Real-Time Systems. Design Principles for Distributed Embedded Applications. Herman Kopetz. Second Edition (811374), страница 99
Текст из файла (страница 99)
The length ofthe interval between a membership point and themoment when the consistent membership informationis available at the other components is a quality ofservice parameter of the membership service (5.3.2).A specification that explains how the data field of amessage is structured into syntactic units and assignsnames to these syntactic units.
The names identify theconcepts that explain the meaning of the data (4.6.2).A microtick of a physical clock is a periodic eventgenerated by this clock (➔ tick) (3.1.2).The non-blocking write protocol (NBW) is a synchronization protocol between a single writing task and manyreading tasks that achieves data consistency withoutblocking the writer (9.4.2).A non-deterministic design construct is a design construct that produces unpredictable result either in thevalue domain or the temporal domain (5.6.3).An observation of a real-time entity is an atomic tripleconsisting of the name of the real-time entity, the instantof the observation, and the value of the real-time entity(5.2).The offset between two events denotes the time difference between these events (3.1.3).A PFSM is an extension of the finite state machinemodel to include the progression of real time (4.1.3).A periodic task is a task that has a constant time intervalbetween successive task request times (➔ aperiodictask, ➔ sporadic task) (10.1.2).Permanence is a relation between a given message andall related messages that have been sent to the samereceiver before this given message has been sent.
Aparticular message becomes permanent at a given component at the moment when it is known that all earliersent related messages have arrived (or will never arrive)(5.5.1).A phase-aligned transaction is a real-time transactionwhere the constituting processing and communicationactions are synchronized (5.4.1).The instant when a real-time entity is observed (1.2.1).GlossaryPositiveAcknowledgmentor-Retransmission(PAR) protocolPrecisionPrimary EventPriority CeilingProtocolProcess LagPropagation DelayProtocolRadio FrequencyIdentification (RFID)Rare EventRate-MonotonicAlgorithmRaw DataReal-Time (RT) EntityReal-Time (RT)ImageReal-TimeComputer System353The Positive-Acknowledgment-or-Retransmission (PAR)protocol is an event-triggered protocol where a messagesent by the sender must be positively acknowledged by thereceiver (7.1.2).The precision of an ensemble of clocks denotes themaximum offset of respective ticks of any two clocksof the ensemble over the period of interest.
The precision is expressed in the number of ticks of the referenceclock (3.1.3).A primary event is the cause of an alarm shower (1.2.1).A scheduling algorithm for scheduling a set of dependent periodic tasks (10.4.2).The delay between applying a step function to an inputof a controlled object and the start of response of thecontrolled object (1.3.1).The propagation delay of a communication channeldenotes the time interval it takes for a single bit totraverse the channel (7.2.2).A protocol is a set of rules that governs the communication among partners (2.2.3).A technology for the identification of objects by electronic means (13.4)A rare event is a seldomly occurring event that is of criticalimportance.
In a number of applications the predictableperformance of a real-time computer system in rare eventsituations is of overriding concern (1.2.1).A dynamic preemptive scheduling algorithm for scheduling a set of independent periodic tasks (10.4.1).A raw data element is an analog or digital data elementas it is delivered by an unintelligent sensor (➔ measureddata, agreed data) (9.6.1).A real-time (RT) entity is a state variable, either in theenvironment of the computational cluster, or in thecomputational cluster itself, that is relevant for thegiven purpose. Examples of RT entities are: the temperature of a vessel, the position of a switch, the setpointselected by an operator, or the intended valve positioncalculated by the computer (5.1).A real-time (RT) image is a current picture of a real-timeentity (5.3).A real-time computer system is a computer system, inwhich the correctness of the system behavior dependsnot only on the logical results of the computations, butalso on the physical time when these results are produced.
A real-time computer system can consist of oneor more computational clusters (1.1).354Real-time Data BaseReal-Time ObjectReal-Time TransactionReasonablenessConditionReference ClockReliabilityReplica DeterminismResource AdequacyRise TimeRiskSafetySafety CaseGlossaryThe real-time database is formed by the set of all temporally accurate real-time images (1.2.1).A real-time (RT) object is a container inside a computerfor a RT entity or a RT image. A clock with a granularitythat is in agreement with the dynamics of the RT objectis associated with every RT object (5.3.2).A real-time (RT) transaction is a sequence of computational and communication actions between a stimulusfrom the environment and a response to the environmentof a computational cluster (1.7.3).The reasonableness condition of clock synchronizationstates that the granularity of the global time must belarger than the precision of the ensemble of clocks(3.2.1).The reference clock is an ideal clock that ticks always inperfect agreement with the international standard oftime (3.1.2).The reliability R (t) of a system is the probability that asystem will provide the specified service until time t,given that the system was operational at t ¼to (1.4.1).Replica Determinism is a desired relation between replicated RT objects.
