Real-time computing
This articleneeds additional citations forverification.(April 2014) |
Real-time computing(RTC) is thecomputer scienceterm forhardwareandsoftwaresystems subject to a "real-time constraint", for example fromeventtosystem response.[1]Real-time programs must guarantee response within specified time constraints, often referred to as "deadlines".[2]
The term "real-time" is also used insimulationto mean that the simulation's clock runs at the same speed as a real clock.
Real-time responses are often understood to be in the order of milliseconds, and sometimes microseconds. A system not specified as operating in real time cannot usuallyguaranteea response within any timeframe, althoughtypicalorexpectedresponse times may be given. Real-time processingfailsif not completed within a specified deadline relative to an event; deadlines must always be met, regardless ofsystem load.
A real-time system has been described as one which "controls an environment by receiving data, processing them, and returning the results sufficiently quickly to affect the environment at that time".[3]The term "real-time" is used inprocess controlandenterprise systemsto mean "without significant delay".
Real-time software may use one or more of the following:synchronous programming languages,real-time operating systems(RTOSes), and real-time networks, each of which provide essential frameworks on which to build a real-time software application.
Systems used for manysafety-criticalapplications must be real-time, such as for control offly-by-wireaircraft, oranti-lock brakes,both of which demand immediate and accurate mechanical response.[4]
History
[edit]The termreal-timederives from its use in earlysimulation,in which a real-world process is simulated at a rate that matched that of the real process (now calledreal-time simulationto avoid ambiguity).Analog computers,most often, were capable of simulating at a much faster pace than real-time, a situation that could be just as dangerous as a slow simulation if it were not also recognized and accounted for.
Minicomputers, particularly in the 1970s onwards, when built into dedicatedembedded systemssuch as DOG (Digital on-screen graphic) scanners, increased the need for low-latency priority-driven responses to important interactions with incoming data and so operating systems such asData General'sRDOS (Real-Time Disk Operating System)and RTOS withbackground and foreground schedulingas well asDigital Equipment Corporation'sRT-11date from this era. Background-foreground scheduling allowed low priority tasks CPU time when no foreground task needed to execute, and gave absolute priority within the foreground to threads/tasks with the highest priority. Real-time operating systems would also be used fortime-sharingmultiuser duties. For example,Data General Business Basiccould run in the foreground or background of RDOS and would introduce additional elements to the scheduling algorithm to make it more appropriate for people interacting viadumb terminals.
Early personal computers were sometimes used for real-time computing. The possibility of deactivating other interrupts allowed for hard-coded loops with defined timing, and the lowinterrupt latencyallowed the implementation of a real-time operating system, giving the user interface and the disk drives lower priority than the real-time thread. Compared to these theprogrammable interrupt controllerof the Intel CPUs (8086..80586) generates a very large latency and the Windows operating system is neither a real-time operating system nor does it allow a program to take over the CPU completely and use its ownscheduler,without using native machine language and thus bypassing all interrupting Windows code. However, several coding libraries exist which offer real time capabilities in a high level language on a variety of operating systems, for exampleJava Real Time.Later microprocessors such as theMotorola 68000and subsequent family members (68010, 68020,ColdFireetc.) also became popular with manufacturers of industrial control systems. This application area is one in which real-time control offers genuine advantages in terms of process performance and safety.[citation needed]
Criteria for real-time computing
[edit]A system is said to bereal-timeif the total correctness of an operation depends not only upon its logical correctness, but also upon the time in which it is performed.[5]Real-time systems, as well as their deadlines, are classified by the consequence of missing a deadline:[6]
- Hard– missing a deadline is a total system failure.
- Firm– infrequent deadline misses are tolerable, but may degrade the system's quality of service. The usefulness of a result is zero after its deadline.
- Soft– the usefulness of a result degrades after its deadline, thereby degrading the system's quality of service.
Thus, the goal of ahard real-time systemis to ensure that all deadlines are met, but forsoft real-time systemsthe goal becomes meeting a certain subset of deadlines in order to optimize some application-specific criteria. The particular criteria optimized depend on the application, but some typical examples include maximizing the number of deadlines met, minimizing the lateness of tasks and maximizing the number of high priority tasks meeting their deadlines.
Hard real-time systemsare used when it is imperative that an event be reacted to within a strict deadline. Such strong guarantees are required of systems for which not reacting in a certain interval of time would cause great loss in some manner, especially damaging the surroundings physically or threatening human lives (although the strict definition is simply that missing the deadline constitutes failure of the system). Some examples of hard real-time systems:
- Acarenginecontrol system is a hard real-time system because a delayed signal may cause engine failure or damage.
