Anonymous memory pages modified in memory but not yet

anonymous
memory

Pages modified in memory but not yet written back to the file system

Mobile systems

Typically don’t support swapping

Instead, demand page from file system and reclaim read-only pages
(such as code)

10.22
Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10 th Edition
Copy-on-Write

Copy-on-Write (COW) allows both parent and child processes to initially
share the same pages in memory

If either process modifies a shared page, only then is the page copied

COW allows more efficient process creation as only modified pages are
copied

In general, free pages are allocated from a pool of zero-fill-on-demand
pages

Pool should always have free frames for fast demand page execution

Don’t want to have to free a frame as well as other processing on
page fault

Why zero-out a page before allocating it?

vfork() variation on fork() system call has parent suspend and child
using copy-on-write address space of parent

Designed to have child call exec()

Very efficient

10.23
Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10 th Edition
Before Process 1 Modifies Page C

10.24
Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10 th Edition
After Process 1 Modifies Page C

10.25
Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10 th Edition
What Happens if There is no Free Frame?

Used up by process pages

Also in demand from the kernel, I/O buffers, etc

How much to allocate to each?

Page replacement – find some page in memory, but not really in
use, page it out

Algorithm – terminate? swap out? replace the page?

Performance – want an algorithm which will result in minimum
number of page faults

Same page may be brought into memory several times

10.26
Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10 th Edition
Page Replacement

Prevent over-allocation of memory by modifying page-
fault service routine to include page replacement

Use modify ( dirty ) bit to reduce overhead of page
transfers – only modified pages are written to disk

Page replacement completes separation between logical
memory and physical memory – large virtual memory can
be provided on a smaller physical memory

10.27
Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10 th Edition
Need For Page Replacement

10.28
Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10 th Edition
Basic Page Replacement
1.
Find the location of the desired page on disk
2.
Find a free frame:
- If there is a free frame, use it
- If there is no free frame, use a page replacement algorithm to
select a victim frame
- Write victim frame to disk if dirty
3.
Bring the desired page into the (newly) free frame; update the page
and frame tables
4.
Continue the process by restarting the instruction that caused the trap
Note now potentially 2 page transfers for page fault – increasing EAT

10.29
Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10 th Edition
Page Replacement

10.30
Silberschatz, Galvin and Gagne ©2018
Operating System Concepts – 10 th Edition
Page and Frame Replacement Algorithms

Frame-allocation algorithm determines
