[petsc-users] Slow convergence while parallel computations.
Barry Smith
bsmith at petsc.dev
Mon Sep 6 20:33:59 CDT 2021
You can use -mat_null_space_test to check it the null space you provide is within the null space of the operator. There is no practical way to test if the null space you provide is exactly the full null space of the operator but at least the check ensures that you are not providing something that is not in the null space.
Barry
Also if you run with GMRES and look at the norms of the residuals -ksp_monitor at each restart iteration (the default restart is 30), if they jump wildly at each restart this can indicate a problem with the nullspace.
> On Sep 3, 2021, at 10:48 AM, Viktor Nazdrachev <numbersixvs at gmail.com> wrote:
>
> Hello Mark and Matthew!
>
> I attached log files for serial and parallel cases and corresponding information about GAMG preconditioner (using grep).
>
> I have to notice, that assembling of global stiffness matrix in code was performed by MatSetValues subrotuine (not MatSetValuesBlocked)
>
> !nnds – number of nodes
> !dmn=3
> call MatCreate(Petsc_Comm_World,Mat_K,ierr)
> call MatSetFromOptions(Mat_K,ierr)
> call MatSetSizes(Mat_K,Petsc_Decide,Petsc_Decide,n,n,ierr_m)
> …
> call MatMPIAIJSetPreallocation(Mat_K,0,dbw,0,obw,ierr)
> …
> call MatSetOption(Mat_K,Mat_New_Nonzero_Allocation_Err,Petsc_False,ierr)
> …
>
> do i=1,nels
> call FormLocalK(i,k,indx,"Kp") ! find local stiffness matrix
> indx=indxmap(indx,2) !find global indices for DOFs
> call MatSetValues(Mat_K,ef_eldof,indx,ef_eldof,indx,k,Add_Values,ierr)
> end do
>
> But nullspace vector was created using VecSetBlockSize subroutine.
>
> call VecCreate(Petsc_Comm_World,Vec_NullSpace,ierr)
> call VecSetBlockSize(Vec_NullSpace,dmn,ierr)
> call VecSetSizes(Vec_NullSpace,nnds*dmn,Petsc_Decide,ierr)
> call VecSetUp(Vec_NullSpace,ierr)
> call VecGetArrayF90(Vec_NullSpace,null_space,ierr)
> …
> call VecRestoreArrayF90(Vec_NullSpace,null_space,ierr)
> call MatNullSpaceCreateRigidBody(Vec_NullSpace,matnull,ierr)
> call MatSetNearNullSpace(Mat_K,matnull,ierr)
>
> I suppose it can be one of the reasons of GAMG slow convergence.
> So I attached log files for parallel run with «pure» GAMG precondtioner.
>
>
> Kind regards,
>
> Viktor Nazdrachev
>
> R&D senior researcher
>
> Geosteering Technologies LLC
>
> пт, 3 сент. 2021 г. в 15:11, Matthew Knepley <knepley at gmail.com <mailto:knepley at gmail.com>>:
> On Fri, Sep 3, 2021 at 8:02 AM Mark Adams <mfadams at lbl.gov <mailto:mfadams at lbl.gov>> wrote:
>
>
> On Fri, Sep 3, 2021 at 1:57 AM Viktor Nazdrachev <numbersixvs at gmail.com <mailto:numbersixvs at gmail.com>> wrote:
> Hello, Lawrence!
> Thank you for your response!
> I attached log files (txt files with convergence behavior and RAM usage log in separate txt files) and resulting table with convergence investigation data(xls). Data for main non-regular grid with 500K cells and heterogeneous properties are in 500K folder, whereas data for simple uniform 125K cells grid with constant properties are in 125K folder.
>
> >> On 1 Sep 2021, at 09:42, Наздрачёв Виктор <numbersixvs at gmail.com <https://lists.mcs.anl.gov/mailman/listinfo/petsc-users>> wrote:
> >>
> >> I have a 3D elasticity problem with heterogeneous properties.
> >
> >What does your coefficient variation look like? How large is the contrast?
>
> Young modulus varies from 1 to 10 GPa, Poisson ratio varies from 0.3 to 0.44 and density – from 1700 to 2600 kg/m^3.
