“…Krylov subspace methods together with various preconditioners are recommended for the saddle point problems in large scale. In the passed decades, there are lots of work on this topic, including block diagonal preconditioner [9,15,16,18], block triangular preconditioner [7,10,13,19,22,24], HSS preconditioner [1,2,4], constraint preconditioner [3,6,11], matrix splitting preconditioner [21] and so on.…”