Transcript Lecture 1: Overview - City University of New York

Lecture 8: Virtual Memory
Operating System
Fall 2006
1
Two characteristics of paging and
segmentation

Memory references are dynamically translated into
physical addresses at run time


A process may be swapped in and out of main memory
such that it occupies different regions
A process may be broken up into pieces that do not
need to located contiguously in main memory

All pieces of a process do not need to be loaded in main
memory during execution
2
Virtual Memory


It is not necessary that all of the pages or all of the segments of
a process be in main memory during execution. As long as the
piece holding the next instruction and the data to be accessed
are in main memory, then execution may proceed.
Use page table to do address translation. If the page is not in
memory, it generates a page fault interrupt, the OS will bring
the page from disk into main memory. When this is done,
resume execution.
3
Advantages of Virtual Memory

More processes may be maintained in main memory



Only load in some of the pieces of each process
With so many processes in main memory, it is very likely a
process will be in the Ready state at any particular time
A process may be larger than all of main memory.
Programs become portable across different platforms.
4
Types of Memory

Real memory


Physical Main memory
Virtual memory



Programmer perceived memory
Memory on disk
Allows for effective multiprogramming and relieves
the user of tight constraints of main memory
5
Principle of Locality




Program and data references within a process tend to
cluster
Only a few pieces of a process will be needed over a
short period of time
Possible to make intelligent guesses about which
pieces will be needed in the future
This suggests that virtual memory may work
efficiently
6
Support Needed for Virtual Memory


Hardware must support paging and
segmentation
Operating system must be able to
management the movement of pages and/or
segments between secondary memory and
main memory
7
Paging




Each process has its own page table
Each page table entry contains the frame number of
the corresponding page in main memory
Presence Bit: A bit is needed to indicate whether the
page is in main memory or not
Modify Bit:


Another bit is needed to indicate if the page has been
altered since it was last loaded into main memory
If no change has been made, the page does not have to be
written to the disk when it needs to be swapped out
8
Page Table Entries
9
Translation Lookaside Buffer


Contains page table entries that have been most
recently used
Functions same way as a memory cache
11
Paging Hardware With TLB
12
Structure of the Page Table

Hierarchical Paging

Hashed Page Tables

Inverted Page Tables
15
Hierarchical Page Tables


Break up the logical address space into multiple
page tables
A simple technique is a two-level page table
16
Two-Level Page-Table Scheme
17
Address-Translation Scheme
18
Memory Protection


Memory protection implemented by associating
protection bit with each frame
Valid-invalid bit attached to each entry in the page
table:


“valid” indicates that the associated page is in the process’
logical address space, and is thus a legal page
“invalid” indicates that the page is not in the process’
logical address space
19
Valid (v) or Invalid (i) Bit In A Page Table
20
Segmentation





May be unequal, dynamic size
Simplifies handling of growing data structures
Allows programs to be altered and recompiled
independently
Lends itself to sharing data among processes
Lends itself to protection
21
Segment Tables




corresponding segment in main memory
Each entry contains the length of the segment
A bit is needed to determine if segment is already in
main memory
Another bit is needed to determine if the segment
has been modified since it was loaded in main
memory
22
Segment Table Entries
23
Segmentation Hardware
24
Combined Paging and Segmentation





Paging is transparent to the programmer
Paging eliminates external fragmentation
Segmentation is visible to the programmer
Segmentation allows for growing data structures,
modularity, and support for sharing and protection
Each segment is broken into fixed-size pages
25
Combined Segmentation and Paging
26
OS Supports for Virtual Memory


Virtual Memory: not all pages of a process
are in main memory
OS needs to decide on the following issues:



Fetch Policy
Placement Policy
Replacement Policy
27
Fetch Policy

Fetch Policy


Determines when a page should be brought into memory
Demand paging – bring pages into main memory only when
it is needed






Many page faults when process first started
Less I/O needed
Less memory needed
Faster response
More users
Prepaging – brings in more pages then needed even though
it is not needed now.


Faster to bring in several pages than one at a time
More efficient to bring in pages that reside contiguously on the
disk
28
Placement Policy

Decides where a process piece reside in main
memory


For paging system, it is a trivial issue
For segmentation system, use first-fit or best-fit to
look for a hole.
29
Replacement Policy



Determines which page to replace when a new page
needs to be brought in and there is no empty page
frame around
Page removed should be the page least likely to be
referenced in the near future
Most policies predict the future behavior on the basis
of past behavior
30
Replacement Policy

Frame Locking





If frame is locked, it may not be replaced
Kernel of the operating system
Control structures
I/O buffers
Associate a lock bit with each frame
31
Replacement Algorithms




Belady’s Optimal Algorithm
Least Recently Used Algorithm (LRU)
First-in-first-out Algorithm (FIFO)
Clock (approximation of LRU)
32
Belady’s Optimal Algorithm

Optimal policy


Selects for replacement that page for which the
time to the next reference is the longest
Impossible to have perfect knowledge of future
events
33
Least Recently Used (LRU)



Replaces the page that has not been referenced for
the longest time
By the principle of locality, this should be the page
least likely to be referenced in the near future
Each page could be tagged with the time of last
reference. This would require a great deal of
overhead.
34
First-in, first-out (FIFO)






Treats page frames allocated to a process as a
circular buffer
Pages are removed in round-robin style
Simplest replacement policy to implement
Page that has been in memory the longest is
replaced
These pages may be needed again very soon
LRU performs better than FIFO, but difficult to
implement.
35
Clock Policy





Additional bit called a use bit
When a page is first loaded in memory, the use bit is
set to 0
When the page is referenced, the use bit is set to 1
When it is time to replace a page, the first frame
encountered with the use bit set to 0 is replaced.
During the search for replacement, each use bit set
to 1 is changed to 0
36
Further Refinement for Clock Policy

Two bits:



Used bit: u bit=1 when used
Modify bit: m bit=1 when the page is written
Four states:




u=0;m=0
u=1;m=0
u=0;m=1
u=1;m=1
–
–
–
–
not used, not modified
used, not modified
not used, modified
used, modified
39
Further Refinement for Clock Policy

Algorithm:
1.
2.
3.
Beginning at the current position of the ptr, scan the frame
buffer. During this scan, make no changes to the use bit.
The first frame encountered with (u=0;m=0) is selected for
replacement.
If step 1 fails, scan again, looking for the frame with
(u=0;m=1). The first such frame encountered is selected for
replacement. During this scan, set the use bit to 0 on each
frame that is bypassed.
If step 2 fails, the ptr should have returned to its original
position and all of the frames in the set will have a use bit of
0. Repeat step 1, and if necessary, step 2. This time, a frame
will be found for the replacement.
40
End of lecture 8
Thank you!
41