Chapter 9: Virtual Memory

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Chapter 9: Virtual Chapter 9: Virtual MemoryMemory

Sections Covered in ChapterSections Covered in Chapter

BackgroundBackground

Demand PagingDemand Paging

Copy-on-WriteCopy-on-Write

Page ReplacementPage Replacement

Allocation of Frames Allocation of Frames

ThrashingThrashing

Memory-Mapped FilesMemory-Mapped Files

Allocating Kernel MemoryAllocating Kernel Memory

Other Considerations Other Considerations (Slide 73 only)(Slide 73 only)

Operating-System ExamplesOperating-System Examples

NoteNote: Skipped slides also indicated in slide notes.: Skipped slides also indicated in slide notes.22

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Chapter 9: Virtual MemoryChapter 9: Virtual MemoryBackgroundBackground





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Other ConsiderationsOther Considerations


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ObjectivesObjectives

To describe the benefits of a virtual To describe the benefits of a virtual memory systemmemory system

To explain the concepts of demand To explain the concepts of demand paging, page-replacement algorithms, and paging, page-replacement algorithms, and the allocation of page framesthe allocation of page frames

To discuss the principle of the To discuss the principle of the working-setworking-set modelmodel

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BackgroundBackgroundVirtual memoryVirtual memory – separation of user logical memory – separation of user logical memory from physical memory.from physical memory. Only part of the program needs to be in memory for Only part of the program needs to be in memory for

executionexecution Logical address space can therefore be much larger than Logical address space can therefore be much larger than

physical address spacephysical address space Allows address spaces to be shared by several processesAllows address spaces to be shared by several processes Allows for more efficient process creationAllows for more efficient process creation

Virtual memory can be implemented via:Virtual memory can be implemented via: Demand pagingDemand paging Demand segmentationDemand segmentation

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Virtual Memory That is Larger Than Virtual Memory That is Larger Than Physical MemoryPhysical Memory

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Virtual-address SpaceVirtual-address Space

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Shared Library Using Virtual Shared Library Using Virtual MemoryMemory

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Demand PagingDemand PagingBring a page into memory only when neededBring a page into memory only when needed Less I/O neededLess I/O needed Less memory needed Less memory needed Faster responseFaster response More usersMore users

Page is needed Page is needed reference to it reference to it invalid reference invalid reference abort abort not-in-memory not-in-memory bring to memory bring to memory

Lazy swapperLazy swapper – never swaps a page into memory – never swaps a page into memory unless page will be neededunless page will be needed Swapper that deals with pages is a Swapper that deals with pages is a pagerpager

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Transfer of a Paged Memory to Transfer of a Paged Memory to Contiguous Disk SpaceContiguous Disk Space

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Valid-Invalid BitValid-Invalid BitWith each page table entry a valid–invalid bit exists (With each page table entry a valid–invalid bit exists (vv in-memory, in-memory, ii not-in-memory) not-in-memory)Initially the valid–invalid bit is set toInitially the valid–invalid bit is set to ii on all entrieson all entries

The above is an example of a page table snapshotThe above is an example of a page table snapshot ..

During address translation, if valid–invalid bit in page table entryDuring address translation, if valid–invalid bit in page table entry isis I I page fault page fault

vvvvi

ii

….

Frame #

valid-invalid bit

page table

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Page Table When Some Pages Are Not Page Table When Some Pages Are Not in Main Memoryin Main Memory

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Page FaultPage FaultIf there is a reference to a page, the first reference If there is a reference to a page, the first reference will trap to the operating system:will trap to the operating system:

1.1.Operating system looks at another table to decide:Operating system looks at another table to decide: Invalid reference Invalid reference abort abort Just not in memoryJust not in memory

2.2.Get empty frameGet empty frame3.3.Swap page into frameSwap page into frame4.4.Reset tablesReset tables5.5.Set validation bit = Set validation bit = vv6.6.Restart the instruction that caused the page faultRestart the instruction that caused the page fault

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Page Fault (Cont.)Page Fault (Cont.)Restart instructionRestart instruction block moveblock move

auto increment/decrement locationauto increment/decrement location

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Steps in Handling a Page FaultSteps in Handling a Page Fault

