CS 333
Introduction to Operating Systems
Class 14 – Page Replacement
Jonathan Walpole
Computer Science
Portland State University
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Page Replacement Algorithms: Understanding FIFO, LRU, and Optimal Algorithms - Prof. Jonat, Study notes of Operating Systems
An overview of page replacement algorithms, focusing on fifo (first in, first out), lru (least recently used), and the optimal algorithm. It explains the concept of page replacement, the need for algorithms, and the limitations of the optimal algorithm. The document also covers various page replacement algorithms, their implementation, and their advantages and disadvantages.
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CS 333
Introduction to Operating Systems^ Class 14 – Page Replacement
Jonathan Walpole Computer Science Portland State University
Page replacement^ ^ Assume a normal page table (e.g., BLITZ)^ ^ User-program is executing^ ^ APageInvalidFault occurs!^ ^
The page needed is not in memory Select some frame and remove the page in it If it has been modified, it must be written back to disk^ •^ the “dirty” bit in its page table entry tells us if this isnecessary Figure out which page was needed from the faulting addr Read the needed page into this frame Restart the interrupted process by r
etrying the same
instruction
The optimal page replacement algorithm^ ^ Idea:^ ^
Select the page that will not be needed for the longesttime
Optimal page replacement^ ^ Replace the page that will not be needed for the longest^ ^ Example:^ Time^
Requests
c^ a^
d^ b^
e^ b^
a^ b^
c^ d
Page^
0 a Frames 1
b 2 c 3 d Page faults
a^ a^
a^ a b^ b^
b^ b c^ c^
c^ c d^ d^
d^ d
X
The optimal page replacement algorithm^ ^ Idea:^ ^
Select the page that will not be needed for the longesttime Problem: Can’t know the future of a program Can’t know when a given page will be needed next The optimal algorithm is unrealizable
The optimal page replacement algorithm^ ^ However:^ ^
We can use it as a control case for simulation studies^ •^ Run the program once^ •^ Generate a log of all memory references^ •^ Use the log to simulate various page replacementalgorithms^ •^ Can compare others to “optimal” algorithm
FIFO page replacement algorithm^ ^ Replace the page that was first brought into memory^ ^ Example: Memory system with 4 frames:^ Time^
Requests
c^ a^
d^ b^
e^ b^
a^ b^
c^ a
Page^
0 a Frames 1
b 2 c 3 d Page faults
a^ a^
a b c^ c^
c^ c d^ d
X
FIFO page replacement algorithm^ ^ Replace the page that was first brought into memory^ ^ Example: Memory system with 4 frames:^ Time^
Requests
c^ a^
d^ b^
e^ b^
a^ b^
c^ a
Page^
0 a Frames 1
b 2 c 3 d Page faults
a^ a^
a^ a^
a b^ b^
b
c^ c^
c^ c^
e^ e d^ d^
d^ d X
FIFO page replacement algorithm^ ^ Always replace the oldest page.^
“Replace the page that has been in memory for thelongest time.” Implementation Maintain a linked list of all pages in memory Keep it in order of when they came into memory The page at the front of the list is oldest Add new page to end of list
FIFO page replacement algorithm^ ^ Disadvantage:^ ^
The oldest page may be needed again soon Some page may be important throughout execution It will get old, but replacing it will cause an immediatepage fault
Page table: referenced and dirty bits^ ^ Each page has a^ ^
Referenced Bit - set by MMU when page read / written Dirty / Modified Bit - set when page is written On Some Hardware... ReadOnly Bit but no Dirty Bit Idea: Software sets the ReadOnly bit for all pages When program tries to update the page...^ •^ A trap occurs^ •^ Software sets the Dirty Bit and clears the ReadOnly bit^ •^ Resumes execution of the program
Not recently used page replacement alg.^ ^ Use the Referenced Bit and the Dirty Bit^ ^ Initially, all pages have^ ^
Referenced Bit = 0 Dirty Bit = 0 Periodically... (e.g. whenever a timer interrupt occurs) Clear the Referenced Bit
Second chance page replacement alg.^ ^ Modification to FIFO^ ^ Pages kept in a linked list^ ^
Oldest is at the front of the list Look at the oldest page If its “referenced bit” is 0...^ •^ Select it for replacement Else^ •^ It was used recently; don’t want to replace it^ •^ Clear its “referenced bit”^ •^ Move it to the end of the list Repeat What if every page was used in last clock tick? Select a page at random
Clock algorithm (same as second chance)^ ^ Maintain a circular list of pages in memory^ ^ Set a bit for the page when a page is referenced^ ^ Clock sweeps over memory looking for a victim page that doesnot have the referenced bit set^ ^
If the bit is set, clear it and move on to the next page Replaces pages that haven’t been referenced for one completeclock revolution
1
(^12) 3 5 4
0
clock bit^ frame #