>
> That is not too bad. Poorly shaped elements are the next thing to worry about. Try to keep the aspect ratio below 10 if possible.
>
>
>
> >> There is unstructured grid with aspect ratio varied from 4 to 25. Zero Dirichlet BCs are imposed on bottom face of mesh. Also, Neumann (traction) BCs are imposed on side faces. Gravity load is also accounted for. The grid I use consists of 500k cells (which is approximately 1.6M of DOFs).
> >>
> >> The best performance and memory usage for single MPI process was obtained with HPDDM(BFBCG) solver and bjacobian + ICC (1) in subdomains as preconditioner, it took 1 m 45 s and RAM 5.0 GB. Parallel computation with 4 MPI processes took 2 m 46 s when using 5.6 GB of RAM. This because of number of iterations required to achieve the same tolerance is significantly increased.
> >
> >How many iterations do you have in serial (and then in parallel)?
>
> Serial run is required 112 iterations to reach convergence (log_hpddm(bfbcg)_bjacobian_icc_1_mpi.txt), parallel run with 4 MPI – 680 iterations.
>
> I attached log files for all simulations (txt files with convergence behavior and RAM usage log in separate txt files) and resulting table with convergence/memory usage data(xls). Data for main non-regular grid with 500K cells and heterogeneous properties are in 500K folder, whereas data for simple uniform 125K cells grid with constant properties are in 125K folder.
>
>
> >> I`ve also tried PCGAMG (agg) preconditioner with ICС (1) sub-precondtioner. For single MPI process, the calculation took 10 min and 3.4 GB of RAM. To improve the convergence rate, the nullspace was attached using MatNullSpaceCreateRigidBody and MatSetNearNullSpace subroutines. This has reduced calculation time to 3 m 58 s when using 4.3 GB of RAM. Also, there is peak memory usage with 14.1 GB, which appears just before the start of the iterations. Parallel computation with 4 MPI processes took 2 m 53 s when using 8.4 GB of RAM. In that case the peak memory usage is about 22 GB.
> >
> >Does the number of iterates increase in parallel? Again, how many iterations do you have?
>
> For case with 4 MPI processes and attached nullspace it is required 177 iterations to reach convergence (you may see detailed log in log_hpddm(bfbcg)_gamg_nearnullspace_4_mpi.txt). For comparison, 90 iterations are required for sequential run(log_hpddm(bfbcg)_gamg_nearnullspace_1_mpi.txt).
>
> Again, do not use ICC. I am surprised to see such a large jump in iteration count, but get ICC off the table.
>
> You will see variability in the iteration count with processor count with GAMG. As much as 10% +-. Maybe more (random) variability , but usually less.
>
> You can decrease the memory a little, and the setup time a lot, by aggressively coarsening, at the expense of higher iteration counts. It's a balancing act.
>
> You can run with the defaults, add '-info', grep on GAMG and send the ~30 lines of output if you want advice on parameters.
>
> Can you send the output of
>
> -ksp_view -ksp_monitor_true_residual -ksp_converged_reason
>
> Thanks,
>
> Matt
>
> Thanks,
> Mark
>
>
>
>
> >> Are there ways to avoid decreasing of the convergence rate for bjacobi precondtioner in parallel mode? Does it make sense to use hierarchical or nested krylov methods with a local gmres solver (sub_pc_type gmres) and some sub-precondtioner (for example, sub_pc_type bjacobi)?
> >
> >bjacobi is only a one-level method, so you would not expect process-independent convergence rate for this kind of problem. If the coefficient variation is not too extreme, then I would expect GAMG (or some other smoothed aggregation package, perhaps -pc_type ml (you need --download-ml)) would work well with some tuning.
>
> Thanks for idea, but, unfortunately, ML cannot be compiled with 64bit integers (It is extremely necessary to perform computation on mesh with more than 10M cells).
>
>
> >If you have extremely high contrast coefficients you might need something with stronger coarse grids. If you can assemble so-called Neumann matrices (https://petsc.org/release/docs/manualpages/Mat/MATIS.html#MATIS <https://petsc.org/release/docs/manualpages/Mat/MATIS.html#MATIS>) then you could try the geneo scheme offered by PCHPDDM.
