Download Virtual Memory: Algorithms, Page Faults, and Performance and more Slides Computer Science in PDF only on Docsity!
Virtual Memory Virtual Memory
COMP755 Advanced OperatingCOMP755 Advanced OperatingSystemsSystems
Virtual Memory Performance Virtual Memory Performance
Size of the PageFetch Policy Size of the Page Fetch Policy Placement PolicyCleaning policy Placement Policy Cleaning policy Resident Set ManagementReplacement Policy Resident Set Management Replacement Policy Multiprogramming Level Multiprogramming Level
Resident Set Management Resident Set Management
Should all programs have an equalShould all programs have an equalnumber of pages? If there are N programsnumber of pages? If there are N programs running, should each program have 1/N ofrunning, should each program have 1/N ofthe total RAM?the total RAM? When a program gets a page fault andWhen a program gets a page fault andneeds another page, should the new pageneeds another page, should the new page replace only a page in that userreplace only a page in that user’or can it replace any page from anyor can it replace any page from any’s programs program program?program?
Resident Set ManagementResident Set Management
Fixed or variable number of pages perprocess. Fixed or variable number of pages perprocess. Pages replaced globally or within aprocess. Pages replaced globally or within aprocess. Working Set Algorithm looks at thenumber of pages recently accessed to Working Set Algorithm looks at thenumber of pages recently accessed to determine how many pages a programshould have.determine how many pages a programshould have.
If free pages are available, it is usuallyIf free pages are available, it is usuallyAllocating Pages^ Allocating Pages
advantageous to give more pages to aadvantageous to give more pages to aprogram.program. When few or no free pages are available,When few or no free pages are available,giving pages to one program means takinggiving pages to one program means taking pages from another.pages from another. Taking pages from another program willTaking pages from another program will likely increase that programlikely increase that program’rate.rate. ’s page faults page fault The goal is to reduce the total number ofThe goal is to reduce the total number ofpage faults in the system.page faults in the system.
Docsity.com
Page Fault Frequency Algorithm Page Fault Frequency Algorithm
If a program is generating more pageIf a program is generating more pagefaults than some limit, give the programfaults than some limit, give the program more pages in RAM.more pages in RAM. If the OS needs to take pages from aIf the OS needs to take pages from a program, remove them from a programprogram, remove them from a programthat is generating less than somethat is generating less than some minimum number of page faults.minimum number of page faults.
Working Set Strategy Working Set Strategy
For each program, the OS keeps residentFor each program, the OS keeps residentall pages that have been accessed in theall pages that have been accessed in the lastlast Increasing the parameterIncreasing the parameter^ ∆∆^ time units.time units. ∆∆ (^) increases theincreases the number of pages.number of pages. Usually implemented so that at a pageUsually implemented so that at a page fault the system keeps those pages thatfault the system keeps those pages thathave been accesses inhave been accesses in (^) ∆∆ time units.time units.
A system is said to be thrashing when it is A system is said to be thrashing when it isThrashing^ Thrashing
spending so much of its resourcesmaintaining the virtual memory that littlespending so much of its resourcesmaintaining the virtual memory that little actual work gets accomplished.As the number of running programsactual work gets accomplished. As the number of running programs increases, each program gets fewer pagesin RAM.increases, each program gets fewer pagesin RAM. With fewer pages in RAM, each programcreates more page faults. With fewer pages in RAM, each programcreates more page faults.
Replacement PolicyReplacement Policy
What page should be overwritten?What page should be overwritten? OptimalOptimal - requires looking forward in time.-requires looking forward in time. Useful for evaluating other algorithms.Useful for evaluating other algorithms. LRULRU (^) - - works well.works well. FIFOFIFO ClockClock^ - denies the concept of locality- - - used by Mac OS.denies the concept of localityused by Mac OS. Page BufferingPage Buffering available and released pages.available and released pages. - - System keeps a list ofSystem keeps a list of
Multiprogramming LevelMultiprogramming Level
How many programs can runsimultaneously? How many programs can runsimultaneously? Swap out idle programs Swap out idle programs
Docsity.com
The Program Accesses PageThe Program Accesses Page
The Program Accesses Page The Program Accesses Page
The Program Accesses PageThe Program Accesses Page
5 4 7 5 6 8 A5 4 7 5 6 8 A
The Program Accesses Page The Program Accesses Page
5 4 7 5 6 8 A 55 4 7 5 6 8 A 5
The Program Accesses PageThe Program Accesses Page
5 4 7 5 6 8 A 5 75 4 7 5 6 8 A 5 7
The Program Accesses Page The Program Accesses Page
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
Docsity.com
The Program Accesses PageThe Program Accesses Page
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
FF
The Program Accesses Page The Program Accesses Page
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
F AF A
The Program Accesses PageThe Program Accesses Page
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
F A 7F A 7
The Program Accesses Page The Program Accesses Page
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
F A 7 AF A 7 A
The Program Accesses PageThe Program Accesses Page
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
F A 7 A 6F A 7 A 6
The Program Accesses Page The Program Accesses Page
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
F A 7 A 6 5F A 7 A 6 5
Docsity.com
LRU Algorithm LRU Algorithm
5 4 7 5 6 8 A 5 7 6 5 4 7 5 6 8 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55
LRU Algorithm LRU Algorithm
