2. What is Virtual Memory?
• Virtual memory is a computer memory management technique that allows an
operating system to use a combination of physical RAM (Random Access Memory) and
secondary storage, such as a hard disk drive, to effectively increase the amount of
usable memory available to programs.
What is the importance of Virtual Memory?
• Virtual memory expands the effective memory capacity of a computer system beyond
its physical limits.
• It facilitates memory sharing, swapping, and paging, optimizing memory usage and
improving overall system performance. It enables multitasking and efficient memory
allocation for running multiple programs simultaneously.
3. Why Do We Need Virtual Memory ?
• Increased Addressable Memory:
1. Virtual memory allows a computer system to access more memory than what is physically
available.
2. This allows programs to utilize more memory than the available physical RAM, enabling
them to run larger and more complex tasks.
• Memory Isolation and Protection:
1. Virtual memory facilitates memory isolation between different processes running on a
computer.
2. This isolation prevents one process from accessing or modifying another process's
memory, enhancing security and stability.
4. • Efficient Memory Management:
1. Virtual memory enables efficient memory management by utilizing techniques like
demand paging and page swapping.
2. Demand paging loads only the necessary portions of a program into physical memory,
allowing for efficient memory usage.
3. Page swapping moves infrequently used pages of memory between RAM and disk
storage, freeing up physical memory for other tasks.
• Simplified Program Development:
1. Virtual memory simplifies program development by providing a uniform and consistent
memory model for applications.
2. Programmers can develop applications using a flat, contiguous address space,
independent of physical memory limitations.
5. What is Demand Paging in
Virtual Memory ?
• Demand paging is a virtual memory management technique where pages are
loaded into memory only when they are explicitly requested by processes.
• Instead of loading the entire program into memory at once, only the required
pages are brought into memory as needed.
• When a process references a page that is not in physical memory, a page fault
occurs, and the operating system brings the required page from disk into
memory.
• Demand paging allows for efficient memory utilization by loading pages on-
demand, reducing the initial memory footprint of processes.
6. Benefits of demand Paging
in Virtual Memory
1. Reduced Memory Footprint:
Demand paging reduces the amount of physical memory required to
run processes by loading only necessary pages.
2. Faster Process Startup Time:
Since demand paging loads only essential pages initially, the
process startup time is reduced as it doesn't have to load the
entire program into memory.
3. Efficient Memory Sharing:
Multiple processes can share the same page in memory, which
reduces memory duplication and allows for better resource
utilization.
4. Improved Overall System Performance:
Demand paging helps to avoid unnecessary I/O operations by
loading pages only when they are needed.
7. Copy-on-Write in Virtual Memory
• Definition:
Copy-on-write (COW) is a memory management technique used in virtual
memory systems to optimize memory usage and improve efficiency. It is
particularly relevant in scenarios where multiple processes or threads share the
same memory resources.
How is Memory Shared with Copy-on-Write ?
When COW is implemented, the operating system initially maps the virtual pages of two or more
processes to the same physical memory page. In this scenario, the processes share the same
underlying memory content. However, if any process attempts to modify the shared memory page,
the operating system triggers a copy operation.
8. Benefits of Copy-on-Write
• Memory Efficiency:
COW reduces memory consumption by deferring the creation of separate copies until
necessary. This is especially advantageous when processes primarily read from shared
memory.
• Performance Improvement:
Copying memory can be a costly operation, particularly when dealing with large
memory pages. By postponing the copy until modifications occur, COW minimizes
the overhead associated with copying.
• Simplified Memory Sharing:
COW allows multiple processes to easily share memory without the need for explicit
coordination or synchronization mechanisms.
9. Usage in Operating Systems and Virtualization Tech.
• Copy-on-write is widely used in various operating systems and virtualization
technologies to optimize memory utilization and enhance system performance in
scenarios involving shared memory. It is a fundamental technique for managing
memory efficiently and improving overall system scalability.
10. Examples of Copy-on-Write Usage
• Copy-on-Write is widely used in operating systems, such as Unix/Linux, to optimize
process creation and memory management.
• Forking a process in Unix/Linux typically employs Copy-on-Write to save memory
and improve performance.
11. Page Replacement in Virtual Memory
Definition:
Page replacement is a crucial aspect of virtual memory management, where the operating system
dynamically moves pages between physical memory and secondary storage (such as a hard disk or
SSD) to efficiently utilize limited physical memory resources. When the physical memory becomes
full and a new page needs to be brought in, the operating system selects a victim page to be
replaced with the new page. This process is known as page replacement.
12. When a page fault occurs the operating system needs to select a page to replace from
physical memory to make room for the required page. This selection is done using a page
replacement algorithm.
There are Four Types of Algorithms using in Page Replacement.
1. FIFO (First-In-First-Out):
The page that has been in memory the longest is selected for
replacement.
2. LRU (Least Recently Used):
This algorithm replaces the page that has not been used for the
longest time.
3. LFU (Least Frequently Used):
This Algorithm replaces the page that has been accessed the fewest number
of times
4. Optimal:
It replaces the page that will not be used for the longest time in furture.
Page Replacement Algorithm
13. ALLOCATION
OF FRAMES
IN
VIRTUAL
MEMORY
Definition:
The allocation of frames in virtual memory refers to
the assignment of physical memory (RAM) frames
to the pages of a process's virtual address space.
The operating system manages this allocation to
enable efficient memory utilization and provide
each process with the necessary memory resources.
14. Memory Allocation Policies and Consideration.
• Memory Management Unit (MMU):
The MMU is responsible for translating virtual addresses to physical addresses using
the page table.
• Page Replacement Algorithms:
The choice of a page replacement algorithm affects frame allocation indirectly. The
algorithm determines which pages are evicted from memory when a new page needs
to be allocated. By selecting appropriate victim frames, the system can optimize memory
utilization and overall performance.
15. Page Table Entries and Frame Allocation
The mapping between virtual pages and physical frames is maintained in the process's page table. Each
entry in the page table represents a virtual page and stores the corresponding physical frame number.
The allocation can be done using following 3 strategies.
1. Contiguous Allocation:
2. Non-Contiguous Allocation
3. Segmentation
