Real Time Terms
The following terms were copied from the Terminology section of the Realtime Technologies Specification of the CELF.
- Context switch
- The act of replacing the running task A by another task B. The state of task A is saved, and A is placed in the ready queue; the state of task B is restored, and B becomes a running task.
- Critical section
- A brief interval during which a task accesses shared data. During this interval, no other tasks are allowed to access the same data. One way of ensuring this is to prevent preemption during the critical section. If the critical section defers preemption for a bounded interval, the resulting priority inversion is bounded.
- Deadline-monotonic priority setting
- The task with the shortest relative deadline is assigned the highest priority.
- Deadline requirement
- A real-time requirement that requires the response to be completed within a fixed time interval from the triggering event. A relative deadline is the duration of the above mentioned interval; an absolute deadline is the moment in time at which the response must be completed.
- Fixed priority preemptive scheduling
- The task with the highest priority is always running. If a task with a higher priority becomes runnable, the running task will be preempted immediately. The priority of a task, process or Interrupt Service Routine (ISR) is explicitly determined at creation, or by an explicit set-priority command. No implicit priority changes by the scheduler are assumed. For an exception to this rule see Priority inheritance. The scheduling policy of SCHED_FIFO in Linux implements fixed priority preemptive scheduling.
Note: Fixed priority scheduling is typically designed to be used for a single coherent application.
- The time range of a certain interval. We can talk about the granularity of a timing requirement, of a non-interruptible code segment, etc. Needs fixing
- Hard deadline requirement
- Missing the deadline is considered an error.
- Hard real-time system
- System with hard real-time requirements.
- Hybrid real-time system
- This refers to a system where a real-time kernel is used in conjunction with a (mostly) unmodified Linux kernel, to provide real-time services to a subset of tasks on the system. This is often also referred to as the "multi-kernel" approach.
- Interrupt latency
- Time passed between interrupt occurrence and activation of interrupt handler.
- Interrupt masking
- Making certain interrupts invisible to the software.
- Interrupt response time (worst-case)
- (Worst-case) time passed between interrupt occurrence and either completion of interrupt service routine (ISR) or wake up of dependent task.
- Jitter – absolute
- Deviation of the occurrence of an event (e.g. completion of frame) from expected occurrence.
- Jitter – relative
- Deviation of the interval between two successive occurrences of an event (e.g. completion of frame) from expected interval.
- Mutual exclusion
- Prevent multiple tasks or ISRs from accessing the same data concurrently. Mutual exclusion is used to protect the integrity of the data.
- A running thread or process can be temporarily suspended. The state of the thread or process (including e.g., program counter, and register values) is saved. When the process or thread is later resumed, the saved state is restored.
- Priority inheritance
- If a high priority-task blocks for a critical section, a low-priority task that holds the lock for the section gets a priority boost. It inherits the priority of the blocked task. This prevents the unbounded priority inversion that could occur if medium priority tasks preempt the lower-priority task that holds the lock. Note that, with priority inheritance, the medium-priority tasks suffer priority inversion as well. But, and this is important in real-time systems, the priority inversion for all tasks is bounded (can be determined without knowing the exact run-time schedule)
- Priority inversion
- The highest priority task is not running. There can be several reasons for priority inversion. One of them is the absence of full preemption. Priority inversion is one of the main reasons for deadlines being missed.
- Real-time requirement
- A requirement on the completion time of a response, generally measured relative to the event that triggered the response.
- Real-time system
- System with one or more real-time requirements.
- Response time (worst-case)
- (worst-case) Time passed between event occurrence and completion of the response to that event. The event may be an interrupt. The response typically involves an interrupt handler and one or more synchronized tasks.
- Runnable task
- (also ready/active task) A task that can run from a logical perspective, but is prevented from running physically.
- A synchronization primitive often used to achieve mutual exclusion.
- Soft deadline
- Missing deadlines is sometimes acceptable. Compared to hard deadlines, where there is no reason to consider the value of a late result, the value of a late result for a soft deadline is of interest. The value of the result may, for instance, decrease linearly after the deadline.
- Soft real-time requirement
- Soft deadline, or average-case response time requirement.
Note that hard and soft real-time requirements are orthogonal to the temporal granularity that is required. Meeting a soft requirement in the microsecond domain may be more difficult than meeting a hard requirement in the milliseconds domain.
- Soft real-time system
- System with soft real-time requirements.
It is relatively well-established that the following events and time periods are of interest when measuring realtime performance:
- 0' - event initiation
- A - hardware interrupt assertion
- This is the time when the hardware interrupt line for an event is raised
- B - Interrupt service routine starts execution
- B' - Task is scheduled
- C - Task starts execution
- D - Result is received from processing the event
Real time performance is often expressed in terms of the maximum, minimum and average duration for certain time periods defined by the events above.
Here are some common terms, expressed relative to those events:
- Interrupt latency
- The time from A to B (from hardware interrupt assertion to ISR start).
- Scheduling latency
- The time from B' to C (from time of task scheduling to process start).
Note that sometimes this term is used to refer to the time from A to C (from interrupt assertion to process start).
- Processing time
- The time from C to D (from process start to completion of processing).
- Response time
- The time from A to D (total time from interrupt assertion to delivery of processed data).
Often, these or similar terms are used with less accuracy to describe the closest approximation one can get, with a particular test framework and instrumentation set.
For example, 'response latency' may be reported as "response time", and refer to the time from when a host program records the time, previous to transmitting a piece of data which will cause an interrupt on the target machine, to the time when a host program records the time after receiving some signal from the target that processing is completed. Of course, time is taken in the operations of recording time, transmiting data, and detecting signals on the host machine. But it may be that timing these individual operations at a finer granularity requires instrumentation or hardware support not reasonably available for a test.
Terms in need of definition
- Reservation system
- Granularity (above definition is weak)