> preconditioned resid norm 2.890735677419e+14 true resid norm
> 2.585072583244e+03 ||r(i)||/||b|| 2.042211631374e+02 and it continues.
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