1. Virtual Memory 3 Fred Kuhns
Department of Computer Science and Engineering
Washington University in St. Louis

2. Basic Paging Policies Fetch Policy: when to bring page into memory
Demand paging: fetch when referenced
Prepaging brings in more pages than needed
Replacement Policy: which page to remove from memory
Page removed should be the page least likely to be referenced in the near future
Most policies predict the future behavior on the basis of past behavior
Frame Locking - If frame is locked, it may not be replaced.
Placement Policy: where fetched page is placed
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3. Demand Paging
Static Paging Algorithms – fixed number of pages allocated to process
Optimal
Not recently used
First-in, First-out and Second chance
Clock algorithm
Least Recently Used
Least Frequently Used
Dynamic Paging Algorithms
Working Set Algorithms
Fred Kuhns ()

4. Static Replacement Algorithms
Optimal
Selects for replacement that page for which the time to the next reference is the longest
Impossible to have perfect knowledge of future events
Not Recently used
recall M and R bits
when page fault occurs divide each page into one of 4 groups 0) ~R & ~M; 1) ~R & M; 2) R & ~M; 3) R & M and replace from the lowest ordered non-empty group.
disadvantage is having to scan all pages when a fault occurs
Fred Kuhns ()

5. Static Replacement Algorithms
First-in, first-out (FIFO)
Treats page frames allocated to a process as a circular buffer
Pages are removed in round-robin style
Simplest replacement policy to implement
But may remove needed pages
Second Chance Algorithm
improvement to FIFO by using the R bit
if R bit is set then clear and move to end of list otherwise replace
may result in expensive list manipulations
Fred Kuhns ()

6. Static Replacement Algorithms
Least Recently Used (LRU)
Replaces the page that has not been referenced for the longest time
By the principle of locality, this should be the page least likely to be referenced in the near future
Each page could be tagged with the time of last reference. This would require a great deal of overhead.
Expensive to implement if not supported in hardware
Fred Kuhns ()

7. Static Replacement Algorithms
Least Frequently Used
software implementation of least recently used
associate counter with each page initialized to 0 and increment by R each clock tick
problem is history ... slow to react to changing reference string
Implementing Least Frequently Used
shift counter right 1 bit and add R to leftmost bit. This is known as aging.
Fred Kuhns ()

8. Static Replacement Algorithms
Clock Policy
Behavior is same as second chance but avoids extra list manipulations
put pages on a circular list in the form of a clock with a “hand” referencing the “current” page
when page fault occurs, if “current” page has R = 0 then replace otherwise set R = o and advance to next page. Keep advancing until locate page with R = 0.
Fred Kuhns ()

9. from William Stallings, “Operating Systems”, 4th edition, Prentice Hall

10. Dynamic Replacement Algorithms
Working set algorithm – aka demand paging
working set – set of pages currently used by process
OS keeps track of processes working set, the working set model, and is modeled as w(k,t), for the k most recent references and process virtual time t.
implementations use virtual time rather then k, the virtual time of a process. Can track the pages referenced in the past x seconds of virtual time
Required scanning entire page list, expensive
Compromise the is the WSClock algorithm: stamp page with
Fred Kuhns ()

11. WSClock Algorithm
Similar to clock algorithm with circular list and clock hand.
scans list and if R = 1 then it is cleared and the current virtual time is written.
advance hand
if R = 0 then check timestamp > T then replace (if dirty schedule for write else put on free list)
Fred Kuhns ()

12. Some Details Stack Algorithms Page Buffering
pages loaded with allocation m is always subset of those loaded with allocation of m+1.
Page Buffering
Replaced page is added to one of two lists
Fixed process page allocation
gives a process a fixed number of pages within which to execute
when a page fault occurs, one of the pages of that process must be replaced
Variable process page allocation
number of pages allocated to a process varies over the lifetime of the process
Fred Kuhns ()

13. Variable Allocation, Global Scope
Easiest to implement
Adopted by many operating systems
Operating system keeps list of free frames
Free frame is added to resident set of process when a page fault occurs
If no free frame, replaces one from another process
Fred Kuhns ()

14. Variable Allocation, Local Scope
When new process added, allocate number of page frames based on application type, program request, or other criteria
When page fault occurs, select page from among the resident set of the process that suffers the fault
Reevaluate allocation from time to time
Fred Kuhns ()

15. Cleaning Policy Demand cleaning Precleaning
a page is written out only when it has been selected for replacement
Precleaning
pages are written out in batches
Best approach uses page buffering
Replaced pages are placed in two lists
Modified and unmodified
Pages in the modified list are periodically written out in batches
Pages in the unmodified list are either reclaimed if referenced again or lost when its frame is assigned to another page
Fred Kuhns ()

16. Load Control
Determines the number of processes that will be resident in main memory
Too few processes, many occasions when all processes will be blocked and much time will be spent in swapping
Too many processes will lead to thrashing
Fred Kuhns ()

17. Process Suspension Lowest priority process Faulting process
this process does not have its working set in main memory so it will be blocked anyway
Last process activated
this process is least likely to have its working set resident
Process with smallest resident set
this process requires the least future effort to reload
Largest process
obtains the most free frames
Process with the largest remaining execution window
Fred Kuhns ()