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Performance of Demand PagingPerformance of Demand PagingPage Fault Rate 0 Page Fault Rate 0 pp 1.0 1.0 if if pp = 0 no page faults = 0 no page faults if if pp = 1, every reference is a fault = 1, every reference is a fault

Effective Access Time (EAT)Effective Access Time (EAT)

EAT = (1 – EAT = (1 – pp) x memory access) x memory access

+ + pp (page fault overhead (page fault overhead

+ swap page out+ swap page out

+ swap page in+ swap page in

+ restart overhead)+ restart overhead)

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Demand Paging ExampleDemand Paging ExampleMemory access time = 200 nanosecondsMemory access time = 200 nanoseconds

Average page-fault service time = 8 Average page-fault service time = 8 millisecondsmilliseconds

EAT = (1 – p) x 200 + p (8 milliseconds) EAT = (1 – p) x 200 + p (8 milliseconds)

= (1 – p x 200 + p x 8,000,000 = (1 – p x 200 + p x 8,000,000

= 200 + p x 7,999,800= 200 + p x 7,999,800

If one access out of 1,000 causes a page fault, If one access out of 1,000 causes a page fault, thenthen

EAT = 8.2 microseconds. EAT = 8.2 microseconds.

This is a slowdown by a factor of 40!!This is a slowdown by a factor of 40!!

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Process CreationProcess Creation

Virtual memory allows other benefits Virtual memory allows other benefits during process creation:during process creation:

- Copy-on-Write- Copy-on-Write

- Memory-Mapped Files (later)- Memory-Mapped Files (later)

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Copy-on-WriteCopy-on-WriteCopy-on-Write (COW) allows both parent and child Copy-on-Write (COW) allows both parent and child processes to initially processes to initially shareshare the same pages in the same pages in memorymemory

If either process modifies a shared page, only then If either process modifies a shared page, only then is the page copiedis the page copied

COW allows more efficient process creation as only COW allows more efficient process creation as only modified pages are copiedmodified pages are copied

Free pages are allocated from a Free pages are allocated from a poolpool of zeroed-out of zeroed-out pagespages

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Before Process 1 Modifies Page CBefore Process 1 Modifies Page C

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After Process 1 Modifies Page CAfter Process 1 Modifies Page C

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What happens if there is no free What happens if there is no free frame?frame?

Page replacement – find some page in Page replacement – find some page in memory, but not really in use, swap it outmemory, but not really in use, swap it out algorithmalgorithm performance – want an algorithm which will performance – want an algorithm which will

result in minimum number of page faultsresult in minimum number of page faults

Same page may be brought into memory Same page may be brought into memory several timesseveral times

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Page ReplacementPage ReplacementPrevent over-allocation of memory by modifying Prevent over-allocation of memory by modifying page-fault service routine to include page page-fault service routine to include page replacementreplacement

Use Use modify (dirty) bitmodify (dirty) bit to reduce overhead of to reduce overhead of page transfers – only modified pages are page transfers – only modified pages are written to diskwritten to disk

Page replacement completes separation Page replacement completes separation between logical memory and physical memory between logical memory and physical memory – a large virtual memory can be provided on a – a large virtual memory can be provided on a smaller physical memorysmaller physical memory

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Need For Page ReplacementNeed For Page Replacement

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Basic Page ReplacementBasic Page Replacement

1.1. Find the location of the desired page on diskFind the location of the desired page on disk

2.2. Find a free frame:Find a free frame: - If there is a free frame, use it - If there is a free frame, use it - If there is no free frame, use a page - If there is no free frame, use a page replacement replacement algorithm to select a algorithm to select a victimvictim frameframe

3.3. Bring the desired page into the (newly) free Bring the desired page into the (newly) free frame; update the page and frame tablesframe; update the page and frame tables

4.4. Restart the processRestart the process

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Page Replacement AlgorithmsPage Replacement AlgorithmsWant lowest page-fault rateWant lowest page-fault rate

Evaluate algorithm by running it on a particular Evaluate algorithm by running it on a particular string of memory references (reference string) string of memory references (reference string) and computing the number of page faults on and computing the number of page faults on that stringthat string