- Medical systems such as heartpacemakers.Even though a pacemaker's task is simple, because of the potential risk to human life, medical systems like these are typically required to undergo thorough testing and certification, which in turn requires hard real-time computing in order to offer provable guarantees that a failure is unlikely or impossible.
- Industrial process controllers, such as a machine on anassembly line.If the machine is delayed, the item on the assembly line could pass beyond the reach of the machine (leaving the product untouched), or the machine or the product could be damaged by activating the robot at the wrong time. If the failure is detected, both cases would lead to the assembly line stopping, which slows production. If the failure is not detected, a product with a defect could make it through production, or could cause damage in later steps of production.
- Hard real-time systems are typically found interacting at a low level with physical hardware, inembedded systems.Early video game systems such as theAtari 2600andCinematronicsvector graphics had hard real-time requirements because of the nature of the graphics and timing hardware.
- Softmodemsreplace a hardware modem with software running on a computer'sCPU.The software must run every few milliseconds to generate the next audio data to be output. If that data is late, the receiving modem will lose synchronization, causing a long interruption as synchronization is reestablished or causing the connection to be lost entirely.
- Many types ofprintershave hard real-time requirements, such asinkjets(the ink must be deposited at the correct time as the printhead crosses the page),laser printers(the laser must be activated at the right time as the beam scans across the rotating drum), and dot matrix and various types ofline printers(the impact mechanism must be activated at the right time as the print mechanism comes into alignment with the desired output). A failure in any of these would cause either missing output or misaligned output.
In the context ofmultitaskingsystems thescheduling policyis normally priority driven (pre-emptiveschedulers). In some situations, these can guarantee hard real-time performance (for instance if the set of tasks and their priorities is known in advance). There are other hard real-time schedulers such asrate-monotonicwhich is not common in general-purpose systems, as it requires additional information in order to schedule a task: namely a bound or worst-case estimate for how long the task must execute. Specific algorithms for scheduling such hard real-time tasks exist, such asearliest deadline first,which, ignoring the overhead ofcontext switching,is sufficient for system loads of less than 100%.[7]New overlay scheduling systems, such as anadaptive partition schedulerassist in managing large systems with a mixture of hard real-time and non real-time applications.
Firm real-time systemsare more nebulously defined, and some classifications do not include them, distinguishing only hard and soft real-time systems. Some examples of firm real-time systems:
- The assembly line machine described earlier ashardreal-time could instead be consideredfirmreal-time. A missed deadline still causes an error which needs to be dealt with: there might be machinery to mark a part as bad or eject it from the assembly line, or the assembly line could be stopped so an operator can correct the problem. However, as long as these errors are infrequent, they may be tolerated.
Soft real-time systemsare typically used to solve issues of concurrent access and the need to keep a number of connected systems up-to-date through changing situations. Some examples of soft real-time systems:
- Software that maintains and updates the flight plans for commercialairliners.The flight plans must be kept reasonably current, but they can operate with the latency of a few seconds.
- Live audio-video systems are also usually soft real-time. A frame of audio that is played late may cause a brief audio glitch (and may cause all subsequent audio to be delayed correspondingly, causing a perception that the audio is being played slower than normal), but this may be better than the alternatives of continuing to play silence, static, a previous audio frame, or estimated data. A frame of video that is delayed typically causes even less disruption for viewers. The system can continue to operate and also recover in the future using workload prediction and reconfiguration methodologies.[8]
- Similarly, video games are often soft real-time, particularly as they try to meet a targetframe rate.As the next image cannot be computed in advance, since it depends on inputs from the player, only a short time is available to perform all the computing needed to generate a frame of video before that frame must be displayed. If the deadline is missed, the game can continue at a lower frame rate; depending on the game, this may only affect its graphics (while the gameplay continues at normal speed), or the gameplay itself may be slowed down (which was common on olderthird-andfourth-generation consoles).
Real-time in digital signal processing
[edit]In a real-timedigital signal processing(DSP) process, the analyzed (input) and generated (output) samples can be processed (or generated) continuously in the time it takes to input and output the same set of samplesindependentof the processing delay.[9]It means that the processing delay must be bounded even if the processing continues for an unlimited time. That means that themeanprocessing time per sample, includingoverhead,is no greater than the sampling period, which is the reciprocal of thesampling rate.This is the criterion whether the samples are grouped together in large segments and processed as blocks or are processed individually and whether there are long, short, or non-existentinput and output buffers.
