On 02/02/2012 16:41, Jed Brown wrote: > On Thu, Feb 2, 2012 at 17:24, gouarin <loic.gouarin at math.u-psud.fr > <mailto:loic.gouarin at math.u-psud.fr>> wrote: > > No. In those examples, you have the same grid for the velocity and > the pressure. If I use 4Q1_Q1 elements, I have not the same grid. > I have one for the velocity and an other for the pressure in the > DMComposite. And for me, it is the difficulty because as you say > after, I have to do my own preallocation step to have the good > off-diagonal blocks. > > > Okay, I misinterpreted your notation. For your mixed elements, you > would couple with DMComposite. > > > This is why I asked what is the best way to construct my matrix. I > hoped that now it is not necessary to do this preallocation. > > > As long as dynamic preallocation is slow, you need to provide it. > > This is also a difficulty to use a multigrid only on the velocity. > > > What is hard about using multigrid only on velocity? > > -pc_type fieldsplit -fieldsplit_velocity_pc_type mg > -fieldsplit_velocity_pc_mg_levels 3 > > will do geometric MG on the velocity block. Note that your elements > are slightly non-standard, so depending on how you want to work, you > might provide your own coarsening and interpolation. It is not hard to use multigrid on velocity if I provide the interpolation as I said in my first email and as you say here. But it is for me too intrusive because I have to do a special case to use a multigrid pcmg whereas I can use boomeramg easily. I don't know if what I say is clear ...
The idea is to test many solvers on this problem. Thanks for your replies. Loic -- Loic Gouarin Laboratoire de Math?matiques Universit? Paris-Sud B?timent 425 91405 Orsay Cedex France Tel: (+33) 1 69 15 60 14 Fax: (+33) 1 69 15 67 18 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120202/01603165/attachment-0001.htm>
