virtual memory

[Former Member]

hello

what is the importance of virtual memory?

[Former Member · Accepted solution]

Virtual memory is a common part of most operating systems on desktop computers. It has become so common because it provides a big benefit for users at a very low cost.

In this article, you will learn exactly what virtual memory is, what your computer uses it for and how to configure it on your own machine to achieve optimal performance.

Most computers today have something like 32 or 64 megabytes of RAM available for the CPU to use (see How RAM Works for details on RAM). Unfortunately, that amount of RAM is not enough to run all of the programs that most users expect to run at once.

For example, if you load the operating system, an e-mail program, a Web browser and word processor into RAM simultaneously, 32 megabytes is not enough to hold it all. If there were no such thing as virtual memory, then once you filled up the available RAM your computer would have to say, "Sorry, you can not load any more applications. Please close another application to load a new one." With virtual memory, what the computer can do is look at RAM for areas that have not been used recently and copy them onto the hard disk. This frees up space in RAM to load the new application.

Because this copying happens automatically, you don't even know it is happening, and it makes your computer feel like is has unlimited RAM space even though it only has 32 megabytes installed. Because hard disk space is so much cheaper than RAM chips, it also has a nice economic benefit.

The read/write speed of a hard drive is much slower than RAM, and the technology of a hard drive is not geared toward accessing small pieces of data at a time. If your system has to rely too heavily on virtual memory, you will notice a significant performance drop. The key is to have enough RAM to handle everything you tend to work on simultaneously -- then, the only time you "feel" the slowness of virtual memory is is when there's a slight pause when you're changing tasks. When that's the case, virtual memory is perfect.

When it is not the case, the operating system has to constantly swap information back and forth between RAM and the hard disk. This is called thrashing, and it can make your computer feel incredibly slow.

The area of the hard disk that stores the RAM image is called a page file. It holds pages of RAM on the hard disk, and the operating system moves data back and forth between the page file and RAM. On a Windows machine, page files have a .SWP extension.

[Former Member]

Hi,

Pls see the link

http://help.sap.com/saphelp_nw04/helpdata/en/66/380fb7d43d11d188bd0000e83539c3/content.htm

For 64 bit kernel SAP suggest 20 GB swap space which is necessary and avoid many low memory dump error.

see the Note 146289.

Regards

Tom

[Esha1]

Virtual memory is swap space and physical RAM. The purpose of virtual memory is to enlarge the address space, the set of addresses a program can utilize.

A large virtual memory enables

1. A process to be larger than physical memory

2. A process to execute even if all of the process is not in memory

3. More processes than in memory to run concurrently

Demand Paging uses memory as a cache for disk

For each page the page table either says:

Page is in memory, and here is the frame number

Page is on disk, and here is the block number

The valid bit is used to distinguish between these two cases.

Hope this helps!!

Thanks and Regards

Esha Rajpal

[Former Member]

Hi nuzhath javeria,

Check this link.

http://help.sap.com/erp2005_ehp_03/helpdata/EN/22/5d980297cb48129e663b0d4a3e844d/frameset.htm

Regards

Ashok

[Former Member]

One of the most important of all concepts related to Memory Management is Virtual Memory. Virtual Memory refers to the concept whereby a process with a larger size than available memory can be loaded and executed by loading the process in parts. The program memory is divided into pages and the available physical memory into frames. If a process attempts to access a page that is not available in the main memory and the information of which does not exist in its page table, a page fault occurs. The Operating System now takes care of swapping this page in to the main memory from the backing store. The Operating System follows its predefined algorithms for page replacement. Demand Paging refers to loading a page of program code from disk into memory as and when it is required by the program. The region of the backing store that is used to store the swapped out pages from the main memory is called swap space. This article discusses Virtual Memory with special reference to Demand Paging and sheds light on some the most commonly used page replacement strategies that are adopted by the Operating Systems.

you can check this out in tcode ST06

Hope this will help you in understanding the comcepts..

if your question is answered please mark it answered and award points

cheers

dEE