Consider anaudio DSPexample; if a process requires 2.01 seconds toanalyze,synthesize,or process 2.00 seconds of sound, it is not real-time. However, if it takes 1.99 seconds, it is or can be made into a real-time DSP process.
A common life analogy is standing in a line orqueuewaiting for the checkout in a grocery store. If the line asymptotically grows longer and longer without bound, the checkout process is not real-time. If the length of the line is bounded, customers are being "processed" and output as rapidly, on average, as they are being inputted then that processisreal-time. The grocer might go out of business or must at least lose business if they cannot make their checkout process real-time; thus, it is fundamentally important that this process is real-time.
A signal processing algorithm that cannot keep up with the flow of input data with output falling farther and farther behind the input, is not real-time. But if the delay of the output (relative to the input) is bounded regarding a process that operates over an unlimited time, then that signal processing algorithm is real-time, even if the throughput delay may be very long.
Live vs. real-time
[edit]Real-time signal processing is necessary, but not sufficient in and of itself, for live signal processing such as what is required inlive event support.Live audio digital signal processing requires both real-time operation and a sufficient limit to throughput delay so as to be tolerable to performers usingstage monitorsorin-ear monitorsand not noticeable aslip sync errorby the audience also directly watching the performers. Tolerable limits to latency for live, real-time processing is a subject of investigation and debate but is estimated to be between 6 and 20 milliseconds.[10]
Real-time bidirectionaltelecommunications delaysof less than 300 ms ( "round trip" or twice the unidirectional delay) are considered "acceptable" to avoid undesired "talk-over" in conversation.
Real-time and high-performance
[edit]Real-time computing is sometimes misunderstood to behigh-performance computing,but this is not an accurate classification.[11]For example, a massivesupercomputerexecuting a scientific simulation may offer impressive performance, yet it is not executing a real-time computation. Conversely, once the hardware and software for an anti-lock braking system have been designed to meet its required deadlines, no further performance gains are obligatory or even useful. Furthermore, if a network server is highly loaded with network traffic, its response time may be slower but will (in most cases) still succeed before it times out (hits its deadline). Hence, such a network server would not be considered a real-time system: temporal failures (delays, time-outs, etc.) are typically small and compartmentalized (limited in effect) but are notcatastrophic failures.In a real-time system, such as theFTSE 100 Index,a slow-down beyond limits would often be considered catastrophic in its application context. The most important requirement of a real-time system is consistent output, not high throughput.
Some kinds of software, such as manychess-playing programs,can fall into either category. For instance, a chess program designed to play in a tournament with a clock will need to decide on a move before a certain deadline or lose the game, and is therefore a real-time computation, but a chess program that is allowed to run indefinitely before moving is not. In both of these cases, however, high performance is desirable: the more work a tournament chess program can do in the allotted time, the better its moves will be, and the faster an unconstrained chess program runs, the sooner it will be able to move. This example also illustrates the essential difference between real-time computations and other computations: if the tournament chess program does not make a decision about its next move in its allotted time it loses the game—i.e., it fails as a real-time computation—while in the other scenario, meeting the deadline is assumed not to be necessary. High-performance is indicative of the amount of processing that is performed in a given amount of time, whereas real-time is the ability to get done with the processing to yield a useful output in the available time.
Near real-time
[edit]The term "near real-time" or "nearly real-time" (NRT), intelecommunicationsandcomputing,refers to the timedelayintroduced, by automateddata processingornetworktransmission, between the occurrence of an event and the use of the processed data, such as for display orfeedbackand control purposes. For example, a near-real-time display depicts an event or situation as it existed at the current time minus the processing time, as nearly the time of the live event.[12]
The distinction between the terms "near real time" and "real time" is somewhat nebulous and must be defined for the situation at hand. The term implies that there are no significant delays.[12]In many cases, processing described as "real-time" would be more accurately described as "near real-time".
Near real-time also refers to delayed real-time transmission of voice and video. It allows playing video images, in approximately real-time, without having to wait for an entire large video file to download. Incompatible databases can export/import to common flat files that the other database can import/export on a scheduled basis so that they can sync/share common data in "near real-time" with each other.
Design methods
[edit]Several methods exist to aid the design of real-time systems, an example of which isMASCOT,an old but very successful method which represents theconcurrentstructure of the system. Other examples areHOOD,Real-Time UML,AADL,theRavenscar profile,andReal-Time Java.
