When Replacements Really Occur

This lesson draws out a realistic view of why and ​how the operating system always keeps​ some memory free through replacements.

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Thus far, the way we’ve described how replacements occur assumes that the OS waits until memory is entirely full, and only then replaces (evicts) a page to make room for some other page. As you can imagine, this is a little bit unrealistic, and there are many reasons for the OS to keep a small portion of memory free more proactively.

Swap daemon or page daemon

To keep a small amount of memory free, most operating systems thus have some kind of high watermark (HW) and low watermark (LW) to help decide when to start evicting pages from memory. How this works is as follows: when the OS notices that there are fewer than LW pages available, a background thread that is responsible for freeing memory runs. The thread evicts pages until there are HW pages available. The background thread sometimes called the swap daemon or page daemon, then goes to sleep, happy that it has freed some memory for running processes and the OS to use.

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