Place sweet potatoes in a large kettle and cover with water; bring to a boil. Boil gently until potatoes can easily be pierced with the tip of a sharp knife, 30-45 minutes. Drain; cool slightly., Preheat oven to 350°. Peel potatoes and place in a large bowl with butter and sugar; mash. Add pineapple, eggs, vanilla, nutmeg and salt; stir to combine., Spoon into a greased 2-qt. baking dish. Top with marshmallows. Bake, uncovered, until a knife inserted in the center comes out clean, 40-45 minutes.