Issues with solving large sparse linear equations

I have some issues solving sparse linear equations Ax = b

My 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?

• You need to be more detailed in your question on what you have already tried. Which solvers did you use, which preconditioners? Is your matrix symmetric? Is it positive definite? – Wolfgang Bangerth Apr 28 '13 at 15:18