I have some issues solving sparse linear equations Ax = bMy matrix A is sparse with dimension of 5 million by 5 million. Actually, it is a combination of two matrices. One is tridiagonal and the other matrix has a bigger band. For example the $29450^{th}$  row has non zero diagonal entry. Plus non zero element at $14725^{th}$, $44175^{th}$, $29451^{th}$,  $29449^{th}$,$29605^{th}$,$29295^{th}$,column. So every row has almost 6 non zero elements plus non zero diagonal entry. I tried to use a couple of iterative solvers in matlab, but they don't seem to give good accuracy. Also if I run the iteration for a long time, I will have round off errors and all. What can I do to get the best estimate?
Issues with solving large sparse linear equations
linear algebra;optimization;sparse