chess - Knights on a Torus

An 8×8 chessboard is in the shape of a torus. (This means that the board "wraps around" - you can go left from a2 and come out on h2, for instance. It also works vertically - you can go up from f8 and come out on f1.) What's the minimum number of knights you can place on the board so every square is attacked, even ones with pieces on them?

Answer

I can do it in 8:

And of course, this is the optimum, since each knight attacks 8 squares and we need to cover at least 64.