See also
[edit]- Autonomous peripheral operation
- Control system
- Failure detector
- Nodal architecture
- Processing modes
- Ptolemy Project
- Real-time data
- Real-time computer graphics
- Real-time operating system
- Real-time testing
- Remote diagnostics
- Scheduling analysis real-time systems
- Synchronous programming language
- Time-utility function
- Ward–Mellor method
- Worst-case execution time
References
[edit]- ^"FreeRTOS – Open Source RTOS Kernel for small embedded systems – What is FreeRTOS FAQ?".FreeRTOS.Retrieved2021-03-08.
- ^Ben-Ari, Mordechai;"Principles of Concurrent and Distributed Programming", ch. 16, Prentice Hall, 1990,ISBN0-13-711821-X,p. 164
- ^Martin, James (1965).Programming Real-time Computer Systems.Englewood Cliffs, New Jersey: Prentice-Hall Incorporated. p.4.ISBN978-0-13-730507-0.
- ^Kant, Krishna (May 2010).Computer-Based Industrial Control.PHI Learning. p. 356.ISBN9788120339880.Retrieved2015-01-17.
- ^Shin, Kang G.;Ramanathan, Parameswaran (Jan 1994)."Real-time computing: a new discipline of computer science and engineering"(PDF).Proceedings of the IEEE.82(1): 6–24.CiteSeerX10.1.1.252.3947.doi:10.1109/5.259423.ISSN0018-9219.
- ^Kopetz, Hermann;Real-Time Systems: Design Principles for Distributed Embedded Applications,Kluwer Academic Publishers, 1997
- ^Liu, Chang L.; and Layland, James W.; "Scheduling Algorithms for Multiprogramming in a Hard Real-time Environment",Journal of the ACM,20(1):46-61, January 1973,http://citeseer.ist.psu.edu/liu73scheduling.html
- ^Menychtas, Andreas; Kyriazis, Dimosthenis; Tserpes, Konstantinos (July 2009). "Real-time reconfiguration for guaranteeing QoS provisioning levels in Grid environments".Future Generation Computer Systems.25(7): 779–784.doi:10.1016/j.future.2008.11.001.
- ^Kuo, Sen M.; Lee, Bob H.; and Tian, Wenshun; "Real-Time Digital Signal Processing: Implementations and Applications", Wiley, 2006,ISBN0-470-01495-4,Section 1.3.4:Real-Time Constraints.
- ^Kudrle, Sara; Proulx, Michel; Carrieres, Pascal; Lopez, Marco; et al. (July 2011). "Fingerprinting for Solving A/V Synchronization Issues within Broadcast Environments".SMPTE Motion Imaging Journal.120(5): 36–46.doi:10.5594/j18059XY.
Appropriate A/V sync limits have been established and the range that is considered acceptable for film is +/- 22 ms. The range for video, according to the ATSC, is up to 15 ms lead time and about 45 ms lag time
- ^Stankovic, John (1988), "Misconceptions about real-time computing: a serious problem for next-generation systems",Computer,vol. 21, no. 10, IEEE Computer Society, p. 11,doi:10.1109/2.7053,S2CID13884580
- ^ab"Federal Standard 1037C: Glossary of Telecommunications Terms".Its.bldrdoc.gov.Retrieved2014-04-26.
Further reading
[edit]- Burns, Alan; Wellings, Andy (2009),Real-Time Systems and Programming Languages(4th ed.), Addison-Wesley,ISBN978-0-321-41745-9
- Buttazzo, Giorgio (2011),Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications,New York, New York: Springer,ISBN9781461406761– viaGoogle Books.
- Liu, Jane W. S.(2000),Real-time systems,Upper Saddle River, New Jersey: Prentice Hall.
- The International Journal of Time-Critical Computing Systems
External links
[edit]- IEEE Technical Committee on Real-Time Systems
- Euromicro Technical Committee on Real-time Systems
- The What, Where and Why of Real-Time Simulation
- Johnstone, R.L."RTOS—Extending OS/360 for real time spaceflight control"(PDF).Bitsavers.RetrievedFebruary 24,2023.
- Coyle, R. J.; Stewart, J. K. (September 1963)."Design of a Real-time Programming System".Computers and Automation.XII(9). Silver Spring, Maryland: Datatrol Corporation: 26–34.
[...] set of notes which will hopefully point up problem areas which should be considered in real time design.