> On Jul 29, 2016, at 1:58 PM, Valeria Mele <[email protected]> wrote: > > Thank you very much Barry. > Apparently I missed many things about "DMShell..." that I didn't find in the > current users manual, and I was trying to create the operators through > matCreateShell() and MatShellSetOperation().
You do need to use MatCreateShell and MatShellSetOperation()! But you need to call these from within the DMShellSetCreateMatrix() and interpolation/restriction routines that you provide. Barry > If I use "DMShellSetCreate..." to define the matrices I shouldn't have any > doubt about the da to refer to. > > Now I can go on. > Thank you again. > > Best, > Valeria > > > --------------------------------------------------------------------------------------------- > PhD Valeria Mele > > University of Naples Federico II > Department of Mathematics and Applications "R. Caccioppoli" > Complesso Universitario M.S. Angelo, Via Cinthia > 80126 Naples > --------------------------------------------------------------------------------------------- > > 2016-07-29 12:19 GMT-05:00 Barry Smith <[email protected]>: > > > On Jul 28, 2016, at 12:48 PM, Valeria Mele <[email protected]> wrote: > > > > Hi everyone, > > this time I am using PETSc to do something that is more complicated than my > > usual and I want to do it at the highest possible abstraction level. > > > > To put it in a nutshell, my intent is to build a parallel multigrid to > > solve a linear system via DM, KSP and PCMG (I would like to use DMMG but > > probably I should have the same problems or more). > > DMMG doesn't exist anymore. It was refactored away many years ago, its > functionality is handled by PCMG and DM. > > > > > > I created the distributed object, da, with DMDACreate3d, even if it is > > distributed (as yet) only in the x-dimension and has 3 dof. > > Then I create the KSP (type KSPRICHARDSON) and set the nonzero initial > > guess and PCMG as preconditioner. Here I start to tune the MG. > > > > The point is that I need to define all the operators as matrix-free, since > > they will do several operations on x to obtain y, and I am not familiar > > with the way to access all the elements or informations in the two levels > > involved and/or among the processors with a so-high level interface. > > > > So please (please please please please) tell me if I correctly understand > > the mechanism or I am on the wrong way and clear my doubts. > > > > That is, let's say that my operation for the shell are: > > • A_mult(Mat mat,Vec x, Vec y) //coefficients matrix > > in this case the level is only one but should I write it taking into > > account only the local data (I think so) and accessing them via the > > informations in da? > > Yes. You can use VecGetDM(x,&da) to get the DMDA object > > > > > For example, if I use DMDAVecGetArray, DMDAVecGetCorners (or > > DMDAGetGhostCorners) and DMDAVecRestoreArray, will they retrieve > > informations from the right level each time (I am pretty sure that in some > > official examples it is done in this way)? > > > > Or should I handle just Vecs as local structures with their usual indices > > (through VecGetArray and VecRestoreArray)? > > No, no, no because then you would need to mange all the structured grid > information yourself, since the DMDA manages it for you you should use it. > > > • P_mult(Mat mat,Vec x, Vec y) //interpolation matrix that is NOT > > conceptually the traspose of Restriction > > in this case x and y will be from two different levels (respectively L and > > L+1), so, if I retrieve informations from the da... how can I access the > > two at different levels? > > Use VecGetDM(x, and VecGetDM(y to get access to both DMDA. > > > > > I am sorry if it seems that they are trivial questions, and I will be > > grateful to anyone will help me. > > Additional information. Since the PCMG will be requesting the matrices and > the interpolation/restriction operations (rather than you setting them into > each level of multigrid) you will need to use DMShellSetCreateMatrix() and > DMShellSetCreateInterpolation() and DMShellSetCreateRestriction() to provide > the routines that will create the Shell matrices you need to represent the > operators on the levels and the restriction and interpolation (Even though > you are using a DMDA you can still call these routines). > > Barry > > > > > Thanks a lot, > > Valeria > > > > > > > > > > > > > > --------------------------------------------------------------------------------------------- > > PhD Valeria Mele > > > > University of Naples Federico II > > Department of Mathematics and Applications "R. Caccioppoli" > > Complesso Universitario M.S. Angelo, Via Cinthia > > 80126 Naples > > --------------------------------------------------------------------------------------------- > > >
