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Improve Article Save Article Like Article A computer has a sufficient amount of physical memory but most of the time we need more so we swap some memory on disk. Swap space is a space on a hard disk that is a substitute for physical memory. It is used as virtual memory which contains process memory images. Whenever our computer runs short of physical memory it uses its virtual memory and stores information in memory on disk. Swap space helps the computer’s operating system in pretending that it has more RAM than it actually has. It is also called a swap file. This interchange of data between virtual memory and real memory is called swapping and space on disk as “swap space”. Virtual memory is a combination of RAM and disk space that running processes can use. Swap space is the portion of virtual memory that is on the hard disk, used when RAM is full. Swap space can be useful to computers in various ways:
Operating systems such as Windows, Linux, etc systems provide a certain amount of swap space by default which can be changed by users according to their needs. If you don’t want to use virtual memory you can easily disable it all together but in case if you run out of memory then the kernel will kill some of the processes in order to create a sufficient amount of space in physical memory. So it totally depends upon the user whether he wants to use swap space or not.
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For every operating system, there is a dedicated amount of RAM available that makes the processing of a program possible. However, the amount of this RAM is limited which is why RAM cannot hold a bulk of data in it. Therefore, there should be a backup option available which can support RAM whenever it runs out of memory. This concept holds for the Windows operating system as well as for Linux. In Windows OS, whenever RAM has an insufficient amount of memory to hold a process, it borrows some amount of memory from the secondary storage. This borrowed memory is known as Virtual Memory. Similarly, whenever RAM runs out of memory in Linux, it borrows some memory from the secondary storage to store its inactive content. In this way, the RAM finds sufficient space to hold a new process within it. Here, the borrowed space from the hard disk is called Swap Memory. In this article, we will try to learn the concept of swap memory in detail. Working of Swap Memory:As explained above, swap memory is the dedicated amount of hard drive that is used whenever RAM runs out of memory. There is a memory management program in Linux that takes care of this process. Whenever RAM is short of memory, the memory management program looks for all those inactive blocks of data present in RAM that have not been used for a long time. When it successfully finds those blocks, it shifts them into the swap memory. In this way, the space of RAM is freed up and hence it can be utilized for some other programs that need processing on an urgent basis. The concept of swapping is very much similar to the concept of paging used in the Windows operating system. Types of Swap Memory:Typically there are two different types of swap memory which are mentioned below:
What should be the Ideal Frequency of Swapping?Linux allows us to set the frequency of swapping on our own i.e. how frequently the process of swapping should take place. You can set the value of swapping between 0 and 100 depending upon your requirements. A low-frequency value of swapping means that the process of swapping will take place very rarely only when it is needed whereas a high-frequency value of swapping means that the swapping process will occur quite often. However, the default and recommended value of swapping frequency is 60. Benefits of using Swap Memory:By learning the working of swap memory, we can easily perceive the benefits of using it. However, some of the main benefits of using swap memory are listed below:
Conclusion:In this article, we have learned the usage and working swap memory along with its numerous benefits. Swap memory acts as a backup option for RAM when it runs short of space. We all know that we cannot have an infinite amount of RAM however; we do realize that today’s high-end applications require a large amount of RAM to operate smoothly. Therefore, we must have a sufficient amount of RAM to avoid our applications from crashing. Also, there is a cost associated with adding in more RAM whereas there is no cost of using swap memory. Moreover, additional RAM can also be plugged in up to a certain limit depending upon your hardware. Hence, the only option we are left with is to use swap memory which can make our system work very efficiently without any cost. This article needs attention from an expert in computing. See the talk page for details.(June 2019)
In computer operating systems, memory paging is a memory management scheme by which a computer stores and retrieves data from secondary storage[a] for use in main memory.[citation needed] In this scheme, the operating system retrieves data from secondary storage in same-size blocks called pages. Paging is an important part of virtual memory implementations in modern operating systems, using secondary storage to let programs exceed the size of available physical memory.
For simplicity, main memory is called "RAM" (an acronym of random-access memory) and secondary storage is called "disk" (a shorthand for hard disk drive, drum memory or solid-state drive, etc.), but as with many aspects of computing, the concepts are independent of the technology used.
Depending on a memory model, paged memory functionality is usually hardwired into a CPU/MCU by using Memory Management Unit (MMU) or Memory Protection Unit (MPU) and separately enabled by priviliged system code often called kernel. In CPUs implementing x86 instruction set architecture (ISA) for instance, the memory paging is enabled via CR0 control register.
In the 1960s, swapping was an early virtual memory technique. An entire program or entire segment would be "swapped out" (or "rolled out") from RAM to disk or drum, and another one would be swapped in (or rolled in).[1][2] A swapped-out program would be current but its execution would be suspended while its RAM was in use by another program; a program with a swapped-out segment could continue running until it needed that segment, at which point it would be suspended until the segment was swapped in.
