3.5 Virtual memory and Swapping

Virtual Memory: Virtual memory is a memory management technique that allows an operating system to give an application the impression that it has access to a large, contiguous block of memory, even if the actual physical memory (RAM) is smaller or fragmented. It provides a logical separation between a process’s memory space and the actual physical memory by using both RAM and a part of the hard disk.

Swapping: Swapping is the process of moving inactive or less-used pages or segments of a process from the main memory (RAM) to a storage device (like a hard disk) and bringing them back into memory when needed. This helps in managing memory when physical memory becomes scarce by offloading less critical memory pages to disk.

Virtual Address Space: Each process is given a large virtual address space. This address space may be much larger than the available physical memory.
Address Translation: Virtual memory uses a mechanism called address translation to map virtual addresses (used by processes) to physical addresses (actual memory locations in RAM). This translation is done via a page table or segment table.
Page Fault: When a process tries to access data that is not currently in RAM but stored on the disk, it generates a page fault. The operating system then retrieves the required data from disk (or swap space) into memory.

Swapping In: If a page or segment needed by a process is currently swapped out to disk, the operating system will swap it back into RAM, making it available for use.
Swapping Out: When memory becomes limited, the operating system swaps out inactive or less frequently used pages of a process from RAM to a special area on the disk called the swap space.

Page Table: A data structure that maps virtual addresses to physical addresses. It keeps track of where the virtual pages are stored (either in RAM or on disk).
Swap Space: A portion of the disk used to store pages that are swapped out of memory. It acts as an extension of the physical memory.
MMU (Memory Management Unit): A hardware device that handles the translation of virtual addresses to physical addresses.

Larger Memory Space: Allows programs to use more memory than what is physically available in RAM.
Memory Isolation: Ensures that processes do not interfere with each other’s memory, enhancing system stability.
Efficient Memory Utilization: Pages not in active use can be swapped out, freeing up physical memory for other tasks.
Simplified Programming Model: Programmers can write code as if there is a large, contiguous block of memory available, without worrying about the physical memory constraints.

Slower Access (due to swapping): Accessing data from the hard disk (swap space) is much slower than accessing data from RAM.
Overhead: The process of swapping and managing page tables introduces some overhead, impacting overall system performance.
Thrashing: If too much of the memory is swapped out and the system continually swaps pages in and out, it leads to thrashing, where the system spends more time swapping than executing applications, causing a significant slowdown.

Scenario: A computer with 8 GB of RAM is running multiple applications that together require 12 GB of memory. Instead of denying some of the applications the memory they need, the operating system will use virtual memory to allocate each application its requested memory, swapping out less active portions of data to disk when the physical RAM fills up.

Virtual Memory: Refers to the entire concept of creating a large logical memory space using both RAM and disk space.
Swapping: Refers specifically to the mechanism of moving data between RAM and disk when memory is needed.

notes-shivam