Solid Hardwood, Engineered and Laminate Flooring - subfloor prep

I'm about to install 3/4" thick, 5" wide solid hardwood flooring in my living and dining rooms. I've stripped off all the carpet and padding, removed all the moldings, and gotten all the furniture out of the rooms, so I'm just about ready to nail in the first planks.

My only concern is a small section of the plywood subfloor that appears to be sagging somwhat. It's sagging as it approaches the wall separating the dining room from the kitchen, and the sag is perpendicular to the joists.

My question is, how much of a sag is allowable, and if this is outside the limits, what's the easiest and best way to correct it.

Thanks for any help you can give.
Joanne

There are self-leveling compounds that can be used to level out sags. The sag could be due to a warped joist. Make sure you don't have any structural problems that may be causing the sag.