RTOS Explained: Run-to-Completion (RTC) Threads

Introduction to RTXC Threads

The RTXC Quadros RTOS offers two different execution entities: Tasks and Threads. You may be familiar with various terms like process or task or thread, depending on your OS experience. In RTXC Quadros Tasks and Threads have very specific functionality. Whether you use Threads or Tasks (or both) depends on the requirements of your application and on the particular RTXC Quadros configuration you are using.

  • Tasks are RTXC program code that performs a defined function or set of functions. In the RTXC Quadros world each task has a priority and its own stack for saving context when preempted or rescheduled. Tasks can block or wait on events. Each task is independent of other tasks but can establish relationships with other tasks in many forms, including data structures, input, output, or other constructs.
  • Threads are lightweight tasks which are designed to run to completion unless preempted by an ISR or a Thread at a higher priority level. The run-to-completion model allows all Threads in an application use a single stack. If preempted they save their context to the common stack. The single stack means that Threads cannot block or wait on events however Threads can be initiated by events.  A Thread has no context upon entry and must perform any required data initialization upon entry. When its operations are complete, the thread returns to the RTXC/ss scheduler without context. Threads have an inherent priority above that of tasks. Thus, the lowest priority Thread is of greater importance than the highest priority Task.

Thread Usage

Threads are ideal for applications with event driven, hard real-time requirements. Because RTXC Threads fit very nicely into fixed priority scheduling methods, thread priorities can be determined using Rate Monotonic Analysis (RMA) to ensure schedulability of the real-time tasks to meet their deadlines. Being lightweight tasks, Threads are adept at handling processing associated with a high frequency of interrupts. They can be scheduled and complete their operating more quickly and with less system overhead.

Threads are also ideal for event-driven execution using Finite State Machines (FSM) or Hierarchical State Machines (HSM). The advantage for state machine implementations is that our implementation of Threads supports the Run-To-Completion execution model inherent in state machine design: the system must complete the processing of an event before it can start processing the next event.

Thread Scheduling and Preemption

Depending on how you set up your system you can operate RTXC/ss as a Cooperative Executive or as a Prioritized, Preemptive Executive.

Cooperative Scheduler: If you implement only a single priority level within RTXC/ss then Threads will run to completion once they are launched. However, during their operation they may be interrupted. You can establish a priority for Threads within a level but this only determines the order in which they will be scheduled. A running Thread cannot be preempted by another Thread in the same level.

Prioritized, Preemptive Scheduler: If you implement multiple priority levels within RTXC/ss then a ready Thread in a higher priority level will preempt the Current Thread. In this case the context of the Current Thread is saved in the common stack to be resumed after all ready Threads in higher levels of priority have run.

For more information on any of these topics request our user manuals or contact your local sales representative.

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