[petsc-users] Problems about GMRES restart and Scaling

[Yingjie Wu]

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?

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
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