In all our examples, the reference string is In all our examples, the reference string is

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 51, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

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Graph of Page Faults Versus The Graph of Page Faults Versus The Number of FramesNumber of Frames

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First-In-First-Out (FIFO) AlgorithmFirst-In-First-Out (FIFO) AlgorithmReference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

3 frames (3 pages can be in memory at a time 3 frames (3 pages can be in memory at a time per process)per process)

4 frames4 framesBelady’s Anomaly: more frames Belady’s Anomaly: more frames more page faults more page faults

1

2

3

1

2

3

4

1

2

5

3

4

9 page faults

1

2

3

1

2

3

5

1

2

4

510 page faults

44 3

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FIFO Page ReplacementFIFO Page Replacement

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FIFO Illustrating Belady’s FIFO Illustrating Belady’s AnomalyAnomaly

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Optimal AlgorithmOptimal Algorithm

Replace page that will not be used for longest period of timeReplace page that will not be used for longest period of time

4 frames example4 frames example

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 51, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

How do you know this?How do you know this?

Used for measuring how well your algorithm performsUsed for measuring how well your algorithm performs

1

2

3

46 page faults

4 5

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Optimal Page ReplacementOptimal Page Replacement

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Least Recently Used (LRU) Least Recently Used (LRU) AlgorithmAlgorithm

Reference string: 1, 2, 3, 4, 1, 2, Reference string: 1, 2, 3, 4, 1, 2, 55, 1, 2, , 1, 2, 33, , 44, , 55

Counter implementationCounter implementation Every page entry has a counter; every time page is referenced Every page entry has a counter; every time page is referenced

through this entry, copy the clock into the counterthrough this entry, copy the clock into the counter When a page needs to be changed, look at the counters to When a page needs to be changed, look at the counters to

determine which are to changedetermine which are to change

5

2

4

3

1

2

3

4

1

2

5

4

1

2

5

3

1

2

4

3

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LRU Page ReplacementLRU Page Replacement

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LRU Algorithm (Cont.)LRU Algorithm (Cont.)

Stack implementation – keep a stack of Stack implementation – keep a stack of page numbers in a double link form:page numbers in a double link form: Page referenced:Page referenced:

move it to the topmove it to the top

requires 6 pointers to be changedrequires 6 pointers to be changed No search for replacementNo search for replacement

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Use Of A Stack to Record The Most Recent Use Of A Stack to Record The Most Recent Page ReferencesPage References

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LRU Approximation AlgorithmsLRU Approximation Algorithms

Reference bitReference bit With each page associate a bit, initially = 0With each page associate a bit, initially = 0 When page is referenced bit set to 1When page is referenced bit set to 1 Replace the one which is 0 (if one exists)Replace the one which is 0 (if one exists)

We do not know the order, howeverWe do not know the order, however

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LRU Approximation AlgorithmsLRU Approximation Algorithms

Second chanceSecond chance Need reference bitNeed reference bit Clock replacementClock replacement If page to be replaced (in clock order) has If page to be replaced (in clock order) has

reference bit = 1 then:reference bit = 1 then:

set reference bit 0set reference bit 0

leave page in memoryleave page in memory

replace next page (in clock order), subject to replace next page (in clock order), subject to same rulessame rules

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Second-Chance (clock) Page-Second-Chance (clock) Page-Replacement AlgorithmReplacement Algorithm

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Counting AlgorithmsCounting Algorithms

Keep a counter of the number of references Keep a counter of the number of references that have been made to each pagethat have been made to each page

LFU AlgorithmLFU Algorithm: replaces page with smallest : replaces page with smallest countcount

MFU AlgorithmMFU Algorithm: based on the argument that : based on the argument that the page with the smallest count was probably the page with the smallest count was probably just brought in and has yet to be usedjust brought in and has yet to be used

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Allocation of FramesAllocation of Frames

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Allocation of FramesAllocation of FramesEach process needs a Each process needs a minimumminimum number of pages number of pages

Example: IBM 370 – 6 pages to handle SS MOVE Example: IBM 370 – 6 pages to handle SS MOVE instruction:instruction: instruction is 6 bytes, might span 2 pagesinstruction is 6 bytes, might span 2 pages 2 pages to handle 2 pages to handle fromfrom 2 pages to handle 2 pages to handle toto

Two major allocation schemesTwo major allocation schemes fixed allocationfixed allocation priority allocationpriority allocation

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Fixed AllocationFixed Allocation

Equal allocation – For example, if Equal allocation – For example, if there are 100 frames and 5 processes, there are 100 frames and 5 processes, give each process 20 frames.give each process 20 frames.

