Process Scheduling

• Scheduling Queues

Job queue – set of all processes in the system.

Ready queue – set of all processes residing in main memory, ready and waiting to execute.

Device queues – set of processes waiting for an I/O device. Process migration between the various queues. Representation of Process Scheduling

• Schedulers
  

Scheduling is a key concept in computer multitasking and multiprocessing operating system design, and in real-time operating system design. In modern operating systems, there are typically many more processes running than there are CPUs available to run them. Scheduling refers to the way processes are assigned to run on the available CPUs. This assignment is carried out by software known as a scheduler.

The scheduler is concerned mainly with:
CPU utilization - to keep the CPU as busy as possible.
Throughput - number of process that complete their execution per time unit.
Turnaround - 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.
Fairness - Equal CPU time to each thread.
In real-time environments, such as mobile devices for automatic control in industry (for example robotics), the scheduler also must ensure that processes can meet deadlines; this is crucial for keeping the system stable. Scheduled tasks are sent to mobile devices and managed through an administrative back end.

• Context Switch

For other uses, see Switch (disambiguation).
A context switch is the computing process of storing and restoring the state (context) of a CPU such that multiple processes can share a single CPU resource. The context switch is an essential feature of a multitasking operating system. Context switches are usually computationally intensive and much of the design of operating systems is to optimize the use of context switches. A context switch can mean a register context switch, a task context switch, a thread context switch, or a process context switch. What constitutes the context is determined by the processor and the operating system.

Contents
1 When to switch?
1.1 Multitasking
1.2 Interrupt handling
1.3 User and kernel mode switching
2 Context switch: steps
3 Software vs hardware context switching
4 External links