mldivide, backslash \ operator options

Hello, I'm solving the system

 x=A\B;

the size of A is is around [1e6 x 1e6] and is a symmetric and sparse matrix with 5 non-zero diagonals (2D laplace equation).

the \ operator is extremely slow, and all other numerical methods (including bicgstab) doesn't Provide a good solution.

what I need to do is to solve this equation many times, every time with different boundary conditions, meaning that A remains unchanged and all I'm changing is B.

my two questions are:

1) is there a way to know which method the \ operator is using so I can predetermine the options to minimize the execution time? 2) is there a way to give an initial guess to this operator so it will converge much faster?

thanks in advance

Answer 1

[Matthaios Doulgerakis-Kontoudis]

https://uk.mathworks.com/matlabcentral/answers/85847-how-can-i-find-out-what-algorithm-mldivide-is-using