1. Chapter 3.3 : OS Policies for Virtual Memory
Fetch policy
Placement policy
Replacement policy
Resident set management
Cleaning policy
Load control
From : Operating Systems by W. Stallings, Prentice-Hall, 1995
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Fetch Policy
Demand Paging: Pages are fetched when needed (ie., when a page fault occurs)
Process starts with a flurry of page faults, eventually locality takes over
Prepaging (anticipatative): Pages other than the one needed are brought in
Prepaging makes use of disk storage characteristics. If pages are stored contiguously it may be more efficient to fetch them in one go
The policy is ineffective if the extra pages are not referenced
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Placement Policy
The placement policy is concerned with determining where in real memory a process piece is to reside
With anything other than pure segmentation this is not an issue (refer to best-fit, first-fit etc.)
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Replacement Policy
All page frames are used. A page fault has occurred. New page must go into a frame.
Which one do you take out?
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5. Replacement Algorithm Objectives
The page being replaced should be the page least likely to be referenced in the near future
There is a link between past history and the future because of locality
Thus most algorithms base their decision on past history
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6. Replacement Algorithms
Optimal
Not-recently-used (NRU)
First-in, first-out (FIFO)
Least recently used (LRU)
Not frequently used (NFU)
Modified NFU (~LRU)
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Scope of Replacements
The set of frames from which these algorithms choose is based on the scope
Local Scope: Only frames belonging to the faulting process can be replaced.
Global Scope: All frames can be replaced
Some frames will be locked (e.g. Kernel, system buffers etc.,)
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8. Optimal Replacement Algorithm
Replace the page which is least likely to be referenced or for which the time to the next reference is the longest (Replace page needed at the farthest point in future)
This algorithm is impossible to implement because OS must have perfect knowledge of future events
This algorithm is used to compare other algorithms
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9. Optimal Replacement ExampleF 2 3 1 5 4
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10. Optimal Replacement Example (Cont.)
1’st page fault : page 1 is replaced by page 5 because page 1 does not appear in the page address stream in the future
2’nd page fault: page 2 is replaced by page 4 because page 2 will be referenced after two (pages 5 and 3) references
3’rd page fault: page 4 is replaced by 2 because page 4 is not in the stream any more
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11. Not-Recently-Used (NRU)
Replace the page which is not used recently
Use the referenced (R) & modified (M) bits in the page table entry
R bit is set when the page is referenced (read or written)
M bit is set when the page is modified (contents changed - written)
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NRU Implementation
Hardware
R & M bits are set by hardware on every memory reference
Software
R & M bits in the page table entry is modified at page faults
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NRU Algorithm
When process starts both R & M bits are cleared
R bit is cleared on every clock interrupt
At a page fault, a page from the lowest numbered nonempty class is removed:
Class 0 : not referenced, not modified
Class 1 : not referenced, modified
Class 2 : referenced, not modified
Class 3 : referenced, modified
Class 1 appears when clock interrupt clears R bits of class 3 processes
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14. First-in First-out (FIFO)
Replace the page which has been in memory for the longest time
Simple to implement (use a circular buffer)
There is a chance that the oldest page may be used heavily (thrashing - page moves back & forth between memory & disk)
Inspect R & M bits (ie., classes) to skip over heavily used pages to decrease thrashing
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FIFO Example 2 3 1 5 4 F F F F F F
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16. Least Recently Used (LRU)
Replace the page which has been unused for the longest time
Does almost as well as optimal
Implementation poses overheads
Implementation uses a time stamp for each page (time of last reference)
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LRU Example F 2 3 1 5 4 F
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LRU Example (Cont.)
