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Virtual Memory in Operating System

Last Updated : 19 Jan, 2024
Virtual Memory is a storage allocation scheme in which secondary
memory can be addressed as though it were part of the main memory.
The addresses a program may use to reference memory are
distinguished from the addresses the memory system uses to identify
physical storage sites and program-generated addresses are
translated automatically to the corresponding machine addresses.

A memory hierarchy, consisting of a computer system’s memory and a

disk, that enables a process to operate with only some portions of its
address space in memory. A virtual memory is what its name
indicates- it is an illusion of a memory that is larger than the real
memory. We refer to the software component of virtual memory as a
virtual memory manager. The basis of virtual memory is the
noncontiguous memory allocation model. The virtual memory
manager removes some components from memory to make room for
other components.

The size of virtual storage is limited by the addressing scheme of the

computer system and the amount of secondary memory available not
by the actual number of main storage locations.

It is a technique that is implemented using both hardware and

software. It maps memory addresses used by a program, called virtual
addresses, into physical addresses in computer memory.

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1. All memory references within a process are logical addresses that

are dynamically translated into physical addresses at run time. This
means that a process can be swapped in and out of the main
memory such that it occupies different places in the main memory
at different times during the course of execution.
2. A process may be broken into a number of pieces and these pieces
need not be continuously located in the main memory during
execution. The combination of dynamic run-time address
translation and the use of a page or segment table permits this.

If these characteristics are present then, it is not necessary that all the
pages or segments are present in the main memory during execution.
This means that the required pages need to be loaded into memory
whenever required. Virtual memory is implemented using Demand
Paging or Demand Segmentation.

Demand Paging
The process of loading the page into memory on demand (whenever a
page fault occurs) is known as demand paging. The process includes
the following steps are as follows:

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Demand Paging

1. If the CPU tries to refer to a page that is currently not available in

the main memory, it generates an interrupt indicating a memory
access fault.
2. The OS puts the interrupted process in a blocking state. For the
execution to proceed the OS must bring the required page into the
memory.
3. The OS will search for the required page in the logical address
space.
4. The required page will be brought from logical address space to
physical address space. The page replacement algorithms are used
for the decision-making of replacing the page in physical address
space.
5. The page table will be updated accordingly.
6. The signal will be sent to the CPU to continue the program
execution and it will place the process back into the ready state.

Hence whenever a page fault occurs these steps are followed by the
operating system and the required page is brought into memory.

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Advantages of Virtual Memory

More processes may be maintained in the main memory: Because
we are going to load only some of the pages of any particular
process, there is room for more processes. This leads to more
efficient utilization of the processor because it is more likely that at
least one of the more numerous processes will be in the ready state
at any particular time.
A process may be larger than all of the main memory: One of the
most fundamental restrictions in programming is lifted. A process
larger than the main memory can be executed because of demand
paging. The OS itself loads pages of a process in the main memory
as required.
It allows greater multiprogramming levels by using less of the
available (primary) memory for each process.
It has twice the capacity for addresses as main memory.
It makes it possible to run more applications at once.
Users are spared from having to add memory modules when RAM
space runs out, and applications are liberated from shared memory
management.
When only a portion of a program is required for execution, speed
has increased.
Memory isolation has increased security.
It makes it possible for several larger applications to run at once.
Memory allocation is comparatively cheap.
It doesn’t require outside fragmentation.
It is efficient to manage logical partition workloads using the CPU.
Automatic data movement is possible.

Disadvantages of Virtual Memory

It can slow down the system performance, as data needs to be
constantly transferred between the physical memory and the hard
disk.
It can increase the risk of data loss or corruption, as data can be lost
if the hard disk fails or if there is a power outage while data is being
transferred to or from the hard disk.
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It can increase the complexity of the memory management system,

as the operating system needs to manage both physical and virtual
memory.

Page Fault Service Time: The time taken to service the page fault is
called page fault service time. The page fault service time includes the
time taken to perform all the above six steps.

Let Main memory access time is: m

Page fault service time is: s
Page fault rate is : p
Then, Effective memory access time = (p*s) + (1-p)*m

Swapping
Swapping is a process out means removing all of its pages from
memory, or marking them so that they will be removed by the normal
page replacement process. Suspending a process ensures that it is not
runnable while it is swapped out. At some later time, the system
swaps back the process from the secondary storage to the main
memory. When a process is busy swapping pages in and out then this
situation is called thrashing.

