> On Mar 20, 2019, at 5:52 AM, Yingjie Wu via petsc-users > <petsc-users@mcs.anl.gov> wrote: > > Dear PETSc developers: > Hi, > Recently, I used PETSc to solve a non-linear PDEs for thermodynamic problems. > In the process of solving, I found the following two phenomena, hoping to get > some help and suggestions. > > 1. Because my problem involves a lot of physical parameters, it needs to call > a series of functions, and can not analytically construct Jacobian matrix, so > I use - snes_mf_operator to solve it, and give an approximate Jacobian matrix > as a preconditioner. Because of the large dimension of the problem and the > magnitude difference of the physical variables involved, it is found that the > linear step residuals will increase at each restart (default 30th linear > step) . This problem can be solved by setting a large number of restart > steps. I would like to ask the reasons for this phenomenon? What knowledge or > articles should I learn if I want to find out this problem? I've seen this behavior. I think in your case it is likely the -snes_mf_operator is not really producing an "accurate enough" Jacobian-Vector product (and the "solution" being generated by GMRES may be garbage). Run with -ksp_monitor_true_residual If your residual function has if () statements in it or other very sharp changes (discontinuities) then it may not even have a true Jacobian at the locations it is being evaluated at. In the sense that the "Jacobian" you are applying via finite differences is not a linear operator and hence GMRES will fail on it. What are you using for a preconditioner? And roughly how many KSP iterations are being used. Barry > > > 2. In my problem model, there are many physical fields (variables are > realized by finite difference method), and the magnitude of variables varies > greatly. Is there any Scaling interface or function in Petsc? > > Thanks, > Yingjie
Re: [petsc-users] Problems about GMRES restart and Scaling
Smith, Barry F. via petsc-users Wed, 20 Mar 2019 10:18:44 -0700
- Re: [petsc-users] Problems about GMRES r... Yingjie Wu via petsc-users
- Re: [petsc-users] Problems about GM... Mark Adams via petsc-users
- Re: [petsc-users] Problems about GM... Smith, Barry F. via petsc-users
- Re: [petsc-users] Problems abou... Mark Adams via petsc-users
- Re: [petsc-users] Problems abou... Yingjie Wu via petsc-users
- Re: [petsc-users] Problems ... Alexander Lindsay via petsc-users