On Thu, Feb 21, 2013 at 4:19 PM, Amneet Bhalla <mail2amneet at gmail.com>wrote:
> Can you guys comment on what example case would be best to start off for > shell > operators with FieldSplit? The examples I am looking into all start with > creating native > PETSc matrices and vectors. > Amneet, do you already have code that applies all the "blocks" of your coupled system or are you starting something new? If it's something new, please don't use MatNest/MatShell just yet. If you have tested existing code, then wrapping it in MatShell/MatNest is fine. If you are working on something new, I recommend the progression below. It will encourage better program structure and better debuggability, and will ultimately be faster than trying to "skip" steps. Step 1: Just write a residual and use -snes_mf to solve all matrix-free without preconditioning. Step 2: Assemble an approximate Jacobian and use -snes_mf_operator Step 3: use fd_coloring (if possible) to see how much solver benefit you could gain by implementing the full Jacobian. Also use fieldsplit solvers to find out which blocks of the Jacobian are most important to assemble. Step 4: Implement those blocks of the Jacobian that you need for effective preconditioning. Step 5: Profile, consider those preconditioners that are most effective and the suitability of the discretization for matrix-free application. If you spend a lot of time/memory in things like MatGetSubMatrix, then add an option to use MatNest. If you have overly heavy matrices (in terms of memory, bandwidth, or assembly time) that need not be completely assembled for effective preconditioning, add an option to use MatShell for those parts. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130221/c5708142/attachment-0001.html>
