Книга: Real-Time Concepts for Embedded Systems
4.4.3 The Context Switch
4.4.3 The Context Switch
Each task has its own context, which is the state of the CPU registers required each time it is scheduled to run. A context switch occurs when the scheduler switches from one task to another. To better understand what happens during a context switch, let’s examine further what a typical kernel does in this scenario.
Every time a new task is created, the kernel also creates and maintains an associated task control block (TCB). TCBs are system data structures that the kernel uses to maintain task-specific information. TCBs contain everything a kernel needs to know about a particular task. When a task is running, its context is highly dynamic. This dynamic context is maintained in the TCB. When the task is not running, its context is frozen within the TCB, to be restored the next time the task runs. A typical context switch scenario is illustrated in Figure 4.3.
As shown in Figure 4.3, when the kernel’s scheduler determines that it needs to stop running task 1 and start running task 2, it takes the following steps:
1. The kernel saves task 1’s context information in its TCB.
2. It loads task 2’s context information from its TCB, which becomes the current thread of execution.
3. The context of task 1 is frozen while task 2 executes, but if the scheduler needs to run task 1 again, task 1 continues from where it left off just before the context switch.
The time it takes for the scheduler to switch from one task to another is the context switch time. It is relatively insignificant compared to most operations that a task performs. If an application’s design includes frequent context switching, however, the application can incur unnecessary performance overhead. Therefore, design applications in a way that does not involve excess context switching.
Every time an application makes a system call, the scheduler has an opportunity to determine if it needs to switch contexts. When the scheduler determines a context switch is necessary, it relies on an associated module, called the dispatcher, to make that switch happen.
Оглавление книги
- 4.4.4 The Dispatcher
- 10.2 The Real-Time Operating System (RTOS)
- About the author
- Chapter 7. The state machine
- Appendix E. Other resources and links
- Example NAT machine in theory
- The final stage of our NAT machine
- Compiling the user-land applications
- The conntrack entries
- Untracked connections and the raw table
- Basics of the iptables command
- Other debugging tools