Hardware Reference
In-Depth Information
Period
The interval of time between the activation of two consecutive instances of a
periodic task.
Periodic task
A type of task that consists of a sequence of identical jobs (instances),
activated at regular intervals.
Phase
The time instant at which a periodic task is activated for the first time, mea-
sured with respect to some reference time.
Polling
A service technique in which the server periodically examines the requests
of its clients.
Port
A general intertask communication mechanism based on a message passing
scheme.
Precedence graph
A directed acyclic graph that describes the precedence relations
in a group of tasks.
Precedence constraint
Dependency relation between two or more tasks that spec-
ifies that a task cannot start executing before the completion of one or more tasks
(called
predecessors
).
Predictability
An important property of a real-time system that allows a program-
mer to analyze and anticipate the consequence of any scheduling decision.
Preemption
An operation of the kernel that interrupts the currently executing job
and assigns the processor to a more urgent job ready to execute.
Preemptive Scheduling
A form of scheduling in which jobs can be interrupted at
any time and the processor is assigned to more urgent jobs that are ready to execute.
Priority
A number associated with a task and used by the kernel to establish an
order of precedence among tasks competing for a common resource.
Priority Inversion
A phenomenon for which a task is blocked by a lower-priority
task for an unbounded amount of time.
Process
A computation in which the operations are executed by the processor one at
a time. A process may consist of a sequence of identical jobs, also called instances.
The word
task
is often used as a synonym.
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