Chapter 3 : Memory Management, Recent Systems - Computer ...

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Chapter 3 : Memory Management,
Recent Systems


Paged Memory Allocation


Demand Paging


Page Replacement Policies


Segmented Memory Allocation


Segmented/Demand Paged
Memory Allocation


Virtual Memory


Paged Memory Allocation

Segmented Demand Paging

Segmented/

Demand Paging

Understanding
Operating Systems

2

Memory Management


Early schemes were limited to storing entire program in
memory.


Fragmentation.


Overhead due to relocation.



More sophisticated memory schemes now that:


Eliminate need to store programs contiguously.


Eliminate need for entire program to reside in memory
during execution.

Problems

Understanding
Operating Systems

3

More Recent Memory Management
Schemes


Paged Memory Allocation



Demand Paging Memory Allocation



Segmented Memory Allocation



Segmented/Demand Paged Allocation


Understanding
Operating Systems

4

Paged Memory Allocation


Divides each incoming job into
pages

of equal size.



Works well if page size = size of memory block size
(
page frames
) = size of disk section (
sector, block
).


Before executing a program, memory manager:


1.

Determines number of pages in program.


2.

Locates enough empty page frames in main memory.


3.

Loads all of the program’s pages into them.



Understanding
Operating Systems

5

Programs Are Split Into Equal
-
sized
Pages
(Figure 3.1)

Understanding
Operating Systems

6

Job 1
(Figure 3.1)

At compilation time every job is divided into pages:



Page 0 contains the first hundred lines.



Page 1 contains the second hundred lines.



Page 2 contains the third hundred lines.



Page 3 contains the last fifty lines.



Program has 350 lines.


Referred to by system as line 0 through line 349.

Understanding OS

7

Paging Requires 3 Tables to Track a
Job’s Pages

1.

Job table (JT)

-

2 entries for each active job.


Size of job & memory location of its page map table.


Dynamic


grows/shrinks as jobs loaded/completed.


2.

Page map table (PMT)

-

1 entry per page.


Page number & corresponding page frame memory
address.


Page numbers are sequential (Page 0, Page 1 …)


3.

Memory map table (MMT)

-

1 entry for each page frame.


Location & free/busy status.

Understanding
Operating Systems

8

Job Table Contains 2 Entries for Each
Active Job
(Table 3.1)

9

Job 1 Is 350 Lines Long & Divided
Into 4 Pages
(Figure 3.2)

Understanding
Operating Systems

10

Displacement
(Figure 3.2)


Displacement (offset
) of a line
--

how far away a line is
from the beginning of its page.


Used to locate that line within its page frame.


Relative factor.



For example, lines 0, 100, 200, and 300 are first lines for
pages 0, 1, 2, and 3 respectively so each has displacement
of zero.


Understanding
Operating Systems

11

To Find the Address of a Given
Program Line


Divide the line number by the page size, keeping the
remainder as an integer.




Page number


Page size


line number to be located





xxx






xxx






xxx







Displacement

Understanding
Operating Systems

12

Address Resolution


Each time and instruction is executed or a data value is
used, the OS or (hardware) must:


Translate the job space address (relative).


Into a physical address (absolute).





Understanding
Operating Systems

13

Pros & Cons of Paging


Allows jobs to be allocated in non
-
contiguous memory
locations.


Memory used more efficiently; more jobs can fit.



Size of page is crucial (not too small, not too large).


Increased overhead occurs.


Reduces, but does not eliminate, internal fragmentation.

Understanding
Operating Systems

14

Demand Paging


Bring a page into memory only when it is needed, so less
I/O & memory needed.


Faster response.



Takes advantage that programs are written sequentially so
not all pages are necessary at once. For example:


User
-
written error handling modules.


Mutually exclusive modules.


Certain program options are either mutually exclusive
or not always accessible.


Many tables assigned fixed amount of address space
even though only a fraction of table is actually used.

Understanding
Operating Systems

15

Demand Paging
-

2


Demand paging made virtual memory widely available
.


Can give appearance of an almost
-
infinite or nonfinite
amount of physical memory
.



Requires use of a high
-
speed direct access storage device
that can work directly with CPU
.



How and when the pages are passed (or “swapped”)
depends on predefined policies that determine when to
make room for needed pages and how to do so
.

Understanding
Operating Systems

16

Tables in Demand Paging


Job Table.


