Scheduling

Intro

• RT Systems must be able to interact with their environment in a timely and predictable manner.
• Therefore an RT system must be analyzable whose timing properties can be proven and mathematically correct.
• Scheduling is the ordering of threads/processes so that the underlying hardware and software resources can be predictably and efficiently used.
• Scheduling consists of three components:
  • Scheduling policy: An algorithm for ordering access to resources.
  • Scheduling mechanism: An algorithm for allocation resources.
  • Schedulability/Feasibility analysis: A means of predicting the worst case behavior of the system when the policy and mechanism are applied. Once the worst case has been predicated, then it can be compared with the systems timing requirements to ensure that all deadlines will be met.

Fixed Priority Scheduling

Policy

• FPS Requires:
• Statically allocate SO's to a single processor.
• Order SO execution on a single processor according to their priority.
• Assign priorities to SO's at their creation time. Usually priorities are assigned according to deadlines.
• Priority Inheritance when accessing resources.

Mechanism

• Pre-emptive priority based dispatching of processes. The resource is always given to the highest priority runnable SO.

Feasibility Analysis

• Many different analysis techniques possible for FPS. Most flexible is Response Time Analysis.