A program might include multiple overlays that occupy the same memory at different times. Overlays are not a method of paging RAM to disk but merely of minimizing the program's RAM use. Subsequent architectures used memory segmentation, and individual program segments became the units exchanged between disk and RAM. A segment was the program's entire code segment or data segment, or sometimes other large data structures. These segments had to be contiguous when resident in RAM, requiring additional computation and movement to remedy fragmentation.[3] Ferranti's Atlas, and the Atlas Supervisor developed at the University of Manchester,[4] (1962), was the first system to implement memory paging. Subsequent early machines, and their operating systems, supporting paging include the IBM M44/44X and its MOS operating system (1964),[5], the SDS 940[6] and the Berkeley Timesharing System (1966), a modified IBM System/360 Model 40 and the CP-40 operating system (1967), the IBM System/360 Model 67 and operating systems such as TSS/360 and CP/CMS (1967), the RCA 70/46 and the Time Sharing Operating System (1967), the GE 645 and Multics (1969), and the PDP-10 with added BBN-designed paging hardware and the TENEX operating system (1969). Those machines, and subsequent machines supporting memory paging, use either a set of page address registers or an in-memory page table to allow the processor to operate on arbitrary pages anywhere in RAM as a seemingly contiguous logical address space. These pages became the units exchanged between disk and RAM. When a process tries to reference a page not currently present in RAM, the processor treats this invalid memory reference as a page fault and transfers control from the program to the operating system. The operating system must:
When all page frames are in use, the operating system must select a page frame to reuse for the page the program now needs. If the evicted page frame was dynamically allocated by a program to hold data, or if a program modified it since it was read into RAM (in other words, if it has become "dirty"), it must be written out to disk before being freed. If a program later references the evicted page, another page fault occurs and the page must be read back into RAM. The method the operating system uses to select the page frame to reuse, which is its page replacement algorithm, is important to efficiency. The operating system predicts the page frame least likely to be needed soon, often through the least recently used (LRU) algorithm or an algorithm based on the program's working set. To further increase responsiveness, paging systems may predict which pages will be needed soon, preemptively loading them into RAM before a program references them. After completing initialization, most programs operate on a small number of code and data pages compared to the total memory the program requires. The pages most frequently accessed are called the working set. When the working set is a small percentage of the system's total number of pages, virtual memory systems work most efficiently and an insignificant amount of computing is spent resolving page faults. As the working set grows, resolving page faults remains manageable until the growth reaches a critical point. Then faults go up dramatically and the time spent resolving them overwhelms time spent on the computing the program was written to do. This condition is referred to as thrashing. Thrashing occurs on a program that works with huge data structures, as its large working set causes continual page faults that drastically slow down the system. Satisfying page faults may require freeing pages that will soon have to be re-read from disk. "Thrashing" is also used in contexts other than virtual memory systems; for example, to describe cache issues in computing or silly window syndrome in networking. A worst case might occur on VAX processors. A single MOVL crossing a page boundary could have a source operand using a displacement deferred addressing mode, where the longword containing the operand address crosses a page boundary, and a destination operand using a displacement deferred addressing mode, where the longword containing the operand address crosses a page boundary, and the source and destination could both cross page boundaries. This single instruction references ten pages; if not all are in RAM, each will cause a page fault. As each fault occurs the operating system needs to go through the extensive memory management routines perhaps causing multiple I/Os which might including writing other process pages to disk and reading pages of the active process from disk. If the operating system could not allocate ten pages to this program, then remedying the page fault would discard another page the instruction needs, and any restart of the instruction would fault again. To decrease excessive paging and resolve thrashing problems, a user can increase the number of pages available per program, either by running fewer programs concurrently or increasing the amount of RAM in the computer. In multi-programming or in a multi-user environment, many users may execute the same program, written so that its code and data are in separate pages. To minimize RAM use, all users share a single copy of the program. Each process's page table is set up so that the pages that address code point to the single shared copy, while the pages that address data point to different physical pages for each process. Different programs might also use the same libraries. To save space, only one copy of the shared library is loaded into physical memory. Programs which use the same library have virtual addresses that map to the same pages (which contain the library's code and data). When programs want to modify the library's code, they use copy-on-write, so memory is only allocated when needed. Shared memory is an efficient way of communication between programs. Programs can share pages in memory, and then write and read to exchange data. The first computer to support paging was the supercomputer Atlas,[7][8][9] jointly developed by Ferranti, the University of Manchester and Plessey in 1963. The machine had an associative (content-addressable) memory with one entry for each 512 word page. The Supervisor[10] handled non-equivalence interruptions[c] and managed the transfer of pages between core and drum in order to provide a one-level