[petsc-users] Fieldsplit with LSC for constrained elasticity/poroelasticity?

[Jed Brown]

Matthew Knepley <knepley at gmail.com> writes:

> On Tue, Oct 28, 2014 at 8:00 AM, Tabrez Ali <stali at geology.wisc.edu> wrote:
>
>>  Matt
>>
>> With MUMPS it is indeed the same but not faster. E.g., see 1.txt and 2.txt
>>
>
> There is a big problem here. This system is never actually being solved.
> Look at 1.txt:
>
> 0 KSP preconditioned resid norm 9.320055451716e+05 true resid norm
> 1.755998647494e+02 ||r(i)||/||b|| 1.000000000000e+00
> 380 KSP preconditioned resid norm 9.034871458425e-05 true resid norm
> 3.118451181896e+09 ||r(i)||/||b|| 1.775884728811e+07
>
> The preconditioned residual did drop 10 order of magnitude, but the true
> residual (b - Ax) is still very very large.
> This looks like the system is singular and the preconditioner is masking
> this. This needs to be figured out first.

It could also be very poor scaling between the fields.  It's important
to nondimensionalize so that you don't ask to solve for velocity 1e15
more accurately than pressure, or vice-versa.
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