2.1 Meaning of Basic Terminology
1. Task, invoking task
The basic logical unit of concurrent program execution is called a “task”. Whereas the programs assigned to one task are executed in sequence, programs assigned to di.erent tasks can be executed in parallel. This concurrent processing is a conceptual phenomenon, from the standpoint of applications; in actual implementation it is accomplished by time-sharing among tasks as controlled by the kernel.
A task that invokes a system call is called the “invoking task”.
2. Dispatch, dispatcher
The switching of tasks executed by the processor is called “dispatching” (or task dispatching). The kernel mechanism by which dispatching is realized is called a “dispatcher” (or task dispatcher).
3. Scheduling, scheduler
The processing to determine which task to execute next is called “scheduling” (or task scheduling).
The kernel mechanism by which scheduling is realized is called a “scheduler” (or task scheduler).
Generally a scheduler is implemented inside system call processing or in the dispatcher.
4. Context
The environment in which a program runs is generally called “context”. For a context to be called identical, at the very least the processor operation mode must be the same and the stack space must be the same (part of the same contiguous area). Note that context is a conceptual matter from the standpoint of applications; even when processing must be executed in independent contexts, in actual implementation both contexts may sometimes use the same processor operation mode and the same stack space.
5. Precedence
The relationship among di.erent processing requests that determines their order of execution is called “precedence”. When a higher-precedence process becomes ready for execution while a low-precedence process is in progress, as a general rule the higher-precedence process is run ahead of the other process.
Priority is a parameter assigned by an application to control the order of task or message processing. Precedence, on the other hand, is a concept used in the speci.cation to make clear the order in which processing is to be executed. Precedence among tasks is determined based on task priority.
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