What does real time mean ?
It means correctness and execution time of the results are guaranteed.
In non-Real time systems normally only the correctness of the result is guaranteed.
In other words: A real time system will also guaranty that a certain deadline is met.
From wikipedia professor also said:
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.
Hard real-time systems are 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:
- A car engine control system is a hard real-time system because a delayed signal may cause engine failure or damage.
- Medical systems such as heart pacemakers. 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 an assembly 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, in embedded systems. Early video game systems such as the Atari 2600 and Cinematronics vector graphics had hard real-time requirements because of the nature of the graphics and timing hardware.
- Softmodems replace a hardware modem with software running on a computer’s CPU. 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 of printers have hard real-time requirements, such as inkjets (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 of line 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 of multitasking systems the scheduling policy is normally priority driven (pre-emptive schedulers). 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 as rate-monotonic which 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 as earliest deadline first, which, ignoring the overhead of context switching, is sufficient for system loads of less than 100%. New overlay scheduling systems, such as an adaptive partition scheduler assist in managing large systems with a mixture of hard real-time and non real-time applications.
Firm real-time systems are 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 as hard real-time could instead be considered firm real-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 systems are 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 commercial airliners. 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’s 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’s 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.
- Similarly, video games are often soft real-time, particularly as they try to meet a target frame 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 older third- and fourth-generation consoles
WCET(Worst Case Execution Time):
It must not be less than the real WCET (otherwise the
deadline will be execced sometime).
2 It should be as near as possible to the real WCET (otherwise
computing power will be wasted −→ the system gets more
Statical WCET Analysis: Calculating execution time
based on the computer program.
Dynamical WCET Analysis: Measurement of an
adequate number of execution runs on a certain