Where is the decision about placement of new tuple made ?

Where in the source is the decision about the placement new tuple (on
which page to put it) made ?

I'd like to take a look at adding "gravity" to that decision, do that I
can make postgres to decide to place new tuple (inserted or updated)
near the beginning of file, in order to make it possible for ordinary
(lazy) vacuum to shrink relations more often, initially controlled by
GUC, maybe later by some other, more automatic hints, like % of empty
pages.

My current jedi mindtricks (seqscan + update pk=pk on last tuple until
its page nr in ctid changes) for shrinking relations do work, but not as
well as I'd like them to ;)

-- 
Hannu Krosing <hannu@skype.net>