It is a page replacement algorithm. This algorithm removes a page at random from the lowest numbered non-empty class. Implicit in this algorithm is that it is better to remove a modified page that has not been referenced in atleast one clock tick than a clean page that is in heavy use.
It is easy to understand, moderately efficient to implement and gives a performance that while certainly not optimal, may be adequate. When page is modified, a modified bit is set. When a page needs to be replaced, the Operating System divides pages into 4 classes.
- 0:- Not Referenced, Not Modified
- 1:- Not Referenced, Modified
- 2:- Referenced, Not Modified
- 3:- Referenced, Modified
Out of above 4 categories, NRU will replace a Not Referenced, Not Modified page, if such page exists. Note that this algorithm implies that a Modified but Not Referenced is less important than a Not Modified and Referenced.
- Case-1 implies to Not Referenced and Modified.
- Case-2 implies to Not Referenced and Not Modified.
- Case-3 implies to Referenced and Modified.
- Case-0 implies to Referenced and Not Modified.
From the given reference string NRU will remove a page at random from the lowest numbered nonempty class. Implicit in this algorithm is that it is better to remove a modified page that has not been referenced in at least one clock tick (typically 20 msec) than a clean page that is in heavy use.
Attention reader! Don’t stop learning now. Get hold of all the important CS Theory concepts for SDE interviews with the CS Theory Course at a student-friendly price and become industry ready.