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Operating System Concepts (4)

Module 4: Processes.• Process Concept.• Process Scheduling.• Operation on Processes.• Cooperating Processes.• Interprocess Communication. 4.1 Silberschatz and Galvin 1999 Process Concept.• An operating system executes a variety of programs:. – Batch system – jobs. – Time-shared systems – user programs or tasks.• Textbook uses the terms job and process almost. interchangeably• Process – a program in execution; process execution must. progress in sequential fashion• A process includes:. – program counter. – stack. – data section. 4.2 Silberschatz and Galvin 1999 Process State.• As a process executes, it changes state. – new: The process is being created – running: Instructions are being executed – waiting: The process is waiting for some event to occur – ready: The process is waiting to be assigned to a process – terminated: The process has finished execution 4.3 Silberschatz and Galvin 1999 Diagram of Process State. 4.4 Silberschatz and Galvin 1999 Process Control Block (PCB).Information associated with each process• Process state.• Program counter.• CPU registers.• CPU scheduling information.• Memory-management information.• Accounting information.• I/O status information. 4.5 Silberschatz and Galvin 1999 Process Control Block (PCB). 4.6 Silberschatz and Galvin 1999 CPU Switch From Process to Process. 4.7 Silberschatz and Galvin 1999 Process 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 4.8 Silberschatz and Galvin 1999 Ready Queue And Various I/O Device Queues. 4.9 Silberschatz and Galvin 1999 Representation of Process Scheduling. 4.10 Silberschatz and Galvin 1999 Schedulers.• Long-term scheduler (or job scheduler) – selects which. processes should be brought into the ready queue• Short-term scheduler (or CPU scheduler) – selects which process. should be executed next and allocates CPU 4.11 Silberschatz and Galvin 1999 Addition of Medium Term Scheduling. 4.12 Silberschatz and Galvin 1999 Schedulers (Cont.).• Short-term scheduler is invoked very frequently (milliseconds). (must be fast)• Long-term scheduler is invoked very infrequently (seconds,. minutes) (may be slow)• The long-term scheduler controls the degree of. multiprogramming• Processes can be described as either:. – I/O-bound process – spends more time doing I/O than. computations, many short CPU bursts – CPU-bound process – spends more time doing. computations; few very long CPU bursts 4.13 Silberschatz and Galvin 1999 Context Switch.• When CPU switches to another process, the system must save. the state of the old process and load the saved state for the new. process• Context-switch time is overhead; the system does no useful work. while switching• Time dependent on hardware support 4.14 Silberschatz and Galvin 1999 Process Creation.•