How do we keep track of where the free pieces of memory are? One idea is to maintain a set of linked-lists of free space; each linked-list will store free chunks of some given size (say, one list for chunks of 4 bytes, one for chunks of 8, 16, etc). This approach resembles the best-?t algorithm, since we can easily ?nd the list with chunks of memory that are the closest to what we need; the difference, of course, is that usually the linked lists store not objects of arbitrary size, but only powers of two. Also, we can never coalesce adjacent elements of the lists, or split elements, since then the very idea of segregating by size classes wouldn't make sense anymore. Another possible technique to keep track of free memory is called Big Bag of Pages (BiBOP). In this technique, a bunch of pages, usually segregated by size classes, is maintained such that there is a header on each page; on this header, among other things, we can ?nd a pointer to the next page of objects of that given size, and also a pointer to the ?rst free "slot" inside that page. Using the right optimizations we can make BiBOP ?nd and manage free slots of memory in O(1).