Processes
- 2. Program, Process, Processor • A progam is a set of instructions. A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity. • Processor is the device which executes programs. • Process needs resources to accomplish its task – CPU, memory, I/O, files – Initialization data • Process termination requires reclaim of any reusable resources
- 3. Threads • A thread is predefined instance of a process. A thread performs a specific function. A process may consist a set of threads • Single-threaded process has one program counter specifying location of next instruction to execute – Process executes instructions sequentially, one at a time, until completion • Multi-threaded process has one program counter per thread • Typically system has many processes, some user, some operating system running concurrently on one or more CPUs – Concurrency by multiplexing the CPUs among the processes / threads
- 4. Single and Multithreaded Processes
- 5. Process Management Activities • The operating system is responsible for the following activities in connection with process management: – Creating and deleting both user and system processes – Suspending and resuming processes – Providing mechanisms for process synchronization – Providing mechanisms for process communication – Providing mechanisms for deadlock handling
- 6. Process Creation • System Initialization – Boots up • User related events – click, double click, drag • Execution of a function call- process creates a child process
- 7. Process Termination • System shut down • Process termination – completes execution • Error condition(Voluntary) • Fatal error(Involuntary • Killed by another process
- 9. • New – just created, arrived in memory • Ready – prepared for execution once the CPU becomes available • Running – Being executed. • Blocked – has suspended running, requires an I/O device. • Terminated – Has finished executed, prepared to exit memory.
- 10. Process Control Block (PCB) Stores Information associated with each process • Process state • Program counter • CPU registers • CPU scheduling information • Memory-management information • Accounting information • I/O status information
- 11. Process Control Block (PCB)
- 12. Process Interruption • CPU Switch From Process to Process
- 13. Process Scheduling Queues • 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 • Processes migrate among the various queues
- 14. Representation of Process Scheduling
- 15. 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
- 16. Process Creation • Parent process create children processes, which, in turn create other processes, forming a tree of processes • Resource sharing may be – Parent and children share all resources – Children share subset of parent’s resources – Parent and child share no resources • Execution mode may be – Parent and children execute concurrently – Parent waits until children terminate
- 17. Process Termination • Process executes last statement and asks the operating system to delete it (exit) – Output data from child to parent (via wait) – Process’ resources are deallocated by operating system • Parent may terminate execution of children processes (abort) if – Child has exceeded allocated resources – Task assigned to child is no longer required – If parent is exiting • Some operating system do not allow child to continue if its parent terminates – All children terminated - cascading termination