Proportional allocation – Allocate Proportional allocation – Allocate according to the size of processaccording to the size of process

mSs

pa

m

sS

ps

iii

i

ii

for allocation

frames of number total

process of size

5964137127

56413710

127

10

64

2

1

2

a

a

s

s

m

i

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Priority AllocationPriority Allocation

Use a proportional allocation scheme Use a proportional allocation scheme using priorities rather than sizeusing priorities rather than size

If process If process PPii generates a page fault, generates a page fault, select for replacement one of its framesselect for replacement one of its frames select for replacement a frame from a select for replacement a frame from a

process with lower priority numberprocess with lower priority number

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Global vs. Local AllocationGlobal vs. Local Allocation

Global replacementGlobal replacement – process selects a – process selects a replacement frame from the set of all frames; replacement frame from the set of all frames; one process can take a frame from anotherone process can take a frame from another

Local replacementLocal replacement – each process selects – each process selects from only its own set of allocated framesfrom only its own set of allocated frames

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ThrashingThrashingIf a process does not have “enough” pages, If a process does not have “enough” pages, the page-fault rate is very high. This leads to:the page-fault rate is very high. This leads to: low CPU utilizationlow CPU utilization operating system thinks that it needs to operating system thinks that it needs to

increase the degree of multiprogrammingincrease the degree of multiprogramming another process is added to the systemanother process is added to the system

ThrashingThrashing a process is kept busy swapping a process is kept busy swapping pages in and outpages in and out

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Thrashing (Cont.)Thrashing (Cont.)

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Demand Paging and Demand Paging and Thrashing Thrashing

Why does demand paging work?Why does demand paging work?Locality modelLocality model Process migrates from one locality to anotherProcess migrates from one locality to another Localities may overlapLocalities may overlap

Why does thrashing occur?Why does thrashing occur? size of locality > total memory size size of locality > total memory size

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Locality In A Memory-Reference Locality In A Memory-Reference PatternPattern

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Working-Set ModelWorking-Set Model working-set window working-set window a fixed number of page a fixed number of page references references Example: 10,000 instructionExample: 10,000 instruction

WSSWSSii (working set of Process (working set of Process PPii) = total number of ) = total number of

pages referenced in the most recent pages referenced in the most recent (varies in time) (varies in time) if if too small will not encompass entire locality too small will not encompass entire locality if if too large will encompass several localities too large will encompass several localities if if = = will encompass entire program will encompass entire program

DD = = WSSWSSii total demand frames total demand frames

if if DD > > mm Thrashing - (m is nr of available frames) Thrashing - (m is nr of available frames)

Policy if Policy if DD > m, then suspend one of the processes > m, then suspend one of the processes

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Working-set modelWorking-set model

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Keeping Track of the Working SetKeeping Track of the Working SetApproximate with interval timer + a reference bitApproximate with interval timer + a reference bit

Example: Example: = 10,000 = 10,000 Timer interrupts after every 5000 time unitsTimer interrupts after every 5000 time units Keep in memory 2 bits for each pageKeep in memory 2 bits for each page Whenever a timer interrupts copy and sets the values of Whenever a timer interrupts copy and sets the values of

all reference bits to 0all reference bits to 0 If one of the bits in memory = 1 If one of the bits in memory = 1 page in working set page in working set

Why is this not completely accurate?Why is this not completely accurate?

