PCTFREE on Pg

I've done a quick hack for a space reservation in a page to use same page on tuple updating. It's similar to Oracle's PCTFREE stuff. heap_update() can use a (reserved) free space on the same page, but heap_insert() can't.
I allocated 1024 bytes for reserve space in each page, then it shows an improvement on the pgbench score 10% or more on my box.

-------------- normal -------------------
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
number of clients: 16
number of transactions per client: 1000
number of transactions actually processed: 16000/16000
tps = 60.434280 (including connections establishing)
tps = 60.461362 (excluding connections establishing)
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
number of clients: 16
number of transactions per client: 1000
number of transactions actually processed: 16000/16000
tps = 57.350695 (including connections establishing)
tps = 57.371082 (excluding connections establishing)
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
number of clients: 16
number of transactions per client: 1000
number of transactions actually processed: 16000/16000
tps = 55.928602 (including connections establishing)
tps = 55.951602 (excluding connections establishing)

-------------- pctfree -------------------
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
number of clients: 16
number of transactions per client: 1000
number of transactions actually processed: 16000/16000
tps = 67.552621 (including connections establishing)
tps = 67.586533 (excluding connections establishing)
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
number of clients: 16
number of transactions per client: 1000
number of transactions actually processed: 16000/16000
tps = 69.260082 (including connections establishing)
tps = 69.290771 (excluding connections establishing)
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
number of clients: 16
number of transactions per client: 1000
number of transactions actually processed: 16000/16000
tps = 69.466603 (including connections establishing)
tps = 69.497294 (excluding connections establishing)