This isn't good enough because size of an allocation says nothing about what its lifetime is. If you know lifetimes or types then you can segregate those and it

This isn't good enough because size of an allocation says nothing about what its lifetime is. If you know lifetimes or types then you can segregate those and it does help. (It does help in that if you have fixed size slabs, you can't waste space on that page, but you can still waste the entire page.)