Hi Matt, I wrote the following line after DMMGSetKSP() in ex22.c ierr =DMMGSetNullSpace(dmmg,PETSC_TRUE,0,PETSC_NULL);
Still I get the difference in the values calculated by single processor and the multiple processors. The two input values for b that I used are 1 and 10 for all the elements in the vector. I am using this code in one of my programs where I assign a random number to b. I get the discrepancy between single and multiple processors there as well. Thanks. Khalid ________________________________ From: Matthew Knepley <[email protected]> To: PETSc users list <petsc-users at mcs.anl.gov> Cc: khalid ashraf <khalid_eee at yahoo.com> Sent: Fri, April 15, 2011 4:38:25 AM Subject: Re: [petsc-users] DMMG with PBC On Fri, Apr 15, 2011 at 4:28 AM, khalid ashraf <khalid_eee at yahoo.com> wrote: Hi, >I am running src/ksp/ksp/examples/tutorials/ex22.c >I matched the output of single processor and multiprocessor results and it >works >fine. >But I want to use a periodic boundary condition. I make the following changes >in >the main function and this works fine with this change as well: > > > ierr = DMMGCreate(PETSC_COMM_WORLD,1,PETSC_NULL,&dmmg);CHKERRQ(ierr); > ierr = >DACreate3d(PETSC_COMM_WORLD,DA_XYZPERIODIC,DA_STENCIL_STAR,10,10,2,PETSC_DECIDE,PETSC_DECIDE,PETSC_DECIDE,1,1,0,0,0,&da);CHKERRQ(ierr); > > > > > >However, when I comment out these following lines since I am using a PBC, then >the result of 1 proc and multi-proc are not the same. They vary within 5 >decimal >points and the difference increases with increasing number of processors. The periodic operator has a null space. You must put that in the solver DMMGSetNullSpace(), so that it is projected out at each step. Matt /* if (i==0 || j==0 || k==0 || i==mx-1 || j==my-1 || k==mz-1){ > v[0] = 2.0*(HxHydHz + HxHzdHy + HyHzdHx); > ierr = >MatSetValuesStencil(B,1,&row,1,&row,v,INSERT_VALUES);CHKERRQ(ierr); > } else */ > > >Could you please tell me what is going wrong here. > > >Thanks. -- 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20110418/046ca4fd/attachment.htm>
