[petsc-dev] sor smoothers

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:
>   I don't see the advantage of keeping the PCMGSetResidual. It seems
>   to me that using the Amat to properly manage the different operator
>   approximations is the clean right way to do it. Messing with shoving
>   a random residual computer function seems prone to errors and
>   confusion about what operator is what.  So is there a particular
>   case where using the hierarchy of Amat and Pmat won't work but
>   PCMGSetResidual will?

Suppose the user is doing geometric multigrid with defect correction
(high order residual, low-order correction) where the low order thing is
Galerkin and the high-order is an expensive matrix-free
rediscretization.  The PCMG Galerkin code currently only handles when
Amat and Pmat are the same on coarse levels, so where would the user put
the code for the high-order residual?

It's not that we can't provide directions to work around this (create
MatShells, set their residual, and put them on appropriate levels, then
make sure that PCSetUp_MG does not overwrite Amat on coarse levels), but
I don't see PCMGSetResidual as being expensive for us to maintain and it
fits a lot of folks' mental picture of how matrix-free MG works.  What
is the tangible value in deleting it (and breaking folks' code)?
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 835 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20130909/5939d52d/attachment.sig>