On Tue, Oct 25, 2016 at 6:22 PM, Barry Smith <[email protected]> wrote:
> > > On Oct 25, 2016, at 5:39 PM, Olivier Mesnard <[email protected]> > wrote: > > > > On 25 October 2016 at 17:51, Barry Smith <[email protected]> wrote: > > > > Olivier, > > > > In theory you do not need to change anything else. Are you using a > different matrix object for the velocity_ksp object than the poisson_ksp > object? > > > > The matrix is different for the velocity_ksp and the poisson_ksp. > > > > The code change in PETSc is very little but we have a report from > another CFD user who also had problems with the change so there may be some > subtle bug that we can't figure out causing things to not behave properly. > > > > First run the 3.7.4 code with -poisson_ksp_view and verify that when > it prints the matrix information it prints something like has attached null > space if it does not print that it means that somehow the matrix is not > properly getting the matrix attached. > > > > When running with 3.7.4 and -poisson_ksp_view, the output shows that > the nullspace is not attached to the KSP (as it was with 3.5.4); however > the print statement is now under the Mat info (which is expected when > moving from KSPSetNullSpace to MatSetNullSpace?). > > Good, this is how it should be. > > > > Though older versions had MatSetNullSpace() they didn't necessarily > associate it with the KSP so it was not expected to work as a replacement > for KSPSetNullSpace() with older versions. > > > > Because our other user had great difficulty trying to debug the > issue feel free to send us at [email protected] your code with > instructions on building and running and we can try to track down the > problem. Better than hours and hours spent with fruitless email. We will, > of course, not distribute the code and will delete in when we are finished > with it. > > > > The code is open-source and hosted on GitHub (https://github.com/ > barbagroup/PetIBM). > > I just pushed the branches `feature-compatible-petsc-3.7` and > `revert-compatible-petsc-3.5` that I used to observe this problem. > > > Thanks, I'll get back to you if I discover anything Obviously GAMG is behaving quite differently (1 vs 2 levels and a much sparser coarse problem in 3.7). Could you try one thing for me before we start running it? Run with -poisson_mg_coarse_sub_pc_type svd and see what happens on 2 procs for 3.7? Thanks, Matt > > > PETSc (both 3.5.4 and 3.7.4) was configured as follow: > > export PETSC_ARCH="linux-gnu-dbg" > > ./configure --PETSC_ARCH=$PETSC_ARCH \ > > --with-cc=gcc \ > > --with-cxx=g++ \ > > --with-fc=gfortran \ > > --COPTFLAGS="-O0" \ > > --CXXOPTFLAGS="-O0" \ > > --FOPTFLAGS="-O0" \ > > --with-debugging=1 \ > > --download-fblaslapack \ > > --download-mpich \ > > --download-hypre \ > > --download-yaml \ > > --with-x=1 > > > > Our code was built using the following commands: > > mkdir petibm-build > > cd petibm-build > > export PETSC_DIR=<directory of PETSc> > > export PETSC_ARCH="linux-gnu-dbg" > > export PETIBM_DIR=<directory of PetIBM git repo> > > $PETIBM_DIR/configure --prefix=$PWD \ > > CXX=$PETSC_DIR/$PETSC_ARCH/bin/mpicxx \ > > CXXFLAGS="-g -O0 -std=c++11" > > make all > > make install > > > > Then > > cd examples > > make examples > > > > The example of the lid-driven cavity I was talking about can be found > in the folder `examples/2d/convergence/lidDrivenCavity20/20/` > > > > To run it: > > mpiexec -n N <path-to-petibm-build>/bin/petibm2d -directory > <path-to-example> > > > > Let me know if you need more info. Thank you. > > > > Barry > > > > > > > > > > > > > > > > > > > On Oct 25, 2016, at 4:38 PM, Olivier Mesnard < > [email protected]> wrote: > > > > > > Hi all, > > > > > > We develop a CFD code using the PETSc library that solves the > Navier-Stokes equations using the fractional-step method from Perot (1993). > > > At each time-step, we solve two systems: one for the velocity field, > the other, a Poisson system, for the pressure field. > > > One of our test-cases is a 2D lid-driven cavity flow (Re=100) on a > 20x20 grid using 1 or 2 procs. > > > For the Poisson system, we usually use CG preconditioned with GAMG. > > > > > > So far, we have been using PETSc-3.5.4, and we would like to update > the code with the latest release: 3.7.4. > > > > > > As suggested in the changelog of 3.6, we replaced the routine > `KSPSetNullSpace()` with `MatSetNullSpace()`. > > > > > > Here is the list of options we use to configure the two solvers: > > > * Velocity solver: prefix `-velocity_` > > > -velocity_ksp_type bcgs > > > -velocity_ksp_rtol 1.0E-08 > > > -velocity_ksp_atol 0.0 > > > -velocity_ksp_max_it 10000 > > > -velocity_pc_type jacobi > > > -velocity_ksp_view > > > -velocity_ksp_monitor_true_residual > > > -velocity_ksp_converged_reason > > > * Poisson solver: prefix `-poisson_` > > > -poisson_ksp_type cg > > > -poisson_ksp_rtol 1.0E-08 > > > -poisson_ksp_atol 0.0 > > > -poisson_ksp_max_it 20000 > > > -poisson_pc_type gamg > > > -poisson_pc_gamg_type agg > > > -poisson_pc_gamg_agg_nsmooths 1 > > > -poissonksp_view > > > -poisson_ksp_monitor_true_residual > > > -poisson_ksp_converged_reason > > > > > > With 3.5.4, the case runs normally on 1 or 2 procs. > > > With 3.7.4, the case runs normally on 1 proc but not on 2. > > > Why? The Poisson solver diverges because of an indefinite > preconditioner (only with 2 procs). > > > > > > We also saw that the routine `MatSetNullSpace()` was already available > in 3.5.4. > > > With 3.5.4, replacing `KSPSetNullSpace()` with `MatSetNullSpace()` led > to the Poisson solver diverging because of an indefinite matrix (on 1 and 2 > procs). > > > > > > Thus, we were wondering if we needed to update something else for the > KSP, and not just modifying the name of the routine? > > > > > > I have attached the output files from the different cases: > > > * `run-petsc-3.5.4-n1.log` (3.5.4, `KSPSetNullSpace()`, n=1) > > > * `run-petsc-3.5.4-n2.log` > > > * `run-petsc-3.5.4-nsp-n1.log` (3.5.4, `MatSetNullSpace()`, n=1) > > > * `run-petsc-3.5.4-nsp-n2.log` > > > * `run-petsc-3.7.4-n1.log` (3.7.4, `MatSetNullSpace()`, n=1) > > > * `run-petsc-3.7.4-n2.log` > > > > > > Thank you for your help, > > > Olivier > > > <run-petsc-3.5.4-n1.log><run-petsc-3.5.4-n2.log><run-petsc- > 3.5.4-nsp-n1.log><run-petsc-3.5.4-nsp-n2.log><run-petsc-3.7. > 4-n1.log><run-petsc-3.7.4-n2.log> > > > > > > -- 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
