Virtual memory uses disk space as an extension of main memory. Since disk access is much slower than memory access, using swap space has a large effect on system performance.
Swap space can reside at two places :
- It can be carved out of the normal file system : In this case, normal file system routines can be used to create it, name it, and to allocate its space. This approach is easy to implement but inefficient because navigating the directory structure and data structures takes time, and extra disk access. In addition to this, external fragmentation greatly increases swapping times. Performance can be improved by caching the block location information in physical memory but the cost of traversing the file system data structures still remains.
- It can be on a separate disk partition : There is no directory structure or file system placed on this space, a separate swap-space storage manager is used to allocate and deallocate the blocks. This manager uses algorithms that are optimized for speed, rather than for storage efficiency. Internal fragmentation may increase but this is acceptable as data in swap space resides for much shorter period. This approach creates a fixed amount of swap space during disk partitioning, adding more space can only be done via repartitioning of the disk.
Tuesday, February 2, 2010
Where is swap space located ?
Monday, February 1, 2010
How to use swap - space ?
Swap space is an area on a high-speed storage device (almost always a disk drive), reserved for use by the virtual memory system for deactivation and paging processes. At least one swap device (primary swap) must be present on the system. Virtual memory uses disk space as an extension of main memory and since memory access is faster than disk access, using swap space has a large effect on system performance.
It is perfectly normal for the swap file or page file to grow in size, sometimes growing several hundred megs in size. Below is a listing of common Microsoft operating system swap file information; however, it is important to realize that this information may vary. Finally, by default the swap files are hidden.
Operating system Swap file name Location
Windows 3.x 386PART.PAR C:\WINDOWS
Windows 95 / 98 / ME WIN386.SWP C:\
Windows NT / 2000 / XP PAGEFILE.SYS C:\
Swap space is used in various ways by different operating systems :
- Systems implementing swapping may use swap space to hold entire process image, including the code and data segments.
- The pages that are moved out of main memory can be stored by paging systems.
- Some operating systems like UNIX allow use of multiple swap spaces. These swap spaces are generally put on separate disks, so the load placed on the I/O system by paging and swapping can be spread over the system's I/O devices.
The amount of swap space needed on a system can vary depending on the amount of physical memory, the amount of virtual memory it is backing, and the way in which the virtual memory is used.