A set of replicated RT objects isreplica determinate if all objects of this set have thesame visible state and produce the same output messages at instants that are at most an interval of d timeunits apart (5.6).A real-time computer system is resource adequate ifthere are enough computing resources available to handle the specified peak load and the faults specified in thefault hypothesis. Guaranteed response systems must bebased on resource adequacy (➔ guaranteed timeliness)(1.5.4).The rise time is the time required for the output of asystem to rise to a specific percentage of its final equilibrium value as a result of step change on the input(1.3.1).Risk is the product of hazard severity and hazard probability.
The severity of a hazard is the worst-case damageof a potential accident related to the hazard (11.4.2).Safety is reliability regarding critical failure modes(1.4.2).A safety case is a combination of a sound set of arguments supported by analytical and experimental evidence substantiating the safety of a given system(11.4.3).GlossarySafety Critical RealTime Computer SystemSamplingSchedulability TestSemantic AgreementSemantic ContentSignal ConditioningSoft DeadlineSoft Real-TimeComputer SystemSparse EventSparse Time BaseSphere of Control(SOC)Sporadic TaskSpoofing AttackState355Synonym to hard real-time computer system (1.1).In sampling, the state of a RT entity is periodicallyinterrogated by the computer system at instants that arein the sphere of control of the computer system.
If amemory element is required to store the effect of anevent, the memory element is outside the sphere ofcontrol of the computer system (1.3.1).A schedulability test determines whether there exists aschedule such that all tasks of a given set will meet theirdeadlines (10.1.1).An agreement among measured variables is calledsemantic agreement if the meanings of the differentmeasured values are related to each other by a processmodel that is based on a priori knowledge about thephysical characteristics and the dynamics of the controlled object (9.6.3).The essential meaning of a statement or variable asunderstood by an end-user. The same semantic contentcan be represented in different syntactic forms (2.2.4).Signal conditioning refers to all processing steps that arerequired to generate a measured data element from araw data element (1.2.1).A deadline for a result is soft if the result has utility evenafter the deadline has passed (1.1).A real-time computer system that concerned with anysoft deadlines only (1.1).an event that occurs in the active interval of a ➔ sparsetime base (3.3).a time-base in a distributed computer systems where thephysical time is partitioned into an infinite sequence ofactive and silent intervals and where sparse events maybe generated only in the active intervals (3.3).The sphere of control of a subsystem is defined by theset of RT entities the values of which are establishedwithin this subsystem (5.1.1).A sporadic task is a task where the task request times arenot known but where it is known that a minimum timeinterval exists between successive requests for execution (➔ periodic task, ➔ aperiodic task) (10.1.2).A security attack where an adversary masquerades as alegitimate user in order to gain unauthorized access to asystem (6.2.2).The state of a component at a given instant is a datastructure that contains all information about the past that356State EstimationState MessageSynchronizationConditionSystem of Systems(SoS)Task DescriptorList (TADL)Task Request TimeTaskTemporal AccuracyTemporal ControlTemporal FailureTemporal OrderThrashingTickGlossaryis considered relevant for the future operation of thecomponent (4.2).State estimation is the technique of building a model of aRT entity inside a RT object to compute the probablestate of a RT entity at a selected future instant, and toupdate the related RT image accordingly (5.4.3).A message is a state message if it contains informationabout states, if a new version of the message replaces theprevious version, and the message is not consumed onreading (➔ event message) (4.3.4).The synchronization condition is a necessary conditionfor the synchronization of clocks.
It relates the convergence function, the drift offset and the precision (3.4.1).A system consisting of a set of nearly autonomousconstituent systems that decide to cooperate in order toachieve a common objective (4.7.3).The task descriptor list (TADL) is a static data structurein a time-triggered operating system that contains theinstants when the tasks have to be dispatched (9.2.1).The task request time is the instant when a task becomesready for execution (10.1.2).A task is the execution of a program (➔ simple task, ➔complex task) (4.2.1).A real-time image is temporally accurate if the timeinterval between the moment “now” and instant whenthe current value of the real-time image was the value ofthe corresponding RT entity is smaller than an application specific bound (5.4).Temporal control is concerned with the determination ofthe real-time instants when a task must be activated orwhen a task must be blocked (➔ logical control) (4.1.3).A temporal failure occurs when a value is presented atthe system-user interface outside the intended intervalof real-time.
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