store[11] to programs. Microsoft WindowsWindows 3.x and Windows 9xPaging has been a feature of Microsoft Windows since Windows 3.0 in 1990. Windows 3.x creates a hidden file named 386SPART.PAR or WIN386.SWP for use as a swap file. It is generally found in the root directory, but it may appear elsewhere (typically in the WINDOWS directory). Its size depends on how much swap space the system has (a setting selected by the user under Control Panel → Enhanced under "Virtual Memory"). If the user moves or deletes this file, a blue screen will appear the next time Windows is started, with the error message "The permanent swap file is corrupt". The user will be prompted to choose whether or not to delete the file (whether or not it exists). Windows 95, Windows 98 and Windows Me use a similar file, and the settings for it are located under Control Panel → System → Performance tab → Virtual Memory. Windows automatically sets the size of the page file to start at 1.5× the size of physical memory, and expand up to 3× physical memory if necessary. If a user runs memory-intensive applications on a system with low physical memory, it is preferable to manually set these sizes to a value higher than default. Windows NTThe file used for paging in the Windows NT family is pagefile.sys. The default location of the page file is in the root directory of the partition where Windows is installed. Windows can be configured to use free space on any available drives for page files. It is required, however, for the boot partition (i.e., the drive containing the Windows directory) to have a page file on it if the system is configured to write either kernel or full memory dumps after a Blue Screen of Death. Windows uses the paging file as temporary storage for the memory dump. When the system is rebooted, Windows copies the memory dump from the page file to a separate file and frees the space that was used in the page file.[12] Fragmentation
In the default configuration of Windows, the page file is allowed to expand beyond its initial allocation when necessary. If this happens gradually, it can become heavily fragmented which can potentially cause performance problems.[13] The common advice given to avoid this is to set a single "locked" page file size so that Windows will not expand it. However, the page file only expands when it has been filled, which, in its default configuration, is 150% of the total amount of physical memory.[citation needed] Thus the total demand for page file-backed virtual memory must exceed 250% of the computer's physical memory before the page file will expand. The fragmentation of the page file that occurs when it expands is temporary. As soon as the expanded regions are no longer in use (at the next reboot, if not sooner) the additional disk space allocations are freed and the page file is back to its original state. Locking a page file size can be problematic if a Windows application requests more memory than the total size of physical memory and the page file, leading to failed requests to allocate memory that may cause applications and system processes to fail. Also, the page file is rarely read or written in sequential order, so the performance advantage of having a completely sequential page file is minimal. However, a large page file generally allows the use of memory-heavy applications, with no penalties besides using more disk space. While a fragmented page file may not be an issue by itself, fragmentation of a variable size page file will over time create several fragmented blocks on the drive, causing other files to become fragmented. For this reason, a fixed-size contiguous page file is better, providing that the size allocated is large enough to accommodate the needs of all applications. The required disk space may be easily allocated on systems with more recent specifications (i.e. a system with 3 GB of memory having a 6 GB fixed-size page file on a 750 GB disk drive, or a system with 6 GB of memory and a 16 GB fixed-size page file and 2 TB of disk space). In both examples, the system uses about 0.8% of the disk space with the page file pre-extended to its maximum. Defragmenting the page file is also occasionally recommended to improve performance when a Windows system is chronically using much more memory than its total physical memory.[citation needed] This view ignores the fact that, aside from the temporary results of expansion, the page file does not become fragmented over time. In general, performance concerns related to page file access are much more effectively dealt with by adding more physical memory. Unix and Unix-like systemsUnix systems, and other Unix-like operating systems, use the term "swap" to describe the act of substituting disk space for RAM when physical RAM is full.[14] In some of those systems, it is common to dedicate an entire partition of a hard disk to swapping. These partitions are called swap partitions. Many systems have an entire hard drive dedicated to swapping, separate from the data drive(s), containing only a swap partition. A hard drive dedicated to swapping is called a "swap drive" or a "scratch drive" or a "scratch disk". Some of those systems only support swapping to a swap partition; others also support swapping to files. LinuxThe Linux kernel supports a virtually unlimited number of swap backends (devices or files), and also supports assignment of backend priorities. When the kernel swaps pages out of physical memory, it uses the highest-priority backend with available free space. If multiple swap backends are assigned the same priority, they are used in a round-robin fashion (which is somewhat similar to RAID 0 storage layouts), providing improved performance as long as the underlying devices can be efficiently accessed in parallel.[15] Swap files and partitionsFrom the end-user perspective, swap files in versions 2.6.x and later of the Linux kernel are virtually as fast as swap partitions; the limitation is that swap files should be contiguously allocated on their underlying file systems. To increase performance of swap files, the kernel keeps a map of where they are placed on underlying devices and accesses them directly, thus bypassing the cache and avoiding filesystem overhead.[16][17] Regardless, Red Hat recommends swap partitions to be used.