It is enough if you use DMPlexDistribute with 1 level of overlap, set the local section for your dofs, and call DMGlobalToLocal . The local vector will contain data for what you call “ghost” cells. I call them “not-owned’ cells.
> On Jun 15, 2020, at 12:09 PM, Shashwat Tiwari <shaswat121...@gmail.com > <mailto:shaswat121...@gmail.com>> wrote: > > The way I'm trying to implement periodic bc is, when I loop over the boundary > faces, say, on the left boundary of the domain to compute flux and residual, > I need solution values from the two cells neighbouring the face, i.e. the > left cell and the right cell (the face normal pointing from the left cell to > the right cell and the right cell being a boundary ghost cell for boundary > faces). For the boundary to be periodic, I need the value that I get from the > right cell (boundary ghost cell) to be the solution value at its periodic > counterpart, i.e. the solution value at left cell of the face on right > boundary of the domain in this case. My question is how do I update the value > at a boundary ghost cell with the value at the real cell which is its > periodic counterpart from the other side of the domain. Is there some kind of > mapping of the boundary ghost cells to their corresponding real cells which I > can use to update the solution values at ghost cells? > > Regards, > Shashwat > > On Sun, Jun 14, 2020 at 5:11 AM Matthew Knepley <knep...@gmail.com > <mailto:knep...@gmail.com>> wrote: > On Fri, Jun 12, 2020 at 3:19 PM Shashwat Tiwari <shaswat121...@gmail.com > <mailto:shaswat121...@gmail.com>> wrote: > Hi, > I am writing a first order 2D solver for unstructured grids with periodic > boundaries using DMPlex. After generating the mesh, I use "DMSetPeriodicity" > function to set periodicity in both directions. After which I partition the > mesh (DMPlexDistribute), construct ghost cells (DMPlexConstructGhostCells), > > These ghost cells are for FVM boundary conditions. If you want cells to be > shared across parallel partitions, then you want to give overlap=1 > to DMPlexDIstribute(). Is that what you want? > > Thanks, > > Matt > > create a section, and set some initial values in the global vector. Then I > use "VecGhostUpdateBegin" to start updating the boundary ghost cell values, > but, I get the following error in case I use multiple processors: > > [0]PETSC ERROR: Invalid argument > [0]PETSC ERROR: Vector is not ghosted > [0]PETSC ERROR: See https://www.mcs.anl.gov/petsc/documentation/faq.html > <https://www.mcs.anl.gov/petsc/documentation/faq.html> for trouble shooting. > > if I run with a single process, there is no error but the values remain empty > (zero) and are not updated. Kindly let me know, if I am missing some crucial > step before I can update the ghost values in order to implement the periodic > bc, or if there is any other approach to achieve it. I am attaching a small > code to demonstrate the issue for your reference. > > Regards, > Shashwat > > > -- > What most experimenters take for granted before they begin their experiments > is infinitely more interesting than any results to which their experiments > lead. > -- Norbert Wiener > > https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
