Why PJFNK calls computeQpResidual() during each linear step?

[Richard-happy]

From Krylov methods, what we need is vectors like ${-R_h, J(-R_h), J^2(-R_h),......}$. So it's OK just call computeQpResidual() once each nonlinear.

From https://mooseframework.inl.gov/workshop/index.html#/9/10 ,it says During each linear step of PJFNK, the computeQpResidual method is called to approximate the action of the Jacobian on the Krylov vector. I'm not understand this.
Dose computeQpResidual() not mean computing $-R_h$?

[GiudGiud]

Hello

if you compute the residual twice, once with a slightly offset solution vector, you can approximate the action of the Jacobian

Guillaume

[Richard-happy]

understand .thank you ,GiudGiud