Beyond Physical Memory: Policies

In a virtual memory manager, life is easy when you have a lot of free memory. A page fault occurs, you find a free page on the free-page list, and assign it to the faulting page. Hey, Operating System, congratulations! You did it again. Unfortunately, things get a littlemore interestingwhen littlemem-ory is free. In such a case, thismemory pressure forces the OS to start paging out pages to make room for actively-used pages. Deciding which page (or pages) to evict is encapsulated within the replace-ment policy of the OS; historically, it was one of the most important decisions the early virtual memory systems made, as older systems had little physical memory. Minimally, it is an interesting set of poli-cies worth knowing a little more about. And thus our problem: THE CRUX: HOW TO DECIDE WHICH PAGE TO EVICT How can the OS decidewhich page (or pages) to evict frommemory? This decision is made by the replacement policy of the system, which usually follows some general principles (discussed below) but also includes certain tweaks to avoid corner-case behaviors.