Sharing Memory - Lecture Slides - Operating Systems | CMSC 412, Study notes of Operating Systems

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Announcements

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Reading:– Today: Chapter 9.4-9.

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Midterm:– Tuesday

Sharing Memory

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Pages can be shared– several processes may share the same code or data– several pages can be associated with the same page frame– given read-only data, sharing is always safe

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when writes occur, decide if processes share data– operating systems often implement “copy on write” - pages

are shared until a process carries out a write

• when a shared page is written, a new page frame is

allocated

• writing process owns the modified page• all other sharing processes own the original page
  • page could be shared
    • processes use semaphores or other means to coordinate

access

What Happens when a virtual address

has no physical address?

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called a

page fault

• a trap into the operating system from the hardware

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caused by: the first use of a page– called

demand paging

• the operating system allocates a physical page and the

process continues

• read code from disk or init data page to zero

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caused by: a reference to an address that is not valid– program is terminated with a “segmentation violation”

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caused by: a page that is currently on disk– read page from disk and load it into a physical page, and

continue the program

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causde by: a copy on write page

–^

NOACCESS: attempts to read, write or execute will cause an accessviolation

-^

READONLY: attempts to write or execute memory in this region causean access violation

-^

READWRITE: attempts to execute memory in this region cause anaccess violation

-^

EXECUTE: Attempts to read or write memory in this region cause anaccess violation

-^

EXECUTE_READ: Attempts to write to memory in this region cause anaccess violation

-^

EXECUTE_READ_WRITE: Do anything to this page

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WRITE_COPY: Attempts to write will cause the system to give aprocess its own copy of the page. Attempts to execute cause accessviolation

-^

EXECUTE_WRITE_COPY: Attempts to write will cause the system togive a process its own copy of a page. Can’t cause an access violation

OS Protection attributes (Win32)

Page Fault – Page is Paged out

PageDirectory

Page Table

PageFrames

P ReferenceTo this page

1. Fault

2. Read from Disk

3. MakeEntry

4. Continue

Page State (hardware view)

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Page frame number (location in memory or on disk)

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Valid Bit^ – indicates if a page is present in memory or stored on disk

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A

modify

or

dirty

bit

• set by hardware on write to a page– indicates whether the contents of a page have been modified

since the page was last loaded into main memory

• if a page has not been modified, the page does not have to

be written to disk before the page frame can be reused

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Reference bit^ – set by the hardware on read/write– cleared by OS– can be used to approximate LRU page replacement

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Protection attributes– read, write, execute

Page Replacement Algorithms

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FIFO–

Replace the page that was brought in longest ago

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However

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old pages may be great pages (frequently used)

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number of page faults may increase when one increases number ofpage frames (discouraging!)

• called belady’s anomaly– 1,2,3,4,1,2,5,1,2,3,4,5 (consider 3 vs. 4 frames)

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Optimal–

Replace the page that will be used furthest in the future

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Good algorithm(!) but requires knowledge of the future

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With good compiler assistance, knowledge of the future issometimes possible

Page Replacement Algorithms

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FIFO–

Replace the page that was brought in longest ago

-^

However

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old pages may be great pages (frequently used)

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number of page faults may increase when one increases number ofpage frames (discouraging!)

• called belady’s anomaly– 1,2,3,4,1,2,5,1,2,3,4,5 (consider 3 vs. 4 frames)

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Optimal–

Replace the page that will be used furthest in the future

-^

Good algorithm(!) but requires knowledge of the future

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With good compiler assistance, knowledge of the future issometimes possible

FIFO Example (3 frames)

• Reference string: 1,2,3,4,1,2,5,1,2,3,4,

•^

access 1 - (1) fault

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access 2 - (1,2) fault

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access 3- (1,2,3) fault

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access 4 - (2,3,4) fault, replacement

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access 1 - (3,4,1) fault, replacement

-^

access 2 - (4,1,2) fault, replacement

-^

access 5 - (1,2,5) fault, replacement

-^

access 1- (1,2,5)

-^

access 2 - (1,2,5)

-^

access 3 - (2,5,3) fault, replacement

-^

access 4 - (5,3,4) fault, replacement

-^

access 5 - (5,3,4)

• 9 page faults