In a large skillet, combine pineapple, sugar, vinegar, lime juice, red onion, cumin, salt & pepper. Bring to a boil & cook over medium-high heat until thickened, 7-10 minutes. If there is still a lot of liquid left, use a slotted spoon to transfer the pineapple to a bowl & continue to cook the liquid over high heat for 5 minutes more, then pour the liquid over the pineapple. Mix in the onions & cilantro. Season with salt to taste.