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CMSC 412 – S02 (lect 13)
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Page Replacement Algorithms in Operating Systems: FIFO, LRU, and Thrashing - Prof. Jeffrey, Study notes of Operating Systems
An overview of page replacement algorithms, specifically fifo (first in, first out) and lru (least recently used), in the context of operating systems. The advantages and disadvantages of each algorithm, as well as the concept of thrashing and swapping. Real-life examples are given to illustrate the concepts.
Typology: Study notes
Pre 2010
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Lecture provided by Bobby Bhattacharjee
Page Replacement Algorithms
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FIFO
Replace the page that was brought in longest ago
However
-^
old pages may be great pages (frequently used)
-^
number of page faults may increase when one increases number ofpage frames (discouraging!)
- called belady’s anomaly– 1,2,3,4,1,2,5,1,2,3,4,5 (consider 3 vs. 4 frames)
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Optimal
Replace the page that will be used furthest in the future
Good algorithm(!) but requires knowledge of the future
With good compiler assistance, knowledge of the future issometimes possible
FIFO Example (3 frames)
- Reference string: 1,2,3,4,1,2,5,1,2,3,4,
•^
access 1 - (1) fault
-^
access 2 - (1,2) fault
-^
access 3- (1,2,3) fault
-^
access 4 - (2,3,4) fault, replacement
-^
access 1 - (3,4,1) fault, replacement
-^
access 2 - (4,1,2) fault, replacement
-^
access 5 - (1,2,5) fault, replacement
-^
access 1- (1,2,5)
-^
access 2 - (1,2,5)
-^
access 3 - (2,5,3) fault, replacement
-^
access 4 - (5,3,4) fault, replacement
-^
access 5 - (5,3,4)
- 9 page faults
LRU Example (3 frames)
- Reference string: 1,2,3,4,1,2,5,1,2,3,4,
•^
access 1 - (1) fault
-^
access 2 - (1,2) fault
-^
access 3- (1,2,3) fault
-^
access 4 - (2,3,4) fault, replacement
-^
access 1 - (3,4,1) fault, replacement
-^
access 2 - (4,1,2) fault, replacement
-^
access 5 - (1,2,5) fault, replacement
-^
access 1- (2,5,1)
-^
access 2 - (5,1,2)
-^
access 3 - (1,2,3) fault, replacement
-^
access 4 - (2,3,4) fault, replacement
-^
access 5 - (3,4,5) fault, replacement
- 10 page faults
FIFO Example (4 frames)
- Reference string: 1,2,3,4,1,2,5,1,2,3,4,
•^
access 1 - (1)
fault
•^
access 2 - (1,2) fault
-^
access 3- (1,2,3) fault
-^
access 4 - (1,2,3,4) fault, replacement
-^
access 1 - (1,2,3,4)
-^
access 2 - (1,2,3,4)
-^
access 5 - (2,3,4,5) fault, replacement
-^
access 1- (3,4,5,1) fault, replacement
-^
access 2 - (4,5,1,2) fault, replacement
-^
access 3 - (5,1,2,3) fault, replacement
-^
access 4 - (1,2,3,4) fault, replacement
-^
access 5 - (2,3,4,5) fault, replacement
- 10 Page faults
Thrashing
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Virtual memory is not “free”
- can allocate so much virtual memory that the system spends
all its time getting pages
- the situation is called thrashing– need to select one or more processes to swap out
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Swapping
- write all of the memory of a process out to disk– don’t run the process for a period of time– part of medium term scheduling
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How do we know when we are thrashing?
- check CPU utilization?– check paging rate?– Answer: need to look at both
- low CPU utilization plus high paging rate --> thrashing