Virtual memory pdf

Processes are then allocated just enough pages to meet their memory requirements. That means that there will always be a small amount of memory wasted, except in the unusual case where a process requires exactly a whole number of pages.

During the normal course of operations, pages i. Virtual Memory Segmentation Segmentation is an alternative approach to memory management, where instead of pages of a fixed size, processes are allocated segments of differing length to exactly meet their requirements.

That means that unlike in a paged system, no memory is wasted in a segment. Segmentation also allows applications to be split up into logically independent address spaces, which can make them easier to share, and more secure. But a problem with segmentation is that because each segment is a different length, it can lead to memory "fragmentation. As these small chunks build up, fewer and fewer segments of useful size can be allocated. And if the OS does start using these small segments then there are a huge number to keep track of, and each process will need to use many different segments, which is inefficient and can reduce performance.

Advantages and Disadvantages of Virtual Memory Even though RAM is now relatively inexpensive compared to its cost when virtual memory was first developed, it is still extremely useful and it is still employed in many, perhaps most, computer systems. The key problem with virtual memory relates to performance. Abhishek Kandel V. When an application including the operating system is running, it stores the location of program threads and other data at a virtual address, while the data is actually stored at a physical address in RAM.

If later that RAM space is needed more urgently by another process, then the data may be swapped out of RAM and into virtual memory. The responsibility for keeping track of all this data as it is swapped between physical and virtual memory falls to the computer's memory manager. The memory manager maintains a table which maps virtual addresses used by the operating system and applications to the physical addresses that data is actually stored in.

When data is swapped between RAM and virtual memory, the table is updated so that a given virtual address always points to the correct physical location. A computer can only run threads and manipulate data that is stored in RAM rather than virtual memory. And it takes a non-negligible amount of time to swap data that is needed into RAM. Consequently, it follows that using virtual memory involves a performance hit.

One potential problem with virtual memory is that if the amount of RAM present is too small compared to the amount of virtual memory then a system can end up spending a large proportion of its CPU resources swapping data back and forth. Operating systems, such as most versions of Windows, generally recommend that users do not increase virtual memory beyond 1.

Moving data to paging file can free up the RAM so your computer can complete its work. In general, the more RAM your computer has, the faster the programs run. If the computer is running pretty slow since there is little free space on the RAM, you can increase virtual memory to fix the problem.

However, your computer can read data from RAM much more quickly than from a hard disk. So increasing the capacity of RAM is a better solution. Virtual memory is, in fact, a giant file called PageFile. Sys, which is invisible in most cases. To see the file, you need to open the Explorer and then close the system file protection function. Recommend: You should not delete the Page Files. Memory will play a big role during the process of running computer because computer needs it to execute all programs.

Thus, Windows would like to use virtual memory to improve computer's memory. SYS file. Since Linux needs complicated commands, we take Windows 10 for example to show how to set virtual memory. Here, the recommended size is equivalent to 1. In fact, setting virtual memory size strictly according to the 1. We had better set it according to actual needs. In the process of setting virtual memory size, you may turn to the Windows built-in performance monitor taking Windows 10 as an example.

In the upper left box, find and expand "Paging File". Then, click "OK" button. Then, click "Next". Right-click on it and choose "Start". After using it for a few hours, the new data collector set can make a basic and complete assessment of your usage. Under "paging file", there is a report about virtual memory usage. Click this report to view it. Therefore, the maximum size of the paging file is set to MB to save free space. It's also important to notice that you had better put the paging file on the drive which does not contain operating system or applications if you have more than one disk.

Keeping applications and paging files on different disks not only can minimize the usage of disk but also can improve the efficiency of disk. However, if you have only one disk and you put the paging files on a different volume in the same disk, it will not improve the efficiency of disk. The hard disk may contain Disk Fragmentation since its files are written in a non-sequential way. Therefore, if the paging file or the virtual memory partition is patchy, the disk seeking time may be increased but disk performance may be decreased.

Some people think that there will be disk fragments if Windows frequently read the pagefile. Actually, it's not. Once pagefile. Reading and writing file will not influence the size of pagefile.

However, the system will create temppf. In Linux operating system, if you divide a separate partition to act as the memory, you can basically eliminate the effects brought by disk fragmentation. In general, Windows XP uses the free space of C drive to act as the virtual memory by default. Thus, the larger free space the C drive has, the faster the system runs. Virtual memory will change dynamically as you use the C drive. Therefore, the C drive is easy to generate disk fragmentation to affect system operation speed.

So well, you had better set another volume like D: as the virtual memory. Here, we take Windows XP for example. After that, you can see the Virtual Memory dialog box. As a matter of fact, you had better set a fixed value for the virtual memory to avoid disk fragmentation. And the specific values depend on your physical memory size.