Whenever a worker thread issues a read request, it gets a list of 64 pages in cache and checks whether the free buffer list is below a certain threshold. If it is, it will try to age-out some pages in its list, which causes any dirty pages to be written to disk.

This is under Dirty Pages at page 17, what I cant understand is why 64 pages (half MB) is allocated and again thats chopped to get more free memory. Ideally SQL Server should first check how much it can afford to dedicate to the thread rather than allocating & reducing that again.