When the system is under memory pressure, the pageout process reuses memory from the free, cache, inact and, if necessary, active pages. When the pageout process runs, it scans memory to see which pages are good candidates to be unmapped and freed up. Thus, the distinction between Active and Inact memory is only used by the pageout process to determine which pool of pages to free first at the time of a memory shortage.
The pageout process first scans the Inact list, and checks whether the pages on this list have been accessed since the time they have been listed here. The pages that have been accessed are moved from the Inact list to the Active list. On the other hand, pages that have not been accessed become prime candidates to be freed by the pageout process. If the pageout process cannot produce enough free pages from the Inact list, pages from the Active list get freed up.
Because the pageout process runs only when the system is under memory pressure, the pages on the Inact list remain untouched – even if they have not been accessed recently – when the amount of Free memory is adequate.
For more information, see Junos OS Routing Protocols Library for Routing Devices