Thanks Mark, if if I see similar performance I'll be very happy!
On 11/08/2019 15:17, Mark Lohry wrote:
Anecdotal: I've been *shocked* at how long I can let the
-snes_lag_preconditioner go with -snes_mf_operator. I have it
configured to only recompute the preconditioner whenever it hits my
linear solver iteration limit, which pretty much never happens on
unsteady problems.
On Fri, Aug 9, 2019 at 11:11 AM Steve via petsc-users
<[email protected] <mailto:[email protected]>> wrote:
Thank you Barry, that's very helpful.
I'll have a play with those various options and see how I get on.
On 09/08/2019 15:43, Smith, Barry F. wrote:
> Steve,
>
> There are two possibilities
>
> 1) completely under user control, when you are asked for a new
Jacobian you can evaluate the current conditions and decide
whether to generate a new one. For example get from SNES the
number of iterations it required and if that is starting to go up
then get a new one or check if the time-step is being cut because
the nonlinear solver is becoming "too hard" and generate a new one.
>
> It is also possible to use -snes_mf_operator (or an inline
version) that uses matrix-free to apply the Jacobian and the
Jacobian you provide to compute the preconditioner. This allows
you to keep the current Jacobian/preconditioner even longer before
rebuilding. Here you can use the increase in the number of linear
iterations to decide when to change the Jacobian.
>
> 2) let PETSc decide when to rebuild the Jacobian. This is more
limited since it has no direct measure of how well the Jacobian is
doing. Some possibilities are
> -snes_lag_jacobian -snes_lag_jacobian_persists
-snes_lag_preconditioner -snes_lag_preconditioner_persists This
introduces yet another parameter under your control; you can lag
the generation of the new preconditioner even when you get a new
preconditioner (this makes sense only when you are not using
-snes_mf_operator),
>
> So, at a high level, you have a great deal of freedom to
control when you recreate the Jacobian (and preconditioner), will
be problem dependent and the optimal value will depend on your
problem and specific integrator. Final note, when you rebuild may
also depend on how far you have integrated, when the nonlinear
effects are strong you probably want to rebuild often but when the
solution is close to linearly evolving less often.
>
> If generating the Jacobian/preconditioner is expensive
relative to everything else a good combination is
-snes_mf_operator and a pretty lagged generation of new Jacobians.
>
> Barry
>
>
>
>
>> On Aug 9, 2019, at 9:25 AM, Steve via petsc-users
<[email protected] <mailto:[email protected]>> wrote:
>>
>> Hi,
>>
>> I'm experimenting with the use of PETSc to replace a DAE solver
in an existing code that I use to solve stiff nonlinear problems.
I expect to use TSBDF in the final instance, and so am currently
playing with it but applied to a simpler linear problem - just to
get some experience with the SNES/KSP/PC controls before diving in
to the hard problem.
>>
>> Below is some output from TSAdapt for the simple linear
problem, using TSBDF and PCLU, so that the linear algebra solve in
the newton loop is direct:
>>
>> TSAdapt basic bdf 0:2 step 0 accepted t=0 +
1.000e-03 dt=2.000e-03 wlte=2.51e-07 wltea= -1 wlter= -1
>> TSResidual...
>> TSJacobian... calculate
>> TSResidual...
>> TSAdapt basic bdf 0:2 step 1 accepted t=0.001 +
2.000e-03 dt=4.000e-03 wlte=2.83e-07 wltea= -1 wlter= -1
>> TSResidual...
>> TSJacobian... calculate
>> TSResidual...
>> TSAdapt basic bdf 0:2 step 2 accepted t=0.003 +
4.000e-03 dt=8.000e-03 wlte=1.22e-07 wltea= -1 wlter= -1
>> TSResidual...
>> TSJacobian... calculate
>> TSResidual...
>>
>> I have added the "TSResidual..." and "TSJacobian..." echoes so
that I can see when PETSc is requesting residuals and Jacobians to
be computed. (This is the Jacobian routine specified via
TSSetIJacobian.)
>>
>> Regarding the above output, it appears that TS / SNES always
requests a new (I)Jacobian at each new timestep (after the first
residual is calculated). I can see mathematically why this would
be the default choice, but had hoped that it might be possible for
out-of-date Jacobians to be used until they become inefficient. My
reason for wanting this is that the Jacobian calculations for the
intended application are particularly expensive, but for small
enough timesteps out-of-date Jacobians may be good enough, for a
few steps.
>>
>> Is there any way of specifying that out-of-date (I)Jacobians
can be tolerated (at the expense of increased Newton iterations,
or smaller timesteps)? Alternatively would it make sense to
include callbacks to TS / SNES from the Jacobian evaluation
function to determine whether sufficiently few iterations have
been used that it might be safe to return the previously
calculated Jacobian (if I store a copy)? If so, is there any
advice on how I should do this?
>>
>> NB. I see that there is an option for TSRHSJacobianSetReuse(),
but this only applies to the RHS component of the DAE (the G(t,u)
part, using the terminology from the manual), but I am not using
this as ultimately I expect to be solving strongly nonlinear
problems with no "slow" G(t,u) part.
>>
>> Any advice would be greatly appreciated.
>>
>>
--
Dr Steven J Benbow
Quintessa Ltd, First Floor, West Wing, Videcom House, Newtown
Road, Henley-on-Thames, Oxfordshire RG9 1HG, UK
Tel: 01491 636246 DD: 01491 630051 Web: http://www.quintessa.org
Quintessa Limited is an employee-owned company registered in
England, Number 3716623.
Registered office: Quintessa Ltd, First Floor, West Wing, Videcom
House, Newtown Road, Henley-on-Thames, Oxfordshire RG9 1HG, UK
If you have received this e-mail in error, please notify
[email protected] <mailto:[email protected]> and delete it
from your system
--
Dr Steven J Benbow
Quintessa Ltd, First Floor, West Wing, Videcom House, Newtown Road,
Henley-on-Thames, Oxfordshire RG9 1HG, UK
Tel: 01491 636246 DD: 01491 630051 Web: http://www.quintessa.org
Quintessa Limited is an employee-owned company registered in England, Number
3716623.
Registered office: Quintessa Ltd, First Floor, West Wing, Videcom House,
Newtown Road, Henley-on-Thames, Oxfordshire RG9 1HG, UK
If you have received this e-mail in error, please notify [email protected]
and delete it from your system