Improvement = 10 bits and interrupt every 1000 Improvement = 10 bits and interrupt every 1000 time unitstime units

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Page-Fault Frequency SchemePage-Fault Frequency SchemeEstablish “acceptable” page-fault rateEstablish “acceptable” page-fault rate If actual rate too low, process loses frameIf actual rate too low, process loses frame If actual rate too high, process gains frameIf actual rate too high, process gains frame

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Working Sets and Page Fault Working Sets and Page Fault RatesRates

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Memory-mapped file I/O allows file I/O to be Memory-mapped file I/O allows file I/O to be treated as routine memory access by treated as routine memory access by mappingmapping a disk block to a page in memorya disk block to a page in memory

A file is initially read using demand paging. A A file is initially read using demand paging. A page-sized portion of the file is read from the page-sized portion of the file is read from the file system into a physical page. Subsequent file system into a physical page. Subsequent reads/writes to/from the file are treated as reads/writes to/from the file are treated as ordinary memory accesses.ordinary memory accesses.

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Simplifies file access by treating file I/O Simplifies file access by treating file I/O through memory rather than through memory rather than read()read() write()write() system calls system calls

Also allows several processes to map the Also allows several processes to map the same file allowing the pages in memory to same file allowing the pages in memory to be sharedbe shared

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Memory Mapped FilesMemory Mapped Files

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Memory-Mapped Shared Memory Memory-Mapped Shared Memory in Windowsin Windows

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Treated differently from user memoryTreated differently from user memory

Often allocated from a free-memory poolOften allocated from a free-memory pool Kernel requests memory for structures of Kernel requests memory for structures of

varying sizesvarying sizes Some kernel memory needs to be contiguousSome kernel memory needs to be contiguous

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Buddy SystemBuddy SystemAllocates memory from fixed-size segment Allocates memory from fixed-size segment consisting of physically-contiguous pagesconsisting of physically-contiguous pages

Memory allocated using Memory allocated using power-of-2 allocatorpower-of-2 allocator Satisfies requests in units sized as power of 2Satisfies requests in units sized as power of 2 Request rounded to next highest power of 2Request rounded to next highest power of 2 When smaller allocation needed than is When smaller allocation needed than is

available, current chunk split into two buddies of available, current chunk split into two buddies of next-lower power of 2next-lower power of 2

Continue until appropriate sized chunk Continue until appropriate sized chunk availableavailable

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Buddy System AllocatorBuddy System Allocator

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Slab AllocatorSlab Allocator

Alternate strategyAlternate strategy

SlabSlab is one or more physically contiguous pagesis one or more physically contiguous pages

CacheCache consists of one or more slabsconsists of one or more slabs

Single cache for each unique kernel data structureSingle cache for each unique kernel data structure Each cache filled with Each cache filled with objectsobjects – instantiations of – instantiations of

the data structurethe data structure

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Slab AllocatorSlab Allocator

When cache is created, it is filled with objects When cache is created, it is filled with objects marked as marked as freefree

When structures are stored, objects marked as When structures are stored, objects marked as usedused

If slab is full of used objects, the next object is If slab is full of used objects, the next object is allocated from an empty slaballocated from an empty slab If there are no empty slabs, a new slab If there are no empty slabs, a new slab

allocatedallocated

Benefits include no fragmentation, fast memory Benefits include no fragmentation, fast memory request satisfactionrequest satisfaction

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Slab AllocationSlab Allocation

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Other Issues -- PrepagingOther Issues -- PrepagingPrepaging Prepaging To reduce the large number of page faults that occurs To reduce the large number of page faults that occurs

at process startupat process startup Prepage all or some of the pages a process will need, Prepage all or some of the pages a process will need,

before they are referencedbefore they are referenced But if prepaged pages are unused, I/O and memory But if prepaged pages are unused, I/O and memory

was wastedwas wasted Assume Assume ss pages are prepaged and pages are prepaged and αα of the pages is of the pages is

usedused

Is cost of Is cost of s * s * αα save pages faults > or < than the cost save pages faults > or < than the cost of prepagingof prepaging s * (1- s * (1- αα) ) unnecessary pagesunnecessary pages? ?