>
>
> I found strange convergence behavior for HPDDM preconditioner. For 1 MPI process BFBCG solver did not converged (log_hpddm(bfbcg)_pchpddm_1_mpi.txt), while for 4 MPI processes computation was successful (1018 to reach convergence, log_hpddm(bfbcg)_pchpddm_4_mpi.txt).
> But it should be mentioned that stiffness matrix was created in AIJ format (our default matrix format in program).
> Matrix conversion to MATIS format via MatConvert subroutine resulted in losing of convergence for both serial and parallel run.
>
> >> Is this peak memory usage expected for gamg preconditioner? is there any way to reduce it?
> >
> >I think that peak memory usage comes from building the coarse grids. Can you run with `-info` and grep for GAMG, this will provide some output that more expert GAMG users can interpret.
>
> Thanks, I`ll try to use a strong threshold only for coarse grids.
>
> Kind regards,
>
> Viktor Nazdrachev
>
> R&D senior researcher
>
> Geosteering Technologies LLC
>
>
>
>
>
> ср, 1 сент. 2021 г. в 12:02, Lawrence Mitchell <wence at gmx.li <mailto:wence at gmx.li>>:
>
>
> > On 1 Sep 2021, at 09:42, Наздрачёв Виктор <numbersixvs at gmail.com <mailto:numbersixvs at gmail.com>> wrote:
> >
> > I have a 3D elasticity problem with heterogeneous properties.
>
> What does your coefficient variation look like? How large is the contrast?
>
> > There is unstructured grid with aspect ratio varied from 4 to 25. Zero Dirichlet BCs are imposed on bottom face of mesh. Also, Neumann (traction) BCs are imposed on side faces. Gravity load is also accounted for. The grid I use consists of 500k cells (which is approximately 1.6M of DOFs).
> >
> > The best performance and memory usage for single MPI process was obtained with HPDDM(BFBCG) solver and bjacobian + ICC (1) in subdomains as preconditioner, it took 1 m 45 s and RAM 5.0 GB. Parallel computation with 4 MPI processes took 2 m 46 s when using 5.6 GB of RAM. This because of number of iterations required to achieve the same tolerance is significantly increased.
>
> How many iterations do you have in serial (and then in parallel)?
>
> > I`ve also tried PCGAMG (agg) preconditioner with ICС (1) sub-precondtioner. For single MPI process, the calculation took 10 min and 3.4 GB of RAM. To improve the convergence rate, the nullspace was attached using MatNullSpaceCreateRigidBody and MatSetNearNullSpace subroutines. This has reduced calculation time to 3 m 58 s when using 4.3 GB of RAM. Also, there is peak memory usage with 14.1 GB, which appears just before the start of the iterations. Parallel computation with 4 MPI processes took 2 m 53 s when using 8.4 GB of RAM. In that case the peak memory usage is about 22 GB.
>
> Does the number of iterates increase in parallel? Again, how many iterations do you have?
>
> > Are there ways to avoid decreasing of the convergence rate for bjacobi precondtioner in parallel mode? Does it make sense to use hierarchical or nested krylov methods with a local gmres solver (sub_pc_type gmres) and some sub-precondtioner (for example, sub_pc_type bjacobi)?
>
> bjacobi is only a one-level method, so you would not expect process-independent convergence rate for this kind of problem. If the coefficient variation is not too extreme, then I would expect GAMG (or some other smoothed aggregation package, perhaps -pc_type ml (you need --download-ml)) would work well with some tuning.
>
> If you have extremely high contrast coefficients you might need something with stronger coarse grids. If you can assemble so-called Neumann matrices (https://petsc.org/release/docs/manualpages/Mat/MATIS.html#MATIS <https://petsc.org/release/docs/manualpages/Mat/MATIS.html#MATIS>) then you could try the geneo scheme offered by PCHPDDM.
>
> > Is this peak memory usage expected for gamg preconditioner? is there any way to reduce it?
>
> I think that peak memory usage comes from building the coarse grids. Can you run with `-info` and grep for GAMG, this will provide some output that more expert GAMG users can interpret.
>
> Lawrence
>
>
>
> --
> 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/>
> <true_residual_logs_and_greped.rar>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20210906/10e91cbd/attachment-0001.html>
More information about the petsc-users
mailing list