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60-449: PostGIS is implemented as a PostgreSQL external extension . The PostGIS implementation is based on "light-weight" geometries and indexes optimized to reduce disk and memory footprint . Using light-weight geometries helps servers increase the amount of data migrated up from physical disk storage into RAM, improving query performance substantially. PostGIS is registered as "implements the specified standard" for "Simple Features for SQL" by

120-472: A page frame 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

180-402: A constrained resource – so much so that developers typically sacrificed efficiency (processing speeds) just to make program footprints small enough to fit into the available RAM. For example, Sun Microsystems brought out a version of its Java Virtual Machine (JVM) for such constrained devices; it goes by the name of KVM . The KVM works on platforms where memory is in kilobytes as opposed to

240-489: 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 include writing other process pages to disk and reading pages of

300-474: 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

360-531: 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 do not 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. macOS uses multiple swap files. The default (and Apple-recommended) installation places them on

420-409: 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. When the system memory is highly insufficient for the current tasks and a large portion of memory activity goes through

480-460: 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 . Swap death can happen due to incorrectly configured memory overcommitment . 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

540-429: 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

600-463: A stable "1.0" version followed on April 19, 2005. In 2006 the OGC registered PostGIS as "implement[ing] the specified standard" for "Simple Features for SQL". Many software products can use PostGIS as a database backend, including: Memory footprint Memory footprint refers to the amount of main memory that a program uses or references while running. The word footprint generally refers to

660-427: 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

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720-411: 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. This view ignores

780-431: 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

840-459: 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

900-463: 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 a value from 0 to 200. 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

960-406: 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 support more virtual memory than physical memory. For instance, many programs may be running concurrently. Together, they may require more physical memory than can be installed on

1020-423: 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

1080-415: 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,

1140-399: Is analogous to a prefetch input queue in a CPU. Swap prefetching will prefetch recently swapped-out pages if there are enough free pages for them. If a program ends, the operating system may delay freeing its pages, in case the user runs the same program again. The free page queue is a list of page frames that are available for assignment. Preventing this queue from being empty minimizes

1200-482: 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. The Linux kernel supports

1260-451: Is enabled via the 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 ). 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

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1320-582: Is loaded indirectly when a Java application launches. In addition, on most operating systems, disk files opened by an application too are read into the application's address space, thereby, contributing to its footprint. During the 1990s, computer memory became cheaper and programs with larger memory footprints became commonplace. This trend has been mostly due to the widespread use of computer software, from large enterprise-wide applications that consume vast amounts of memory (such as databases ), to memory intensive multimedia authoring and editing software. To tackle

1380-498: 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. In most systems, the size of a process's virtual address space is much larger than the available main memory. For example: A computer with true n -bit addressing may have 2 addressable units of RAM installed. An example

1440-509: Is requested. (This is often in combination with pre-cleaning, which guesses which pages currently in RAM are not likely to be needed soon, and pre-writing them out to storage). When a page fault occurs, anticipatory paging systems will not only bring in the referenced page, but also other pages that are likely to be referenced soon. A simple anticipatory paging algorithm will bring in the next few consecutive pages even though they are not yet needed (a prediction using locality of reference ); this

1500-459: 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,

1560-434: 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. 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

1620-475: Is used up. 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

1680-605: Is used, pages are loaded only when they are referenced. A program from a memory mapped file begins execution with none of its pages in RAM. As the program commits page faults, the operating system copies the needed pages from a file, e.g., memory-mapped file , paging file, or a swap partition containing the page data into RAM. Some systems use only demand paging —waiting until a page is actually requested before loading it into RAM. Other systems attempt to reduce latency by guessing which pages not in RAM are likely to be needed soon, and pre-loading such pages into RAM, before that page

1740-931: The IBM M44/44X and its MOS operating system (1964), the SDS 940 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

1800-472: The TENEX operating system (1969). Those machines, and subsequent machines supporting memory paging, use either a set of page address registers or in-memory page tables 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 mapped to

1860-494: The megabytes (or even gigabytes ) of memory available on a regular home PC or more modern mobile phones and tablets. Memory paging In computer operating systems , memory paging (or swapping on some Unix-like systems) is a memory management scheme by which a computer stores and retrieves data from secondary storage for use in main memory . In this scheme, the operating system retrieves data from secondary storage in same-size blocks called pages . Paging

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1920-471: The OGC. PostGIS has not been certified as compliant by the OGC. May of 2001, at that point I was running a small consulting company here in Victoria, working for the provincial government, mostly working in the geospatial field. Because all of our contracts came from the provincial government, we were very tied to the cycle of when they would release and sign contracts. And the government changed that year, so all