Page Map Table (with 3 new fields).

1.
Determines if requested page is already in memory.

2.
Determines if page contents have been modified.

3.
Determines if the page has been referenced recently.


Used to determine which pages should remain in main
memory and which should be swapped out.


Memory Map Table.

Understanding
Operating Systems

17

Page Map Table

Understanding
Operating Systems

18

Hardware Instruction

Processing Algorithm


1.
Start processing instruction

2.
Generate data address

3.
Compute page number

4.
If page is in memory



Then


get data and finish instruction


advance to next instruction


return to step 1


Else



generate page interrupt

call page fault handler


19

Page Fault Handler Algorithm

1.
If there is no free page frame

Then

Select page to be swapped out using page removal algorithm






Update job’s page map table





If content of page had been changed then






Write page to disk


End if

End if

2.

Use page number from step 3 from the hardware instruction processing
algorithm to get disk address where the requested page is stored.

3.

Read page into memory.

4.

Update job’s page map table.

5.

Update memory map table.

6.

Restart interrupted instruction.


Understanding
Operating Systems

20

Thrashing Is a Problem

With Demand Paging


Trashing



an excessive amount of page swapping back
and forth between main memory and secondary storage.


Operation becomes inefficient
.


Caused when a page is removed from memory but is
called back shortly thereafter.


Can occur across jobs, when a large number of jobs are
vying for a relatively few number of free pages.


Can happen within a job (e.g., in loops that cross page
boundaries).


Page fault



a failure to find a page in memory.

Understanding
Operating Systems

21

Page Replacement Policies



Policy that selects page to be removed is crucial to system
efficiency.


Selection of algorithm is critical.



First
-
in first
-
out (FIFO)

policy*



best page to remove is
one that has been in memory the longest.


Least
-
recently
-
used (LRU)

policy*



chooses pages least
recently accessed to be swapped out.


Most recently used (MRU) policy.



Least frequently used (LFU) policy.

* Most well known policies

Understanding
Operating Systems

22

FIFO policy.

When program calls for Page C, Page A is moved out of 1st page
frame to make room for it (solid lines). When Page A is needed again,
it replaces Page B in 2nd page frame (dotted lines)
.

Understanding
Operating Systems

23

How each page requested is swapped into 2 available page frames
using FIFO. When program is ready to be processed all 4 pages are
on secondary storage. Throughout program, 11 page requests are
issued. When program calls a page that isn’t already in memory, a
page interrupt is issued (shown by *). 9 page interrupts result.


Understanding
Operating Systems

24

FIFO


High failure rate shown in previous example caused by:


limited amount of memory available.


order in which pages are requested by program (can’t
change).



There is no guarantee that buying more memory will
always result in better performance (
FIFO anomaly

or
Belady's anomaly
).

Understanding
Operating Systems

25

LRU Policy

For program in Figure 3.8. Throughout the program 11 page requests
are issued, but they cause only 8 page interrupts.


Understanding
Operating Systems

26

LRU


The efficiency of LRU is only slightly better than with
FIFO.



LRU is a
stack algorithm

removal policy


increasing
main memory causes either a decrease in or same number
of page interrupts.


LRU doesn’t have same anomaly that FIFO does.


Understanding
Operating Systems

27

Mechanics of Paging :

Page Map Table


Status bit

indicates if page is currently in memory or not.


Referenced bit

indicates if page has been referenced recently.


Used by LRU to determine which pages should be swapped out.


Modified bit

indicates if page contents have been altered


Used to determine if page must be rewritten to secondary storage
when it’s swapped out.

Understanding
Operating Systems

28

Four Possible Combinations of
Modified and Referenced Bits




Modified


Referenced


Meaning


Case 1


0


0


Not modified AND not referenced


Case 2


0


1


Not modified BUT was referenced


Case 3


1


0


Was modified BUT not referenced
(impossible?)


Case 4


1


1


Was modified AND referenced


Understanding
Operating Systems

29

Page Replacement : The Working Set



Working set



set of pages residing in memory that can be
accessed directly without incurring a page fault
.


Improves performance of demand page schemes.



Locality of reference

occurs with well
-
structured
programs.


During any phase of its execution program references only a small
fraction of its pages
.



System must decide:


How many pages comprise the working set?


What’s the maximum number of pages the operating system will
allow for a working set?