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Swappinghierar

Thrashing
At any given time, only a few pages of any process are in the main
memory, and therefore more processes can be maintained in memory.
Furthermore, time is saved because unused pages are not swapped in
and out of memory. However, the OS must be clever about how it
manages this scheme. In the steady state practically, all of the main
memory will be occupied with process pages, so that the processor
and OS have direct access to as many processes as possible. Thus
when the OS brings one page in, it must throw another out. If it throws
out a page just before it is used, then it will just have to get that page
again almost immediately. Too much of this leads to a condition called
Thrashing. The system spends most of its time swapping pages rather
than executing instructions. So a good page replacement algorithm is
required.

In the given diagram, the initial degree of multiprogramming up to

some extent of point(lambda), the CPU utilization is very high and the
system resources are utilized 100%. But if we further increase the
degree of multiprogramming the CPU utilization will drastically fall

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down and the system will spend more time only on the page
replacement and the time taken to complete the execution of the
process will increase. This situation in the system is called thrashing.

Causes of Thrashing

1. High Degree of Multiprogramming: If the number of processes

keeps on increasing in the memory then the number of frames
allocated to each process will be decreased. So, fewer frames will be
available for each process. Due to this, a page fault will occur more
frequently and more CPU time will be wasted in just swapping in and
out of pages and the utilization will keep on decreasing.

For example:
Let free frames = 400
Case 1: Number of processes = 100
Then, each process will get 4 frames.

Case 2: Number of processes = 400

Each process will get 1 frame.
Case 2 is a condition of thrashing, as the number of processes is
increased, frames per process are decreased. Hence CPU time will be
consumed just by swapping pages.

2. Lacks of Frames: If a process has fewer frames then fewer pages of

that process will be able to reside in memory and hence more frequent
swapping in and out will be required. This may lead to thrashing.

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Hence a sufficient amount of frames must be allocated to each process

in order to prevent thrashing.

Recovery of Thrashing
Do not allow the system to go into thrashing by instructing the
long-term scheduler not to bring the processes into memory after
the threshold.
If the system is already thrashing then instruct the mid-term
scheduler to suspend some of the processes so that we can recover
the system from thrashing.

Performance in Virtual Memory

Let p be the page fault rate( 0 <= p <= 1).
if p = 0 no page faults
if p =1, every reference is a fault.

Effective access time (EAT) = (1-p)* Memory Access Time + p *

Page fault time.

Page fault time = page fault overhead + swap out + swap in

+restart overhead

The performance of a virtual memory management system depends

on the total number of page faults, which depend on “paging policies”
and “frame allocation“

Frame Allocation
A number of frames allocated to each process in either static or
dynamic.

Static Allocation: The number of frame allocations to a process is

fixed.
Dynamic Allocation: The number of frames allocated to a process
changes.

Paging Policies

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Fetch Policy: It decides when a page should be loaded into

memory.
Replacement Policy: It decides which page in memory should be
replaced.
Placement Policy: It decides where in memory should a page be
loaded.

Frequently Asked Questions

Q.1: How does virtual memory work?

Answer:

Virtual memory works by dividing the virtual address space used

by a program into smaller units called pages. The operating
system manages a mapping between virtual addresses used by
the program and physical addresses in the actual RAM or disk.
When a program accesses a virtual address that is not currently
in physical memory, a page fault occurs. The operating system
then retrieves the required page from disk and brings it into
RAM, evicting other pages if necessary.

Q.2: What are the benefits of virtual memory?

Answer:

Virtual memory provides several benefits:

1. Increased memory capacity: It allows programs to use more

memory than is physically available, enabling the execution of
larger programs or multiple programs simultaneously.
2. Memory isolation: Each program operates in its own virtual
address space, ensuring that one program cannot access or
modify the memory of another program.

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3. Simplified memory management: Virtual memory simplifies

memory management for both the operating system and
application developers by providing a uniform memory model.
4. Improved system stability: Virtual memory helps prevent
crashes and system instability by allowing the operating
system to handle memory shortages and prioritize memory
usage efficiently.

Q.3: What is a page fault?

Answer:

A page fault occurs when a program references a virtual memory

page that is not currently resident in physical memory. This
happens when the required page has been paged out to disk or
has not been accessed yet. When a page fault occurs, the
operating system handles it by fetching the required page from
disk and updating the page tables to reflect the new mapping.

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