αα near zero near zero prepaging loses prepaging loses

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Other Issues – Page SizeOther Issues – Page Size

Page size selection must take into Page size selection must take into consideration:consideration: fragmentationfragmentation table size table size I/O overheadI/O overhead localitylocality

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Other Issues – TLB Reach Other Issues – TLB Reach

TLB ReachTLB Reach - The amount of memory accessible from - The amount of memory accessible from the TLBthe TLB

TLB Reach = (TLB Size) X (Page Size)TLB Reach = (TLB Size) X (Page Size)

Ideally, the working set of each process is stored in Ideally, the working set of each process is stored in the TLBthe TLB Otherwise there is a high degree of page faultsOtherwise there is a high degree of page faults

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Increase the Page SizeIncrease the Page Size This may lead to an increase in fragmentation This may lead to an increase in fragmentation

as not all applications require a large page as not all applications require a large page sizesize

Provide Multiple Page SizesProvide Multiple Page Sizes This allows applications that require larger This allows applications that require larger

page sizes the opportunity to use them page sizes the opportunity to use them without an increase in fragmentationwithout an increase in fragmentation

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Other Issues – Program Other Issues – Program StructureStructure

Program structureProgram structure Int[128,128] data;Int[128,128] data; Each row is stored in one page Each row is stored in one page Program 1 Program 1

for (j = 0; j <128; j++)for (j = 0; j <128; j++) for (i = 0; i < 128; i++) for (i = 0; i < 128; i++) data[i,j] = 0; data[i,j] = 0; 128 x 128 = 16,384 page faults 128 x 128 = 16,384 page faults

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Other Issues – Program Other Issues – Program StructureStructure

Program 2 Program 2

for (i = 0; i < 128; i++)for (i = 0; i < 128; i++) for (j = 0; j < 128; j++) for (j = 0; j < 128; j++) data[i,j] = 0; data[i,j] = 0;

128 page faults in contrast to 128 x 128 128 page faults in contrast to 128 x 128 = 16,384 page faults != 16,384 page faults !

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Other Issues – I/O interlockOther Issues – I/O interlock

I/O InterlockI/O Interlock – Pages must sometimes be – Pages must sometimes be locked into memorylocked into memory

Consider I/O - Pages that are used for copying Consider I/O - Pages that are used for copying a file from a device must be locked from being a file from a device must be locked from being selected for eviction by a page replacement selected for eviction by a page replacement algorithmalgorithm

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Reason Why Frames Used For I/O Reason Why Frames Used For I/O Must Be In MemoryMust Be In Memory

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Operating System ExamplesOperating System ExamplesWindows XPWindows XP

Solaris Solaris

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Windows XPWindows XP

Uses demand paging with Uses demand paging with clusteringclustering. . Clustering brings in pages surrounding the Clustering brings in pages surrounding the faulting pagefaulting page

Processes are assigned a Processes are assigned a working setworking set minimumminimum and and working set maximumworking set maximum

Working set minimum is the minimum Working set minimum is the minimum number of pages the process is guaranteed number of pages the process is guaranteed to have in memoryto have in memory

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A process may be assigned as many A process may be assigned as many pages up to its working set maximumpages up to its working set maximum

When the amount of free memory in the When the amount of free memory in the system falls below a threshold, system falls below a threshold, automaticautomatic working set trimmingworking set trimming is performed to is performed to restore the amount of free memoryrestore the amount of free memory

Working set trimming removes pages from Working set trimming removes pages from processes that have pages in excess of processes that have pages in excess of their working set minimumtheir working set minimum

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Solaris Solaris

Maintains a list of free pages to assign Maintains a list of free pages to assign faulting processesfaulting processes

LotsfreeLotsfree – threshold parameter (amount of – threshold parameter (amount of free memory) to begin pagingfree memory) to begin paging

DesfreeDesfree – threshold parameter to increasing – threshold parameter to increasing pagingpaging

MinfreeMinfree – threshold parameter to being – threshold parameter to being swappingswapping

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Paging is performed by a Paging is performed by a pageoutpageout process process

PageoutPageout scans pages using modified clock scans pages using modified clock algorithmalgorithm

ScanrateScanrate is the rate at which pages are is the rate at which pages are scanned. This ranges from scanned. This ranges from slowscanslowscan to to fastscanfastscan

Pageout is called more frequently Pageout is called more frequently depending upon the amount of free depending upon the amount of free memory availablememory available

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Solaris 2 Page ScannerSolaris 2 Page Scanner

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End of Chapter 9End of Chapter 9

Chapter 9: Virtual Memory

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