1

Virtual Memory in the Real World

Implementing exact LRU Approximating LRU

Hardware Support Clock Algorithm

Thrashing Cause Working Set

2

Implementing Exact LRU

On each reference, time stamp page

When we need to evict: select oldest page= least-recently usedA, B, C, B, C, C, D

3

Implementing Exact LRU

On each reference, time stamp page

When we need to evict: select oldest page= least-recently usedA1

A, B, C, B, C, C, D

4

Implementing Exact LRU

On each reference, time stamp page

When we need to evict: select oldest page= least-recently usedA1

B2

A, B, C, B, C, C, D

5

Implementing Exact LRU

On each reference, time stamp page

When we need to evict: select oldest page= least-recently usedA1

B2

C3

A, B, C, B, C, C, D

6

Implementing Exact LRU

On each reference, time stamp page

When we need to evict: select oldest page= least-recently usedA1

B4

C3

A, B, C, B, C, C, D

7

Implementing Exact LRU

On each reference, time stamp page

When we need to evict: select oldest page= least-recently usedA1

B4

C5

A, B, C, B, C, C, D

8

Implementing Exact LRU

On each reference, time stamp page

When we need to evict: select oldest page= least-recently usedA1

B4

C6

A, B, C, B, C, C, D

9

Implementing Exact LRU

On each reference, time stamp page When we need to evict: select oldest page

= least-recently used

A1

B4

C6

A, B, C, B, C, C, DD7

LRU page

How should we implement this?

10

Implementing Exact LRU:Data Structures Hash table: optimize the common case

(memory hit) Location of a page in memory: apply hash

function to a page number update: O(1), eviction: O(n) (n: # of pages to

process) Expensive: on every reference, compute

hash of page address; update time stamp doubly-linked list

Move items to front when referenced LRU items at end of list Still too expensive:

Linear lookup time to find a page 4-6 pointer updates per reference

11

Virtual Memory in the Real World

Implementing exact LRU Approximating LRU

Reference-bit algorithm Clock Algorithm

Thrashing Cause Working Set

12

Reference-bit algorithm

Hardware support: reference bit Maintain reference bit for every

page On each access, set reference bit to

1 Periodically resets reference bits Evict page with reference bit = 0

13

Reference-bit algorithm : example

Maintain reference bit for every page On each access, set reference bit to

1 Periodically resets reference bits Evict page with reference bit = 0A

1B1

C1

A, B, C, B, C, C, D

14

Reference-bit algorithm: example

Maintain reference bit for every page On each access, set reference bit to

1 Periodically resets reference bits Evict page with reference bit = 0A

0B0

C0

A, B, C, B, C, C, D

reset reference bits

15

Reference-bit algorithm: example

Maintain reference bit for every page On each access, set reference bit to

1 Periodically resets reference bits Evict page with reference bit = 0A

0B1

C0

A, B, C, B, C, C, D

16

Reference-bit algorithm: example

Maintain reference bit for every page On each access, set reference bit to

1 Periodically resets reference bits Evict page with reference bit = 0A

0B1

C1

A, B, C, B, C, C, D

17

Reference-bit algorithm: example

Maintain reference bit for every page On each access, set reference bit to

1 Periodically resets reference bits Evict page with reference bit = 0A

0B1

C1

A, B, C, B, C, C, D

18

Reference-bit algorithm: example

Maintain reference bit for every page On each access, set reference bit to 1 Periodically resets reference bits Evict page with reference bit = 0

A0

B1

C1

A, B, C, B, C, C, DD1

19

Virtual Memory in the Real World

Implementing exact LRU Approximating LRU

Reference-bit algorithm Clock Algorithm

Thrashing Cause Working Set

20

The Clock Algorithm Set reference bit to 1 for an access Consider frames in circle Pointer:

0: initially Only advance pointer when page fault

happens On page fault, OS:

Repeat Checks reference bit of the current pointer If reference bit = 0, replace page, set bit to

1; advance pointer to next frame; break; If reference bit = 1, set bit to 0, advance

pointer to next frame

21

The Clock Algorithm

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

B1

C1

A1

D1

A, B, C, D, B, C, E, F, C, G

22

The Clock Algorithm

B1

C1

A1

D1

A, B, C, D, B, C, E, F, C, G

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

23

The Clock Algorithm

B1

C1

A1

D1

A, B, C, D, B, C, E, F, C, G

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

24

The Clock Algorithm

B1

C1

A0

D1

A, B, C, D, B, C, E, F, C, G

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

25

The Clock Algorithm

B0

C1

A0

D1

A, B, C, D, B, C, E, F, C, G

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

26

The Clock Algorithm

B0

C0

A0

D1

A, B, C, D, B, C, E, F, C, G

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

27

The Clock Algorithm

B0

C0

A0

D0

A, B, C, D, B, C, E, F, C, G

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

28

The Clock Algorithm

B0

C0

A0

D0

A, B, C, D, B, C, E, F, C, G

E1

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

29

The Clock Algorithm

B0

C0

A0

D0

A, B, C, D, B, C, E, F, C, G

E0

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

30

The Clock Algorithm

B0

C0

A0

D0

A, B, C, D, B, C, E, F, C, G

E0

F1

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

31

The Clock Algorithm

B0

C0

A0

D0

A, B, C, D, B, C, E, F, C, G

E0

F1

C1

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

32

The Clock Algorithm

B0

C0

A0

D0

A, B, C, D, B, C, E, F, C, G

E0

F0

C1

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

33

The Clock Algorithm

B0

C0

A0

D0

A, B, C, D, B, C, E, F, C, G

E0

F0

C1C0

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

34

The Clock Algorithm

B0

C0

A0

D0

A, B, C, D, B, C, E, F, C, G

E0

F0

C1C0

G1

Consider frames in circle

On page fault, repeat: Checks reference bit If reference bit = 0,

replace page, set bit to 1; advance pointer to next frame; break;

If reference bit = 1, set bit to 0, advance pointer to next frame

35

The Clock Algorithm: Summary

Variant of FIFO & LRU LRU: ? FIFO: ?

No need to reset referenece bit periodically

36

Enhancing Clock

Recall: we don’t write back unmodified pages Idea: favor eviction of unmodified pages Extend hardware to keep another bit:

modified bit Total order of tuples: (ref bit, mod

bit) (0,0), (0,1), (1,0), (1,1) Evict page from lowest nonempty class

37

Page Replacementin Enhanced Clock

OS scans at most three times Page (0,0) – replace that page Page (0,1) – write out page Page (1,0), (1,1)

Fast, but still coarse approximation of LRU

38

Virtual Memory in the Real World

Implementing exact LRU Approximating LRU

Reference-bit algorithm Clock

Thrashing Cause Working Set

39

Thrashing

A process is busy swapping pages in and out; no useful work is done low CPU utilization

OS adds processes →

even more page swapping & lower CPU utilization

40

Cause of Thrashing

Locality A set of pages that are actively used

together Process migrates from one locality to

another Total locality of processes > total

memory size Process cannot keep in memory all

pages that it is currently using

41

Working Set Strategy

Working set = pages referred in last references (approximate of locality)

OS monitors working set of each process; allocate enough frames to process Another process can be started if

there are enough extra frames Suspend process(es) if the sum of

working-set sizes exceeds RAM

42

Working Set Problems

Algorithm relies on key parameter, How do we set ? Is there one correct ?

Different processes have different timescales over which they refer pages

Not acceptable (or necessarily possible) to suspend processes altogether

Not really used Very rough variant used in Windows