How can you design a scheduling algorithm for a hard real-time system?

1 Task characteristics

A task is a unit of work that the system must execute. A task can have different attributes, such as its arrival time, its execution time, its deadline, its priority, and its resource requirements. A scheduling algorithm must know these attributes to assign tasks to processors and ensure that they finish before their deadlines. A task can also be periodic or aperiodic, meaning that it repeats at regular intervals or occurs at unpredictable times. A scheduling algorithm must handle both types of tasks and balance their demands.

2 System constraints

A system constraint is a limitation or requirement that affects the scheduling algorithm. For example, a system constraint can be the number and type of processors, the memory and power consumption, the communication and synchronization overhead, or the fault tolerance and reliability level. A scheduling algorithm must respect these constraints and optimize the system resources. A system constraint can also be the schedulability, which is the ability of the system to meet all the task deadlines. A scheduling algorithm must guarantee the schedulability of the system or detect and handle any violations.

3 Performance objectives

A performance objective is a goal or metric that evaluates the quality of the scheduling algorithm. For example, a performance objective can be the utilization, which is the ratio of the total execution time to the total available time. A scheduling algorithm must maximize the utilization and avoid wasting processor cycles. Another performance objective can be the response time, which is the time between the arrival and the completion of a task. A scheduling algorithm must minimize the response time and reduce the task waiting time.

4 Scheduling techniques

A scheduling technique is a method or strategy that determines how to assign tasks to processors. There are two main types of scheduling techniques: static and dynamic. A static scheduling technique makes the assignment decisions before the system starts running, based on the known task attributes and system constraints. A static scheduling technique is simpler and faster, but less flexible and adaptable. A dynamic scheduling technique makes the assignment decisions during the system execution, based on the current task attributes and system constraints. A dynamic scheduling technique is more complex and slower, but more flexible and adaptable.

5 Scheduling policies

A scheduling policy is a rule or criterion that decides which task to execute next on a processor. Common policies include rate monotonic, deadline monotonic, earliest deadline first, least laxity first, fixed priority, and round robin. Each policy has its advantages and disadvantages, with some more suitable for hard real-time systems than others. Therefore, when selecting a scheduling algorithm, it is important to choose a policy that meets the performance objectives and guarantees the schedulability of the system.

6 Scheduling challenges

Scheduling algorithms must be robust and adaptable to cope with uncertainties and changes in the system, as well as provide solutions or trade-offs for the various scheduling challenges faced. Common challenges include overload, where the system has more tasks than it can handle; jitter, where tasks have variable arrival and execution times; preemption, when a task is interrupted by one with higher priority; migration, when a task is moved from one processor to another; resource contention, when multiple tasks compete for the same resource; and dependencies, when tasks have logical or temporal relationships with each other.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?