Virtual Memory

VIRTUAL MEMORY
V.V.SUBRAHMANYAM
SOCIS, IGNOU
DATE: 02-09-07
TIME: 14-30 TO 15-15

Introduction
 Storage allocation has always been an
important consideration in computer
programming due to the high cost of
the main memory and the relative
abundance and lower cost of
secondary storage.

 Program code and data required for
execution of a process must reside in
the main memory but the main memory
may not be large enough to
accommodate the needs of an entire
process.
 Early computer programmers divided
programs into the sections that were
transferred into the main memory for
the period of processing time.

 Early computer programmers divided
programs into the sections that were
transferred into the main memory for
the period of processing time.
 As the program proceeded, new
sections moved into the main memory
and replaced sections that were not
needed at that time.
 In this early era of computing, the
programmer was responsible for
devising this overlay system.

 As higher-level languages became
popular for writing more complex
programs and the programmer became
less familiar with the machine, the
efficiency of complex programs
suffered from poor overlay systems.
The problem of storage allocation
became more complex.

 Two theories for solving the problem of
inefficient memory management
emerged - static and dynamic
allocation.
 Static allocation assumes that the
availability of memory resources and
the memory reference string of a
program can be predicted.
 Dynamic allocation relies on memory
usage increasing and decreasing with
actual program needs, not on
predicting memory needs.

 Program objectives and machine
advancements in the 1960s made the
predictions required for static
allocation difficult, if not impossible.
 Therefore, the dynamic allocation
solution was generally accepted, but
opinions about implementation were
still divided.

 One group believed the programmer
should continue to be responsible for
storage allocation, which would be
accomplished by system calls to
allocate or deal locate memory.
 The second group supported automatic
storage allocation performed by the
operating system, because of
increasing complexity of storage
allocation and emerging importance of
multiprogramming.

Virtual Memory
 In 1961, two groups proposed a one-
level memory store.
 One proposal called for a very large
main memory to alleviate any need for
storage allocation. This solution was
not possible due to its very high cost.
 The second proposal is known as
virtual memory.

Definition
 Virtual memory is a technique that allows the
execution of processes that may not be
completely in memory.
 One major advantage of this scheme is that
the program can be larger than physical
memory.
 Of course, in order to do this sensibly it is
highly desirable to have a protection scheme
that restricts a process to be able to access
only those blocks that are assigned to it.

 Virtual memory enables a program to ignore
the physical location of any desired block of
its address space; a process can simply seek
to access any block of its address space
without concern for where that block might
be located.
 If the block happens to be located in the main
memory, access is carried out smoothly and
quickly; else, the virtual memory has to bring
the block in from secondary storage and
allow it to be accessed by the program.

Virtual Memory Vs Cache
 The technique of virtual memory is similar to
a degree with the use of processor caches.
 However, the differences lie in the block size
of virtual memory being typically much larger
(64 kilobytes and up) as compared with the
typical processor cache (128 bytes and up).
 The hit time, the miss penalty (the time taken
to retrieve an item that is not in the cache or
primary storage), and the transfer time are all
larger in case of virtual memory.
 However, the miss rate is typically much
smaller.

Advantages
 It often reduces the time taken to launch a
program, since not all the program code and
data need to be in physical memory before
the program execution can be started.
 Virtual memory automatically manages two
levels of the memory hierarchy, representing
the main memory and the secondary storage,
in a manner that is invisible to the program
that is running.

Types of Virtual Memory Systems
 Virtual memory systems are of two
basic kinds—those using fixed-size
blocks called pages, and those that use
variable-sized blocks called segments.

Principles of Operation
 To facilitate copying virtual memory into real
memory, the operating system divides virtual
memory into pages, each of which contains a
fixed number of addresses.
 Each page is stored on a disk until it is
needed.
 When the page is needed, the operating
system copies it from disk to main memory,
translating the virtual addresses into real
addresses.

Contd…
 Addresses generated by programs are virtual
addresses.
 The actual memory cells have physical
addresses.
 A piece of hardware called a memory
management unit (MMU) translates virtual
addresses to physical addresses at run-time.
 The process of translating virtual addresses
into real addresses is called mapping. The
copying of virtual pages from disk to main
memory is known as paging or swapping.

Contd…
 Some physical memory is used to keep
a list of references to the most recently
accessed information on an I/O
(input/output) device, such as the hard
disk.
 The optimization it provides is that it is
faster to read the information from
physical memory than use the relevant
I/O channel to get that information. This
is called caching. It is implemented
inside the OS.

Virtual Memory Manager
 Virtual memory provides the services
to map the logical and physical
address spaces to one another and
when it is required.

Functions of Virtual memory manager
It is responsible to:
 Make portions of the logical address
space resident in physical RAM
 Make portions of the logical address
space immovable in physical RAM
 Map logical to physical addresses
 Defer execution of application-defined
interrupt code until a safe time.

Abstract model of Virtual to Physical address mapping
VPFN 7
VPFN 6
VPFN 5
VPFN 4
VPFN 3
VPFN 2
VPFN 1
VPFN 0
Process Y
VPFN 7
VPFN 6
VPFN 5
VPFN 4
VPFN 3
VPFN 2
VPFN 1
VPFN 0
Process X
PFN 4
PFN 3
PFN 2
PFN 1
PFN 0
Process X
Page Table
Process Y
Page Table
VIRTUAL MEMORY PHYSICAL MEMORY VIRTUAL MEMORY

Paging scheme supporting the sharing of program code
editor 1
editor 2
editor 3
data 1
3
4
6
1
editor 1
editor 2
editor 3
data 3
3
4
6
2
editor 1
editor 2
editor 3
data 2
3
4
6
7
Process 1
Page table 1
Process 3
Page table 3
Page table 2
Process 2
…
data 1
data 3
editor 1
editor 2
….
editor 3
data 2
…
…
Physical memory

Pure Demand Paging
 In the extreme case, a process without pages
in memory could be executed.
 Page fault trap would occur with the first
instruction. After this page was brought into
memory, the process would continue to
execute.
 In this way, page fault trap would occur
further until every page that is needed was in
memory. This kind of paging is called pure
demand paging. Pure demand paging says
that “never bring a page into memory until it
is required”.

Page Replacement Policy
 When a process needs a non-resident
page, the operating system must
decide which resident page is to be
replaced by the requested page. The
part of the virtual memory which makes
this decision is called the replacement
policy.

Page Replacement Policies
 First in first out (FIFO)
 Second Chance (SC)
 Least Recently Used (LRU)
 Optimal Algorithm (OPT)
 Least Frequently Used (LFU)

Thrashing
 Thrashing occurs when a system
spends more time processing page
faults than executing transactions.
 While processing page faults it is
necessary to be in order to appreciate
the benefits of virtual memory,
thrashing has a negative effect on the
system.

 As the page fault rate increases, more
transactions need processing from the
paging device.
 The queue at the paging device
increases, resulting in increased
service time for a page fault.
 While the transactions in the system
are waiting for the paging device, CPU
utilisation, system throughput and
system response time decrease,
resulting in below optimal performance
of a system.

Thrashing becomes a greater threat as the degree of
multiprogramming of the system increases.

Demand Segmentation
 Same idea as demand paging applied
to segments.

Combined Systems
The combined systems are of two types.
They are:
 Segmented Paging
 Paged Segmentation

Virtual Memory

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