I feel that chapter 21 of Samuel is out of order as it relates to the narrative. It isn't likely, though it isn't impossible, that the famine in the land over Saul's sin would happen in the last 20% of David's reign. During his reign, Saul sought to put the Gibeonites to death. These were the people Joshua and company had made a covenant with upon entering the Promised Land. These were the ones who had disguised themselves in worn-out clothing, with stale and moldy bread and cracked wineskins in a successful effort to deceive Joshua and avoid death. The three-year famine was brought to an end by delivering seven of Saul's descendants to death. The entire story is strange, and I'm not altogether sure what we should do with it.