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4 changes: 2 additions & 2 deletions operating-systems/memory-management.md
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Expand Up @@ -290,15 +290,15 @@ At that point, control is handed back to the process that issued this reference,
We may require a page to be constantly present in memory, or maintain its original physical address throughout its lifetime. We will have to **pin** the page. In order words, we disable swapping. This is useful when the CPU is interacting with devices that support direct memory access, and therefore don't pass through the MMU.

## Page Replacement
###### When should pages be swapped out of main memory and on to disk?
###### When should pages be swapped out of main memory and onto disk?
Periodically, when the amount of occupied memory reaches a particular threshold, the operating system will run some **page out daemon** to look for pages that can be freed.

Pages should be swapped when the memory usage in the system exceeds some threshold and the CPU usage is low enough so that this daemon doesn't cause too much interruption of applications.

###### Which pages should be swapped out?
Obviously, the pages that won't be used in the future! Unfortunately, we don't know this directly. However, we can use historic information to help us make informed predictions.

A common algorithm to determine if a page should be swapped out is too look at how recently a page has been used, and use that to inform a prediction about the page's future use. The intuition here is that a page that has been more recently used is more likely to be needed in the immediate future whereas a page that hasn't been accessed in a long time is less likely to be needed.
A common algorithm to determine if a page should be swapped out is to look at how recently a page has been used, and use that to inform a prediction about the page's future use. The intuition here is that a page that has been more recently used is more likely to be needed in the immediate future whereas a page that hasn't been accessed in a long time is less likely to be needed.

This policy is known as the **Least-Recently Used** (LRU) policy. This policy uses the **access bit** that is available on modern hardware to keep track of whether or not the page has been referenced.

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