Explain Context switch along with a suitable diagram.Name and briefly define the scheduling criteria. List out the circumstances under which a CPU scheduling decision may have to be taken by the CPU scheduler.

4 years ago

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 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 -switch times are highly dependent on hardware support. For instance, some processors (such as the Sun UltraSPARC) provide multiple sets of registers. A context switch simply includes changing the pointer to the current register set. Of course, if active processes exceed register sets, the system resorts to copying register data to and from memory, as before. Also, the more complex the operating system, the more work must be done during a context switch.

Scheduling Criteria

CPU utilization – keep the CPU as busy as possible
Throughput – Number of processes that complete their execution per time unit
Turnaround time – amount of time to execute a particular process
Waiting time – amount of time a process has been waiting in the ready queue
Response time – amount of time it takes from when a request was submitted until the first response is
produced, not output (for time-sharing environment)

Optimization Criteria