1980-420: 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

2040-487: 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

2100-698: The cache and avoiding filesystem overhead. 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. Swappiness

2160-423: The civil servants were very risk averse in terms of signing new contract contracts. They didn't know whether what their budget situation was going to be that year. So we ended up having like three months where we got almost no direct revenue. We, you know, didn't all go on vacation for three months. We were kind of young, excited about the field. So we thought, you know, let's see if we can build something to store

2220-410: The computing necessary to service a page fault. Some operating systems periodically look for pages that have not been recently referenced and then free the page frame and add it to the free page queue, a process known as "page stealing". Some operating systems support page reclamation ; if a program commits a page fault by referencing a page that was stolen, the operating system detects this and restores

2280-413: The concepts are independent of the technology used. Depending on the memory model , paged memory functionality is usually hardwired into a CPU/MCU by using a Memory Management Unit (MMU) or Memory Protection Unit (MPU) and separately enabled by privileged system code in the operating system 's kernel . In CPUs implementing the x86 instruction set architecture (ISA) for instance, the memory paging

2340-454: The current process, etc., that the program ever needs while executing and will be loaded at least once during the entire run. Larger programs have larger memory footprints. An application's memory footprint is roughly proportionate to the number and sizes of shared libraries or classes it loads, whereas static libraries, executable programs and static data areas contribute to a fixed (constant) portion. Programs themselves often do not contribute

2400-415: 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

2460-603: The ever increasing memory needs, virtual memory systems were introduced that divide the available memory into equally sized portions and loads them from " pages " stored on the hard-disk on an as-and-when required basis. This approach to support programs with huge-memory-footprints has been quite successful. Most modern operating systems including Microsoft Windows , Apple's macOS , and all versions of Linux and Unix provide virtual memory systems. Traditionally, low-memory-footprint programs were of importance to running applications on embedded systems where memory would often be

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2520-597: The extent of physical dimensions that an object occupies, giving a sense of its size. In computing, the memory footprint of a software application indicates its runtime memory requirements, while the program executes. This includes all sorts of active memory regions like code segment containing (mostly) program instructions (and occasionally constants), data segment (both initialized and uninitialized), heap memory , call stack , plus memory required to hold any additional data structures, such as symbol tables , debugging data structures , open files, shared libraries mapped to

2580-434: 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 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. In some of those systems, it

2640-484: 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"

2700-441: 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. In 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

2760-441: The largest portions to their own memory footprints; rather, structures introduced by the run-time environment take up most of the memory. For example, a C++ compiler inserts vtables , type info objects and many temporary and anonymous objects that are active during a program's execution. In a Java program, the memory footprint is predominantly made up of the runtime environment in the form of Java virtual machine (JVM) itself that

2820-442: 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. 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. The common advice given to avoid this

2880-430: 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

2940-600: 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, and may steal page frames from pages that have been unreferenced for a long time, making them available. Some systems clear new pages to avoid data leaks that compromise security; some set them to installation defined or random values to aid debugging. When pure demand paging

3000-572: The page frame without having to read the contents back into RAM. The operating system may periodically pre-clean dirty pages: write modified pages back to disk even though they might be further modified. This minimizes the amount of cleaning needed to obtain new page frames at the moment a new program starts or a new data file is opened, and improves responsiveness. (Unix operating systems periodically use sync to pre-clean all dirty pages; Windows operating systems use "modified page writer" threads.) After completing initialization, most programs operate on

3060-481: 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

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3120-482: The root partition, though it is possible to place them instead on a separate partition or device. AmigaOS 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

3180-539: 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 means of communication between programs. Programs can share pages in memory, and then write and read to exchange data. The first computer to support paging

3240-452: 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. The file used for paging in the Windows NT family is pagefile.sys . The default location of the page file is in

3300-480: The spatial data in a database. And we had used Postgres for one of our data processing projects the year previous, so we were kind of familiar with it. And that experimental work in the spring of 2001 ended up being released at the end of May 2001 as PostGIS version 0.1. Refractions Research released the first version of PostGIS in 2001 under the GNU General Public License . After six release candidates ,

3360-500: The underlying devices can be efficiently accessed in parallel. From 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

3420-628: 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 . Ferranti 's Atlas , and the Atlas Supervisor developed at the University of Manchester , (1962), was the first system to implement memory paging. Subsequent early machines, and their operating systems, supporting paging include

3480-454: 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 (even if it does not exist). 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

3540-402: 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

3600-402: Was the supercomputer Atlas , 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 handled non-equivalence interruptions and managed the transfer of pages between core and drum in order to provide a one-level store to programs. Paging has been

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