Understanding
Operating Systems

30

Pros & Cons of Demand Paging


First scheme in which a job was no longer constrained by
the size of physical memory (virtual memory).


Uses memory more efficiently than previous schemes
because sections of a job used seldom or not at all aren’t
loaded into memory unless specifically requested.



Increased overhead caused by tables and page interrupts
.

Understanding
Operating Systems

31

Segmented Memory Allocation


Based on common practice by programmers of structuring
their programs in
modules (
logical groupings of code
).


A
segment

is a
logical

unit such as: main program,
subroutine, procedure, function, local variables, global
variables, common block, stack, symbol table, or array.



Main memory is
not

divided into page frames because size
of each segment is different.


Memory is allocated dynamically.

Understanding
Operating Systems

32

Segment Map Table (SMT)


When a program is compiled, segments are set up
according to program’s structural modules.



Each segment is numbered and a
Segment Map Table
(SMT)

is generated for each job.


Contains segment numbers, their lengths, access rights,
status, and (when each is loaded into memory) its
location in memory
.


Understanding
Operating Systems

33

Tables Used in Segmentation


Memory Manager needs to track segments in memory:


1.
Job Table (JT)
lists every job in process (one for whole
system).

2.
Segment
Map Table lists details about each segment (one
for each job).

3.
Memory Map Table monitors allocation of main memory
(one for whole system).


Understanding
Operating Systems

34

Pros & Cons of Segmentation


Compaction.


External fragmentation.


Secondary storage handling.



Memory is allocated dynamically.



Understanding
Operating Systems

35

Segmented/Demand Paged

Memory Allocation



Evolved from combination of segmentation and demand
paging.


Logical benefits of segmentation.


Physical benefits of paging.



Subdivides each segment into pages of equal size, smaller
than most segments, and more easily manipulated than
whole segments.



Eliminates many problems of segmentation because it uses
fixed length pages.

Understanding
Operating Systems

36

4 Tables Are Used in
Segmented/Demand Paging

1.
Job
Table lists every job in process (one for whole
system).

2.
Segment Map Table lists details about each segment (one
for each job).


E.g., protection data, access data.

3.
Page Map Table lists details about every page (one for
each segment).


E.g., status, modified, and referenced bits .

4.
Memory Map Table monitors allocation of page frames in
main memory (one for whole system).


Understanding
Operating Systems

37

Pros & Cons of Segment/Demand
Paging


Overhead required for the extra tables


Time required to reference segment table and page table.



Logical benefits of segmentation.


Physical benefits of paging



To minimize number of references, many systems use
associative memory

to speed up the process.


Understanding
Operating Systems

38

Virtual Memory (VM)


Even though only a portion of each program is stored in
memory, virtual memory gives appearance that programs
are being completely loaded in main memory during their
entire processing time
.



Shared programs and subroutines are loaded “on demand,”
reducing storage requirements of main memory.



VM is implemented through demand paging and
segmentation schemes
.



Understanding
Operating Systems

39

Comparison of VM With Paging and
With Segmentation

Understanding
Operating Systems

40

Advantages of VM


Works well in a multiprogramming environment because most
programs spend a lot of time waiting.


Job’s size is no longer restricted to the size of main memory (or the
free space within main memory).


Memory is used more efficiently.


Allows an unlimited amount of multiprogramming.


Eliminates external fragmentation when used with paging and
eliminates internal fragmentation when used with segmentation.


Allows a program to be loaded multiple times occupying a different
memory location each time.


Allows the sharing of code and data.


Facilitates dynamic linking of program segments.

Understanding
Operating Systems

41

Disadvantages of VM


Increased processor hardware costs
.



Increased overhead for handling paging interrupts
.



Increased software complexity to prevent thrashing
.

Understanding
Operating Systems

42

Key Terms


address resolution


associative memory


demand paging


displacement


FIFO anomaly


first
-
in first
-
out (FIFO) policy


Job Table (JT)


least
-
recently
-
used (LRU)
policy


locality of reference


Memory Map Table (MMT)


page


page fault


page fault handler


page frame


Page Map Table (PMT)


page replacement policy


page swap


paged memory allocation


reentrant code


segment


Segment Map Table (SMT)


Understanding
Operating Systems

43

Key Terms
-

2


segmented memory allocation


segmented/demand paged
memory allocation


thrashing


virtual memory


working set