Princess Nora University
Faculty of Computer & Information Systems
Computer science Department
Operating Systems
(CS 340 D)
(Chapter-9)
Virtual Memory
Chapter 9: Virtual Memory
1. Background
2. Virtual Memory Implementation
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OBJECTIVES:
 To describe the benefits of a virtual memory system
 To explain the concepts of demand paging
 To explain the concepts of page-replacement algorithms
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Background
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Background
In chapter of memory management ,


various memory-management strategies used in computer systems
were discussed and all of these strategies have the same goal: to
keep many processes in memory simultaneously to allow
multiprogramming.
However, they tend to require that an entire process be in memory
before it can execute.
Virtual memory is a technique that allows the execution of
processes that are not completely in memory.
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Background
Advantages
1. programs can be larger than physical
memory.
2. Map main memory into an extremely
large, uniform array of storage,
3. separating logical memory as viewed
by the user from physical memory.
4. This technique frees programmers
from the concerns of memorystorage limitations.
Virtual Memory That is Larger Than Physical
Memory
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Background

Advantages (cont)
o
Virtual memory also allows processes to share files easily and to implement
shared memory.
Shared Library Using Virtual Memory
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Background

Disadvantage

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Virtual memory is not easy to implement, however, and may
substantially decrease performance if it is used carelessly
Virtual Memory
Implementation
Demand Paging
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Virtual memory can be implemented via:
1.
2.
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Demand paging
Demand segmentation
1-Demand Paging
Consider how an executable program might be loaded from disk into
memory.

One option is to load the entire program in physical memory at
program execution time. However, a problem with this approach
 is that we may not initially need the entire program in memory.
Ex: Suppose a program starts with a list of available options from which the user
is to select. Loading the entire program into memory results in loading the
executable code for all options, regardless of whether an option is ultimately
selected by the user or not.

An alternative strategy is to load pages only as they are needed during
program excution. This technique is known as demand paging and is
commonly used in virtual memory systems.
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1-Demand Paging


A demand-paging system is similar to a paging system with
swapping But rather than swapping the entire process into
memory, we use a lazy swapper.
lazy swapper never swaps a page into memory unless that page
will be needed.
Since we are now viewing a process as a sequence of pages, rather
than as one
large contiguous address space, use of the term swapper is
technically incorrect.


A swapper manipulates entire processes, whereas a pager is
concerned with the individual pages of a process. We thus use
pager, rather than swapper, in connection with demand paging.
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1-Demand Paging
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
Bring a page into memory only when it is needed
 Less I/O needed
 Less memory needed
 Faster response
 More users

Page is needed  reference to it
 invalid reference  abort
 not-in-memory  bring to memory
Transfer of a Paged Memory to Contiguous Disk Space
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1-Demand Paging (cont..)
 With this scheme, we need some form of hardware support to distinguish
between
1.
the pages that are in memory
2.
the pages that are on the disk.
 The valid–invalid bit scheme can be used for this purpose.
 “valid,”
: the associated page is both legal and in memory.
 “invalid,”  the page either is not valid (that is, not in the logical
address space of the process) or is valid but is currently on the disk.
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1-Demand Paging(cont..)
Valid-Invalid Bit
With each page table entry a valid–
invalid bit is associated
(v  in-memory, i  not-in-memory)


Initially valid–invalid bit is set to i on
all entries
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page table
Page Table When Some Pages Are Not in Main Memory
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Page Fault

1.
2.
3.
4.
5.
6.
If there is a reference to a page, first reference to that page will
trap to operating system:
page fault
Operating system looks at another table to decide:
 Invalid reference  abort
 Just not in memory
Get empty frame
Swap page into frame
Reset tables
Set validation bit = v
Restart the instruction that caused the page fault
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Steps in Handling a Page Fault
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Performance of Demand Paging
 Demand paging can significantly affect the performance of a
computer system. why ?
•
if no page faults, the effective access time is equal to the memory
access time.
•
If, however, a page fault occurs, we must first read the relevant
page from disk and then access the desired word.
 Let p be the probability of a page fault (0 ≤ p ≤ 1).
expect p to be close to only a few page faults.
effective access time = (1 − p) × ma + p × page fault time.
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Demand Paging Example

Memory access time = 200 nanoseconds

Average page-fault service time = 8 milliseconds

EAT = (1 – p) x 200 + p (8 milliseconds)
= (1 – p x 200 + p x 8,000,000
= 200 + p x 7,999,800

If one access out of 1,000 causes a page fault, then
EAT = 8.2 microseconds.
This is a slowdown by a factor of 40!!
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Page Replacement
Algorithms
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Page Replacement


If we increase our degree of multiprogramming, we are overallocating memory.
Ex: If we run six processes, each of which is ten pages in size
but actually uses only five pages, we have higher CPU utilization
and throughput, with ten frames to spare.


It is possible, however, that each of these processes, for a particular data set,
may suddenly try to use all ten of its pages, resulting in a need for sixty
frames when only forty are available.
Further, consider that system memory is not used only for holding
program pages. Buffers for I/O also consume a considerable
amount of memory. This use can increase the strain on memoryplacement algorithms.
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Need For Page Replacement
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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
3. Bring the desired page into the (newly) free frame; update the
page and frame tables
4. Restart the process
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Page Replacement
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Page Replacement Algorithms
1.
2.
3.
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FIFO Page Replacement
Optimal Page Replacement
Least Recently Used (LRU) Algorithm
FIFO Page Replacement
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FIFO Page Replacement algorithm
 The simplest page-replacement algorithm is a first-in, first-
out (FIFO)
 FIFO  each page has memory load start time
 or create FIFO queue to hold all pages in memory
 When a page must be replaced, the oldest page is chosen.
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FIFO Page Replacement
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Optimal Page Replacement
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Optimal Algorithm


Replace page that will not be used for longest period of time
4 frames example
1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
1
4
2
3
4



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How do you know this?
Used for measuring how well your algorithm performs
Not easy to implement
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Optimal Page Replacement
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Least Recently Used (LRU)
Page Replacement
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Least Recently Used (LRU) Algorithm


Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
1
1
1
1
5
2
2
2
2
2
3
5
5
4
4
4
4
3
3
3
Counter implementation
o
o
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Every page entry has a counter; every time page is referenced through this
entry, copy the clock into the counter
When a page needs to be changed, look at the counters to determine
which are to change
LRU Page Replacement
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LRU Algorithm (Cont.)

Stack implementation – keep a stack of page numbers in a
double link form:
 Page referenced:
 move it to the top
 requires 6 pointers to be changed
 No search for replacement
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Thank you
End of
Chapter 9
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