Re: [deal.II] Re: PETScWrappers::SparseDirectMUMPS for symmetry system matrix and factorization resue

[shanglong zhang]

Hi Wolfgang,

Thank you for the explanation. Then everything make sense.

Best,
Shanglong

在 2016年7月27日星期三 UTC-4下午6:13:16，bangerth写道：
>
> On 07/27/2016 01:43 PM, Alexander wrote: 
> > 
> > step-17 claims to solve same problem as step-8. If one produces symmetry 
> > matrix and the other does not or both do not -- this is suspicious. 
>
> The reason why the matrix in step-17 is not symmetric is actually 
> explained in the code: 
>
>  // The last argument to the call to 
>  // MatrixTools::apply_boundary_values() below allows for some 
>  // optimizations. It controls whether we should also delete 
>  // entries (i.e., set them to zero) in the matrix columns 
>  // corresponding to boundary nodes, or to keep them (and passing 
>  // true means: yes, do eliminate the columns). If we 
>  // do eliminate columns, then the resulting matrix will be 
>  // symmetric again if it was before; if we don't, then it 
>  // won't. The solution of the resulting system should be the same, 
>  // though. The only reason why we may want to make the system 
>  // symmetric again is that we would like to use the CG method, 
>  // which only works with symmetric matrices. The reason why we may 
>  // not want to make the matrix symmetric is because this 
>  // would require us to write into column entries that actually 
>  // reside on other processes, i.e., it involves communicating 
>  // data. This is always expensive. 
>  // 
>  // Experience tells us that CG also works (and works almost as 
>  // well) if we don't remove the columns associated with boundary 
>  // nodes, which can be explained by the special structure of this 
>  // particular non-symmetry. To avoid the expense of communication, 
>  // we therefore do not eliminate the entries in the affected 
>  // columns. 
>  std::map boundary_values; 
>  VectorTools::interpolate_boundary_values (dof_handler, 
>0, 
>ZeroFunction(dim), 
>boundary_values); 
>  MatrixTools::apply_boundary_values (boundary_values, 
>  system_matrix, 
>  solution, 
>  system_rhs, 
>  false); 
>
> In other words, it is expected that the matrix is not symmetric, and 
> that consequently when you run a direct solver, you can't use the 
> symmetric setting. 
>
> Best 
>   Wolfgang 
>

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[deal.II] Re: PETScWrappers::SparseDirectMUMPS for symmetry system matrix and factorization resue

[shanglong zhang]

Hi Alexander,

I switch to :
dealii 8.4.1
petsc 3.6.4
mumps 5.0.1

The result is still not correct.
The modification I did for the code is replacing the following code in 
step-17.cc:

*SolverControl   solver_control (solution.size(),*
*1e-8*system_rhs.l2_norm());*
*PETScWrappers::SolverCG cg (solver_control,*
*mpi_communicator);*
*PETScWrappers::PreconditionBlockJacobi preconditioner(system_matrix);*
*cg.solve (system_matrix, solution, system_rhs,*
*  preconditioner);*
with
*  SolverControl solver_control (**solution.size()**, 1e-12);*
*  PETScWrappers::SparseDirectMUMPS solver(solver_control, 
mpi_communicator);*
*  solver.set_symmetric_mode(true);*
*  solver.solve(system_matrix, solution, system_rhs);*

Thank you very much.
Regards,
Shanglong

在 2016年7月26日星期二 UTC-4上午3:40:09，Alexander写道：
>
>
> deal.II 8.4.0
>> petsc 3.3.0.3
>> MUMPS 4.10.0-p3
>>
>
> This PETSc and MUMPS > 4 years old. Update to the latest versions and try 
> again. If problem persists, provide the relevant snippet of the code.
>  
>
>> Sorry, I am not familiar with the petsc library. You meant if I call 
>> *solver.solve(system_matrix,solution,system_rhs)* twice, internally, it 
>> would use the factorization from the first time for the second solving 
>> process? (of course the system matrix is the same.) Do i understand this 
>> correctly? Thanks.
>>
>
> Yes, correct.
>
> @Denis: I agree, the logic is somewhat deficient. I also think 
> documentation and interface itself (e.g. I have no idea why does method 
> set_solver_type 
> 
>  
> exist and exposed to the public) could be improved. 
>

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[deal.II] Re: PETScWrappers::SparseDirectMUMPS for symmetry system matrix and factorization resue

[shanglong zhang]

Hi Denis,

Thank you for your information.

Best,
Shanglong

在 2016年7月25日星期一 UTC-4下午12:02:09，Denis Davydov写道：
>
> Hi,
>
> I can't comment on the first part of your question, but as for the second 
> part -- you are right, 
> currently you can't reuse factorization. I you want it, it should be 
> relatively easy to add.
> See Direct solver in TrilinosWrappers which was recently modified to 
> support this: 
>
> https://www.dealii.org/developer/doxygen/deal.II/classTrilinosWrappers_1_1SolverDirect.html#a6c0fecc921bc93f2c005e265cfbdd882
>
> Regards,
> Denis.
>
> On Monday, July 25, 2016 at 5:52:06 PM UTC+2, shanglong zhang wrote:
>>
>> Dear all,
>>
>> I modify the step-17 so that it uses PETScWrappers::SparseDirectMUMPS as 
>> solver. However, when I active the option of set_symmetric_mode, the 
>> solution has discrepancy from CG or MUMPS without the symmetry option. (The 
>> solutions from CG and MUMPS without symmetry option are identical though.) 
>> Any suggestion?
>>
>> And it seems to me that  PETScWrappers::SparseDirectMUMPS doesn't have 
>> the functionality of reusing the factorization of system matrix for 
>> different right-hand side vector. Do I miss anything?
>>
>> Thank you very much.
>>
>> Best,
>> Shanglong
>>
>

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