[18] When residing on HDDs, which are rotational magnetic media devices, one benefit of using swap partitions is the ability to place them on contiguous HDD areas that provide higher data throughput or faster seek time. However, the administrative flexibility of swap files can outweigh certain advantages of swap partitions. For example, a swap file can be placed on any mounted file system, can be set to any desired size, and can be added or changed as needed. Swap partitions are not as flexible; they cannot be enlarged without using partitioning or volume management tools, which introduce various complexities and potential downtimes. SwappinessSwappiness is a Linux kernel parameter that controls the relative weight given to swapping out of runtime memory, as opposed to dropping pages from the system page cache, whenever a memory allocation request cannot be met from free memory. Swappiness can be set to values between 0 and 200 (inclusive).[19] A low value causes the kernel to prefer to evict pages from the page cache while a higher value causes the kernel to prefer to swap out "cold" memory pages. The default value is 60; setting it higher can cause high latency if cold pages need to be swapped back in (when interacting with a program that had been idle for example), while setting it lower (even 0) may cause high latency when files that had been evicted from the cache need to be read again, but will make interactive programs more responsive as they will be less likely to need to swap back cold pages. Swapping can also slow down HDDs further because it involves a lot of random writes, while SSDs do not have this problem. Certainly the default values work well in most workloads, but desktops and interactive systems for any expected task may want to lower the setting while batch processing and less interactive systems may want to increase it.[20] Swap deathWhen the system memory is highly insufficient for the current tasks and a large portion of memory activity goes through a slow swap, the system can become practically unable to execute any task, even if the CPU is idle. When every process is waiting on the swap, the system is considered to be in swap death.[21][22] Swap death can happen due to incorrectly configured memory overcommitment.[23][24][25] The original description of the "swapping to death" problem relates to the X server. If code or data used by the X server to respond to a keystroke is not in main memory, then if the user enters a keystroke, the server will take one or more page faults, requiring those pages to read from swap before the keystroke can be processed, slowing the response to it. If those pages don't remain in memory, they will have to be faulted in again to handle the next keystroke, making the system practically unresponsive even if it's actually executing other tasks normally.[26] macOSmacOS uses multiple swap files. The default (and Apple-recommended) installation places them on the root partition, though it is possible to place them instead on a separate partition or device.[27] AmigaOS 4AmigaOS 4.0 introduced a new system for allocating RAM and defragmenting physical memory. It still uses flat shared address space that cannot be defragmented. It is based on slab allocation method and paging memory that allows swapping. Paging was implemented in AmigaOS 4.1 but may lock up system if all physical memory is used up.[28] Swap memory could be activated and deactivated any moment allowing the user to choose to use only physical RAM. The backing store for a virtual memory operating system is typically many orders of magnitude slower than RAM. Additionally, using mechanical storage devices introduces delay, several milliseconds for a hard disk. Therefore, it is desirable to reduce or eliminate swapping, where practical. Some operating systems offer settings to influence the kernel's decisions.
Many Unix-like operating systems (for example AIX, Linux, and Solaris) allow using multiple storage devices for swap space in parallel, to increase performance. Swap space sizeIn some older virtual memory operating systems, space in swap backing store is reserved when programs allocate memory for runtime data. Operating system vendors typically issue guidelines about how much swap space should be allocated. Paging is one way of allowing the size of the addresses used by a process, which is the process's "virtual address space" or "logical address space", to be different from the amount of main memory actually installed on a particular computer, which is the physical address space. Main memory smaller than virtual memoryIn most systems, the size of a process's virtual address space is much larger than the available main memory.[31] For example:
Main memory the same size as virtual memoryA computer with true n-bit addressing may have 2n addressable units of RAM installed. An example is a 32-bit x86 processor with 4 GB and without Physical Address Extension (PAE). In this case, the processor is able to address all the RAM installed and no more. However, even in this case, paging can be used to create a virtual memory of over 4 GB. For instance, many programs may be running concurrently. Together, they may require more than 4 GB, but not all of it will have to be in RAM at once. A paging system makes efficient decisions on which memory to relegate to secondary storage, leading to the best use of the installed RAM. Although the processor in this example cannot address RAM beyond 4 GB, the operating system may provide services to programs that envision a larger memory, such as files that can grow beyond the limit of installed RAM. The operating system lets a program manipulate data in the file arbitrarily, using paging to bring parts of the file into RAM when necessary. Main memory larger than virtual address spaceA few computers have a main memory larger than the virtual address space of a process, such as the Magic-1,[31] some PDP-11 machines, and some systems using 32-bit x86 processors with Physical Address Extension. This nullifies a significant advantage of paging, since a single process cannot use more main memory than the amount of its virtual address space. Such systems often use paging techniques to obtain secondary benefits:
The size of the cumulative total of virtual address spaces is still limited by the amount of secondary storage available.
Wikisource has original text related to this article: The Paging Game
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