mldivide, backslash \ operator options

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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