Virtual Machines

In computing, a virtual machine (VM) is an emulation of a particular computer system. Virtual machines operate based on the computer architecture and functions of a real or hypothetical computer, and their implementations may involve specialized hardware, software, or a combination of both.

Classification of virtual machines can be based on the degree to which they implement functionality of targeted real machines. That way, system virtual machines (also known as full virtualization VMs) provide a complete substitute for the targeted real machine and a level of functionality required for the execution of a complete operating system. On the other hand, process virtual machines are designed to execute a single computer program by providing an abstracted and platform-independent program execution environment.

A virtual machine (VM) is a software implementation of a machine (for example, a computer) that executes programs like a physical machine. Virtual machines are separated into two major classes, based on their use and degree of correspondence to any real machine:

• A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS). These usually emulate an existing architecture, and are built with the purpose of either providing a platform to run programs where the real hardware is not available for use (for example, executing on otherwise obsolete platforms), or of having multiple instances of virtual machines leading to more efficient use of computing resources, both in terms of energy consumption and cost effectiveness (known as hardware virtualization, the key to a cloud computing environment), or both.

• A process virtual machine (also, language virtual machine) is designed to run a single program, which means that it supports a single process. Such virtual machines are usually closely suited to one or more programming languages and built with the purpose of providing program portability and flexibility (amongst other things). An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine—it cannot break out of its virtual environment.

Virtual Memory

Virtual memory is a feature of an operating system (OS) that allows a computer to compensate for shortages of physical memory by temporarily transferring pages of data from random access memory (RAM) to disk storage.

Eventually, the OS will need to retrieve the data that was moved to temporarily to disk storage -- but remember, the only reason the OS moved pages of data from RAM to disk storage to begin with was because it was running out of RAM. To solve the problem, the operating system will need to move other pages to hard disk so it has room to bring back the pages it needs right away from temporary disk storage. This process is known as paging or swapping and the temporary storage space on the hard disk is called a pagefile or a swap file.

Swapping, which happens so quickly that the end user doesn't know it's happening, is carried out by the computer’s memory manager unit (MMU). The memory manager unit may use one of several algorithms to choose which page should be swapped out, including Least Recently Used (LRU), Least Frequently Used (LFU) or Most Recently Used (MRU).

Page Tables

Page tables are used to translate the virtual addresses seen by the application into physical addresses used by the hardware to process instructions; such hardware that handles this specific translation is often known as the memory management unit. Each entry in the page table holds a flag indicating whether the corresponding page is in real memory or not. If it is in real memory, the page table entry will contain the real memory address at which the page is stored. When a reference is made to a page by the hardware, if the page table entry for the page indicates that it is not currently in real memory, the hardware raises a page fault exception, invoking the paging supervisor component of the operating system.

Systems can have one page table for the whole system, separate page tables for each application and segment, a tree of page tables for large segments or some combination of these. If there is only one page table, different applications running at the same time use different parts of a single range of virtual addresses. If there are multiple page or segment tables, there are multiple virtual address spaces and concurrent applications with separate page tables redirect to different real addresses.

Thrashing

When paging and page stealing are used, a problem called "thrashing" can occur, in which the computer spends an unsuitably large amount of time transferring pages to and from a backing store, hence slowing down useful work. A task's working set is the minimum set of pages that should be in memory in order for it to make useful progress. Thrashing occurs when there is insufficient memory available to store the working sets of all active programs. Adding real memory is the simplest response, but improving application design, scheduling, and memory usage can help. Another solution is to reduce the number of active tasks on the system. This reduces demand on real memory by swapping out the entire working set of one or more processes