1 virtual memory in the real world implementing exact lru approximating lru hardware support clock...

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Virtual Memory in the Real World

Implementing exact LRU Approximating LRU

Hardware Support Clock Algorithm

Thrashing Cause Working Set

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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

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

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

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B2

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

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B2

C3

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

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B4

C3

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

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B4

C5

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

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B4

C6

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

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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?

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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

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Reference-bit algorithm Clock Algorithm


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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

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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

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Reference-bit algorithm: example



0B0

C0

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

reset reference bits

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0B1

C0

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

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0B1

C1

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

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0B1

C1

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

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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

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Reference-bit algorithm Clock Algorithm


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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

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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

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The Clock Algorithm

B1

C1

A1

D1

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





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The Clock Algorithm

B1

C1

A1

D1

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





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The Clock Algorithm

B1

C1

A0

D1

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





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The Clock Algorithm

B0

C1

A0

D1

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





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The Clock Algorithm

B0

C0

A0

D1

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





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The Clock Algorithm

B0

C0

A0

D0

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





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The Clock Algorithm

B0

C0

A0

D0

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

E1





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The Clock Algorithm

B0

C0

A0

D0

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

E0





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The Clock Algorithm

B0

C0

A0

D0

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

E0

F1





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The Clock Algorithm

B0

C0

A0

D0

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

E0

F1

C1





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The Clock Algorithm

B0

C0

A0

D0

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

E0

F0

C1





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The Clock Algorithm

B0

C0

A0

D0

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

E0

F0

C1C0





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The Clock Algorithm

B0

C0

A0

D0

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

E0

F0

C1C0

G1





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The Clock Algorithm: Summary

Variant of FIFO & LRU LRU: ? FIFO: ?

No need to reset referenece bit periodically

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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

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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

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Reference-bit algorithm Clock


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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

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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

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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

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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

1 virtual memory in the real world implementing exact lru approximating lru hardware support clock...

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