1’st page fault: page 5 replaces page 3 because page 3 hasn’t been referenced in the last two references
2’nd page fault: page 4 replaces page 1 because page 1 hasn’t been referenced in the last two references
3’rd page fault: page 3 replaces page 2 because page 2 hasn’t been referenced in the last two references
4’th page fault: page 2 replaces page 4 because page 4 hasn’t been referenced in the last two references
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19. Hardware Implementation of LRU
Maintain a linked list of all pages ordered from the most recently used to the least recently used. Maintaining a list on every instruction execution is very expensive and time consuming
An approximate solution
A hardware counter is incremented after each instruction
Page table also has a field to store the number of references (counter value)
At a page fault remove the page which has the minimum number of references
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20. Software Implementation of LRU : Not Frequently Used (NFU)
At every clock interrupt the R bit value is added to the fields in page tables
At a page fault, replace the page with the minimum counter value
Since the counter update is done at every clock interrupt, the algorithm is an approximation
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NFU Example
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Modified NFU : ~LRU
Aging of pages
Algorithm
Shift right counter values (in page table) by 1
Put R bit value (0 - not referenced or 1- referenced ) as the new leftmost bit at every clock tick
At a page fault replace the page with the lowest counter value (this is the least recently used page)
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23. Modified NFU : ~LRU (Cont.)
Example
Suppose counter is
& R bit = 0
New counter becomes
Leading zeros represent the number of clock ticks that the page has not been referenced
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24. Differences Between NFU & ~LRU
NFU counts the number of times that a page is accessed in a given period
~LRU incorporates a time factor by shifting right (aging) the counter value
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25. Resident Set Management
Resident Set: Set of a process' pages which are in main memory
OS must manage the size and allocation policies which effect the resident set
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Factors Include
Smaller resident set per process, implies more processes in memory, implies OS will always find at least one ready process (if not swapping must occur)
If process' resident set is small, page fault frequency will be higher
Increasing the resident set size beyond some limit does not effect page fault frequency
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27. Resident Set Management Policies
Fixed allocation policy
Each process has a fixed number of pages
Variable allocation policy
The number of pages per process can change
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Allocation vs Scope
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29. Fixed Allocation, Local Scope
Frame number per process is decided beforehand and can't be changed
Too Small: High page faults, poor performance
Too Large: Small number of processes, high processor idle time and/or swapping
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30. Variable Allocation, Global Scope
Easiest to implement
Processes with high page fault rates will tend to grow. However replacement problems exist
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31. Variable Allocation, Local Scope
Allocate new process a resident set size.
Prepaging or demand to fill up allocation
Select replacement from within faulting process
Re-evaluate allocation occasionally
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32. Working Set (Denning’s)1 2 3 5 6 7 8 4
Routine 2
Routine 1
Main program
Main loop
Program Execution
Start with main program (page 8)
Main program loads the main loop (pages 4,5,6,7)
Program executes in the main loop for 20 seconds
Then routine 1 is called (page 1) which executes 2 seconds
Finally routine 2 (pages 2, 3) is called to execute for 3 seconds
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Working Set (Cont.)
In the previous example, the process needs pages 4, 5, 6, 7 for most of the time (20 seconds in a total of 25 seconds execution time)
If these pages are kept in memory the number of page faults will decrease. Otherwise, thrashing may occur
The set of pages that a process is currently using is called its working set
Rule: Do not run a process if its working set can not be kept in memory
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34. The Working Set Page Replacement Algorithm
The working set is the set of pages used by the k most recent memory references
w(k,t) is the size of the working set at time, t
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35. Working Set Implementation
Use aging as in ~LRU
Pages with 1 (referenced) in “n” clock ticks are assumed to be a member of the working set
“n” is experimentally determined
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Cleaning Policy
Cleaning Policy: Deciding when a modified page should be written out to secondary memory
Demand Precleaning: Page is written out only when it has been selected for replacement
Means a page fault may imply two I/O operations which severely effects performance
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Precleaning: Pages written before frames are needed (so they can be written out in batches)
No sense writing out batches of pages and then finding them changed again
Page Buffering is a nice compromise
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Load Control
Load Control: Controlling the number of processes that are resident in memory
Too few processes imply lack of ready processes, implies swapping
Too many processes implies high page fault frequency which leads to thrashing
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