Garbage collecting 'expensive' userdata

[John Dunn <John.Dunn@...>]

If I have a userdata which is wrapping a limited resource ( file handle, socket, a huge block of memory ) is there a way to make the userdata exert pressure on the GC causing it to be cleaned up a quickly as possible? Ideally I'd like it to be cleaned up as soon as there are no references to the data but I don't want to have to call a global collectgarbage call all the time ( or at all ).

John