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 44
LRU Algorithm LRU Algorithm
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 44 77
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 6 8 A 5 7 66 8 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 44 77
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 8 A 5 7 68 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 44 77 66
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 88 A 5 7 6A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
(^55887766)
Docsity.com
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 88 AA 5 7 65 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 88 AA 66
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 88 AA 55 7 67 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 88 AA 66
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 88 AA^77
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
(^5566) AA (^77)
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 66 FF^77
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F AF A 7 A 6 5 4 A F 57 A 6 5 4 A F 5
AA 66 FF (^77)
Docsity.com
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F AF A 77 AA 66 55 44 AA FF 5 5
AA FF 55 44
LRU Algorithm LRU Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F AF A 77 AA 66 55 44 AA FF 5 5
AA FF 55 44 13 total page faults
Optimal AlgorithmOptimal Algorithm
The optimal algorithm looks forward inThe optimal algorithm looks forward intime and replaces the page that will not betime and replaces the page that will not be referenced again or will be referenced thereferenced again or will be referenced thefarthest in the future.farthest in the future. Cannot be implemented.Cannot be implemented. Provides a theoretical minimum number ofProvides a theoretical minimum number of page faultspage faults
Optimal Algorithm Optimal Algorithm
5 4 7 5 6 8 A 5 7 6 5 4 7 5 6 8 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
(^55)
Optimal AlgorithmOptimal Algorithm
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 44
Optimal Algorithm Optimal Algorithm
5 4 7 5 6 8 A 5 7 65 4 7 5 6 8 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 44 77
Docsity.com
Optimal AlgorithmOptimal Algorithm
5 4 7 55 4 7 5 6 8 A 5 7 66 8 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 44 77
Optimal Algorithm Optimal Algorithm
5 4 7 55 4 7 5 66 8 A 5 7 68 A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 44 77 66
Optimal AlgorithmOptimal Algorithm
5 4 7 55 4 7 5 66 88 A 5 7 6A 5 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 88 77 66
Optimal Algorithm Optimal Algorithm
5 4 7 55 4 7 5 66 88 AA 5 7 65 7 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 AA 77 66
Optimal AlgorithmOptimal Algorithm
5 4 7 55 4 7 5 66 88 AA 55 7 67 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 AA^7766
Optimal Algorithm Optimal Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F A 7 A 6 5 4 A F 5F A 7 A 6 5 4 A F 5
55 AA 77 66
Docsity.com
Optimal AlgorithmOptimal Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F AF A 77 AA 66 55 4 A F 54 A F 5
FF AA 55 66
Optimal Algorithm Optimal Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F AF A 77 AA 66 55 44 A F 5A F 5
FF AA 55 44
Optimal AlgorithmOptimal Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F AF A 77 AA 66 55 44 AA F 5F 5
FF AA (^5544)
Optimal Algorithm Optimal Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F AF A 77 AA 66 55 44 AA FF 5 5
FF AA 55 44
Optimal AlgorithmOptimal Algorithm
5 4 7 55 4 7 5 66 88 AA 55 77 6 6
F AF A 77 AA 66 55 44 AA FF 5 5
FF AA (^5544) 9 Total Page Faults
Clock AlgorithmClock Algorithm
Considers if a page has been referencedand if it has been modified. Considers if a page has been referencedand if it has been modified. Unmodified pages are replaced beforemodified pages to avoid having to write the Unmodified pages are replaced beforemodified pages to avoid having to write the old page back to disk.Pages are kept in a circular queue with aold page back to disk. Pages are kept in a circular queue with a pointer to the page after last replacedpage.pointer to the page after last replacedpage.
Docsity.com
Clock AlgorithmClock Algorithm
Scan 1Scan 1 –pages are scanned for one that has notpages are scanned for one that has not– Beginning with the pointer, theBeginning with the pointer, the been referenced or modified. The first pagebeen referenced or modified. The first pagefound is replaced.found is replaced. Scan 2Scan 2 –a page that has not been referenced buta page that has not been referenced but– Starting from the pointer, look forStarting from the pointer, look for has been modified. As the pages arehas been modified. As the pages arechecked, clear the referenced bit.checked, clear the referenced bit.
Clock AlgorithmClock Algorithm
Scan 3 -pages are scanned for one that has not Scan 3pages are scanned for one that has not- Starting from the pointer, theStarting from the pointer, the been referenced or modified.Scan 4 -been referenced or modified. Scan 4- Starting from the pointer, look forStarting from the pointer, look for a page that has not been referenced buthas been modified.a page that has not been referenced buthas been modified.
Page Buffering Page Buffering
The OS maintains a list of free pages.The OS maintains a list of free pages. Pages are taken from the free list wheneverPages are taken from the free list whenever a new page is needed.a new page is needed. When a page is replaced, it is put on the endWhen a page is replaced, it is put on the end of the free list if it has not been modified.of the free list if it has not been modified. If the page was modified, it is put on aIf the page was modified, it is put on a modified list.modified list.
Page BufferingPage Buffering
Pages on the modified list are written todisk when the system is idle. Cleaned Pages on the modified list are written todisk when the system is idle. Cleaned pages are put on the free list.When a page fault occurs, the systempages are put on the free list. When a page fault occurs, the system checks these lists to see if the page is stillin RAM.checks these lists to see if the page is stillin RAM.
Docsity.com
