Virtual Memory Three - Operating Systems - Lecture Slides, Slides of Computer Science

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Operating Systems

Lecture 24:

Virtual Memory (cont)

1

Ryan?

2

Clock Policy

• Uses and additional bit called a “use bit”

• When a page is first loaded in memory or

referenced, the use bit is set to 1

• When it is time to replace a page, the OS

scans the set flipping all 1’s to 0

• The first frame encountered with the use bit

already set to 0 is replaced.

5

Clock Policy

Algorithmically identical with another algorithm called “2nd^ chance”.

Only the implementation is different.

7

The 2

nd

Chance Algorithm

• Holds a FIFO list of pages, each page has a bit R=1 if referenced.
• Evict the oldest page not referenced in last tick.
• Repeat while oldest has R=1:
  • Move to end of list (as if just paged in)
  • Set R=
• At worst, all pages have R=1, so degenerates to regular FIFO.
• Page list if fault occurs at time 20, A has R bit set (numbers above pages are loading times)

8

The Clock Page Replacement Alg

• The clock algorithm on 2 bits: referenced R and modified M
• Hardware sets corresponding R on every memory reference
• At process start, M and R set to 0
• Each circular pass, R bits set to 0.
• Pages are classified
  1. not referenced, not modified
  2. not referenced, modified
  3. referenced, not modified
  4. referenced, modified
• NRU removes page from lowest- numbered non-empty class

Combined Examples

Comparison

Results based on the execution of 0.25 x 10^6 references in a FORTRAN program, using

a page size of 256 words. It is assumed that the number of page frames assigned to a

process is fixed.

13

Thrashing

• If a process does not have “enough” pages, the page-

fault rate is very high. This leads to:

• low CPU utilization
• operating system thinks that it needs to increase the

degree of multiprogramming

• another process added to the system

• Thrashing ≡ a process is busy swapping pages in and

out

14

Thrashing (Cont.)

16

Allocation of Frames

• Each process needs minimum number of pages

• Example: 6 pages needed by one instruction

• instruction might span 2 pages
• 1 st^ argument spans 2 pages
• 2 nd^ argument spans 2 pages

• Major allocation schemes

• Fixed vs. Priority allocation
• Global vs. Local allocation

17

Fixed Allocation

• Equal allocation: e.g, 100 frames and 5

processes, give each process 20 frames.

• Proportional allocation – Allocate according to

the size of process

m S

s a p

m

S s

s p

i i i

i

i i

= = ×

allocation for

totalnumber of frames

sizeofprocess

2

1

2

= × ≈

= × ≈

a

a

s

s

m

i

19

Global vs. Local Allocation

• Global replacement – process selects a

replacement frame from the set of all

frames; one process can take a frame from

another

• Local replacement – each process selects

from only its own set of allocated frames

20

• A, B, C processes in memory
• Process A page faults requesting A6.
• LRU used

Global vs. Local Allocation

(cont)

Time last reference