# Preconditioner for PETSc Mat **URL:** \ **Category:** General Discussion\ **Created:** [July 16, 2021, 8:13pm UTC](https://community.freefem.org/t/preconditioner-for-petsc-mat/1096 "2021-07-16T20:13:01Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![yongxing](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/yongxing/32/3632_2.png) [@yongxing](https://community.freefem.org/u/yongxing)\ **Post date:** [July 16, 2021, 8:13pm UTC](https://community.freefem.org/t/preconditioner-for-petsc-mat/1096/1 "2021-07-16T20:13:01Z")
Deat all, How to specify a block preconditioner in PETSc. For example, specify preconditioner P= A 0 0 M for solving Kx=b, with K= A B B’ 0 Best, Yongxing. ---
**Author:** ![zhaog6](https://avatars.discourse-cdn.com/v4/letter/z/4af34b/32.png) [@zhaog6](https://community.freefem.org/u/zhaog6)\ **Post date:** [July 17, 2021, 1:52am UTC](https://community.freefem.org/t/preconditioner-for-petsc-mat/1096/2 "2021-07-17T01:52:17Z")
You can use FieldSplit preconditioner in PETSc and refer to those examples with the “fieldsplit” keyword in FreeFEM. ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [July 17, 2021, 4:52am UTC](https://community.freefem.org/t/preconditioner-for-petsc-mat/1096/3 "2021-07-17T04:52:39Z")
There is also a general introduction to fieldsplitting in FreeFEM [there](http://jolivet.perso.enseeiht.fr/FreeFem-tutorial/main.pdf#page=162). ---
**Author:** ![yongxing](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/yongxing/32/3632_2.png) [@yongxing](https://community.freefem.org/u/yongxing)\ **Post date:** [July 18, 2021, 10:04am UTC](https://community.freefem.org/t/preconditioner-for-petsc-mat/1096/4 "2021-07-18T10:04:56Z")
@zhaog6 @prj Thank you both very much. I will look at this later. ---
**Author:** ![yongxing](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/yongxing/32/3632_2.png) [@yongxing](https://community.freefem.org/u/yongxing)\ **Post date:** [July 18, 2021, 5:47pm UTC](https://community.freefem.org/t/preconditioner-for-petsc-mat/1096/5 "2021-07-18T17:47:00Z")
@prj Dear @zhaog6 , Thanks for your help! I have tried the “fieldsplit”, however it’s easy to come across a problem. The first step works fine, and both velocity and pressure field are correct. However, in the second step, the solver complains: **To use Schur complement preconditioner you must have exactly 2 fields…** , which I believe it should have 2 fields, because the first step works fine and the matrix has the same structure in the second step-- really strange. ![image](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/1X/eb26e39b4cc692d7d61f39b5d4005b7dd5bbd9c9.png) [Ths\_cylinder.mesh](https://community.freefem.org/uploads/short-url/kho8Ob8YUCkpsUtRUbzQYiQJNST.mesh) (373.3 KB) [Th\_cylinder.mesh](https://community.freefem.org/uploads/short-url/tuRWcVeOFDGFB0zmQNx6dp3KTwH.mesh) (639.7 KB) [fsi\_3d\_vector.edp](https://community.freefem.org/uploads/short-url/4cMnHEaa4LjZRqFCKUHh4OUicqr.edp) (6.2 KB) Do you have any idea? BTW, the direct LU solver has no problem. Best, Yongxing. ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [July 18, 2021, 6:30pm UTC](https://community.freefem.org/t/preconditioner-for-petsc-mat/1096/6 "2021-07-18T18:30:06Z")
You are probably calling `set(..., sparams = "...")` multiple times on the same matrix. ---
**Author:** ![yongxing](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/yongxing/32/3632_2.png) [@yongxing](https://community.freefem.org/u/yongxing)\ **Post date:** [July 18, 2021, 7:25pm UTC](https://community.freefem.org/t/preconditioner-for-petsc-mat/1096/7 "2021-07-18T19:25:07Z")
@prj I did try to put **A=fluid(…) and set (…, sparams=…)** outside the loop, however I came across other issues. A related question is : If the mesh **Ths** is moved , should I call **transferMat(Th, Pk, A, Ths, Pk, B, P)** again and again (inside the time loop)? Both **Th** and **A** are static, but the boundary condition (inlet velocity) depends on time, should I put assembly of **A** outside the time loop? ---
**Author:** ![yongxing](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/yongxing/32/3632_2.png) [@yongxing](https://community.freefem.org/u/yongxing)\ **Post date:** [July 18, 2021, 10:06pm UTC](https://community.freefem.org/t/preconditioner-for-petsc-mat/1096/8 "2021-07-18T22:06:53Z")
@zhaog6 @prj Indeed, I really want to call set(…) only once (outside the time loop). However, when I arrange the following code just outside the time loop: Mat A,B; { macro def(i)[i, i#B, i#C, i#D]// macro init(i)[i, i, i, i]// createMat(Th, A, Pk) createMat(Ths, B, Pk) } and Mat P; transferMat(Th, Pk, A, Ths, Pk, B, P) inside the loop. I came across the following error: ![image](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/1X/cc10d2be98b5b291e9e7f77eb6db7b27c49372a7.png) **I have no idea which array size is 1?** all arrays do have a size of 4. [fsi\_3d\_vector\_opti.edp](https://community.freefem.org/uploads/short-url/bQARuwb6X3aPoyxW7oVx4QkKLbs.edp) (6.0 KB)