Virtual Memory

THE VIRTUAL MEMORY

What is Virtual Memory?


A technique that allows a program to be executed even though they are not stored. Demand paging made it possible for a program to execute even though only a part of it was loaded into main memory. In effect it removed the restriction imposed on maximum program size. This capability of moving pages at will between main memory and secondary storage gave way to a new concept appropriate named VIRTUAL MEMORY.

Virtual memory is used by the operating system in order to enhance the storage capacity of the working memory, without requiring the installation of additional Random Access Memory modules.


How Virtual Memory Works?


When a computer is running, many programs are simultaneously sharing the CPU. Each running program, plus the data structures needed to manage it, is called a process.
Each process is allocated an address space. This is a set of valid addresses that can be used. This address space can be changed dynamically. For example, the program might request additional memory (from dynamic memory allocation) from the operating system.
If a process tries to access an address that is not part of its address space, an error occurs, and the operating system takes over, usually killing the process (core dumps, etc).
Load/store create data addresses, while fetching an instruction creates instruction addresses. Of course, RAM doesn't distinguish between the two kinds of addresses. It just sees it as an address.

In essence, the operating system uses a portion of the hard drive as "false memory," or virtual memory, to make applications believe they have more memory to work with than they actually do. Virtual memory begins by the computer comprehending the RAM available to it, then it calculates the amount of hard drive space available and tells the software that there is so much RAM available between the two sources of storage. Therefore, when a word processor looks for room to store information temporarily, the operating system tells it that there is more available than just the memory of the RAM. There is hard drive space for usage that functions logically like more RAM.



How Virtual Memory Works in Windows?

Windows is an example of a typical operating system that has virtual memory. Windows 98 has an intelligent virtual memory manager that uses a default setting to help Windows allocate hard drive space for virtual memory as needed. For most circumstances, this should meet your needs, but you may want to manually configure virtual memory, especially if you have more than one physical hard drive or speed-critical applications.


Virtual memory is one of the most important, and accordingly confusing, pieces of an operating system. Understanding the basics of virtual memory is required to understand operating system performance. Beyond the basics, a deeper understanding allows a system administrator to interpret system profiling tools better, leading to quicker troubleshooting and better decisions.

The concept of virtual memory is generally taught as though it’s only used for extending the amount of physical RAM in a system. Indeed, paging to disk is important, but virtual memory is used by nearly every aspect of an operating system.

How Virtual Memory Works in Linux?

Nearly every VMM interaction involves the MMU, or Memory Management Unit, excluding the disk subsystem. The MMU allows the operating system to access memory through virtual addresses by using data structures to track these translations. Its main job is to translate these virtual addresses into physical addresses, so that the right section of RAM is accessed.

The Zoned Buddy Allocator interacts directly with the MMU, providing valid pages when the kernel asks for them. It also manages lists of pages and keeps track of different categories of memory addresses.

ages of memory are either Free (available to allocate), Active (in use), or Inactive. Inactive pages of memory are either dirty or clean, depending on if it has been selected for removal yet or not. An inactive, dirty page is no longer in use, but is not yet available for re-use. The operating system must scan for dirty pages, and decide to deallocate them. After they have been guaranteed sync’d to disk, an inactive page my be “clean,” or ready for re-use.

How Virtual Memory Works in OSX and Mac OSX?


In Mac OS X, each process has its own sparse 32-bit or 64-bit virtual address space. For 32-bit processes, each process has an address space that can grow dynamically up to a limit of four gigabytes. For 64-bit processes, the address space can grow dynamically up to a limit of approximately 18 exabytes. As an application uses up space, the virtual memory system allocates additional swap file space on the root file system.

The virtual address space of a process consists of mapped regions of memory. Each region of memory in the process represents a specific set of virtual memory pages. A region has specific attributes controlling such things as inheritance (portions of the region may be mapped from “parent” regions), write-protection, and whether it is “wired” (that is, it cannot be paged out). Because regions contain a given number of pages, they are page-aligned, meaning the starting address of the region is also the starting address of a page and the ending address also defines the end of a page.

The kernel associates a VM object with each region of the virtual address space. The kernel uses the VM object to track and manage the resident and nonresident pages of that region. A region can map either to an area of memory in the backing store or to a specific file-mapped file in the file system.

The VM object maps regions in the backing store through the default pager and maps file-mapped files through the vnode pager. The default pager is a system manager that maps the nonresident virtual memory pages to backing store and fetches those pages when requested. The vnode pager implements file mapping. The vnode pager uses the paging mechanism to provide a window directly into a file. This mechanism lets you read and write portions of the file as if they were located in memory.

A VM object may point to a pager or to another VM object. The kernel uses this self referencing to implement a form of page-level sharing known as copy-on-write. Copy-on-write allows multiple blocks of code (including different processes) to share a page as long as none write to that page. If one process writes to the page, a new, writable copy of the page is created in the address space of the process doing the writing. This mechanism allows the system to copy large quantities of data efficiently.