context switching and dispatcher

Context Switching

  • Switching the CPU to another process requires saving the state of the old process and loading the saved state for the new process. This task is known as a context switch.
  • The context of a process is represented in the Process Control Block(PCB) of a process; it includes the value of the CPU registers, the process state and memory-management information. When a context switch occurs, the Kernel saves the context of the old process in its PCB and loads the saved context of the new process scheduled to run.
  • Context switch time is pure overhead, because the system does no useful work while switching. Its speed varies from machine to machine, depending on the memory speed, the number of registers that must be copied, and the existence of special instructions(such as a single instruction to load or store all registers). Typical speeds range from 1 to 1000 microseconds.
  • Context Switching has become such a performance bottleneck that programmers are using new structures(threads) to avoid it whenever possible.

Dispatcher

Another component involved in the CPU scheduling function is the dispatcher. The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler. This function involves:

  • Switching context
  • Switching to user mode
  • Jumping to the proper location in the user program to restart that program

The dispatcher should be as fast as possible, given that it is invoked during every process switch. The time it takes for the dispatcher to stop one process and start another running is known as the dispatch latency. Dispatch Latency can be explained using the below figure:

Dispatcher

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