4.4.1 Schedulable Entities

A schedulable entity is a kernel object that can compete for execution time on a system, based on a predefined scheduling algorithm. Tasks and processes are all examples of schedulable entities found in most kernels.

A task is an independent thread of execution that contains a sequence of independently schedulable instructions. Some kernels provide another type of a schedulable object called a process. Processes are similar to tasks in that they can independently compete for CPU execution time. Processes differ from tasks in that they provide better memory protection features, at the expense of performance and memory overhead. Despite these differences, for the sake of simplicity, this book uses task to mean either a task or a process.

Note that message queues and semaphores are not schedulable entities. These items are inter-task communication objects used for synchronization and communication. Chapter 6 discusses semaphores, and Chapter 7 discusses message queues in more detail.

So, how exactly does a scheduler handle multiple schedulable entities that need to run simultaneously? The answer is by multitasking. The multitasking discussions are carried out in the context of uniprocessor environments.