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Process Description and Control Process concepts Definitions replaces task, job – program in execution – entity that can be assigned to and executed on a processor – Process states running – ready – blocked – The main responsibility of an OS is to control the execution of processes OS objectives and processes Operating systems must interleave the execution of a number of processes to maximize processor use – while providing reasonable response rate – Operating systems must allocate resources to processes based on priorities – while avoiding deadlock – Operating systems may support interprocess communications – user creation of processes – A two-state process model Dispatch Enter Exit Not running Running Pause Process creation – – – – new batch job interactive login OS created to provide a service spawned by existing process Process termination – – normal completion abnormal end (abend) time related protection error program error data error I/O failure memory error Process state transitions with suspend and resume I/O or event completion Blocked Dispatch Ready A C T I V E I/O or event wait Timerrunout Running Suspend Suspend Resume Suspended ready Suspend I/O or event completion Resume Suspended blocked SWAPPED Other related concepts Suspend and swapping not immediately available – may or may not be waiting for event or I/O – blocked may be independent of suspend – process was placed on suspend by an agent – OS and processes the OS schedules and dispatches processes for execution by the processor, and allocates resources to processes – OS is the entity that manages the use of system resources by processes – OS control structures What does the OS needs (a) to be able to control processes and (b) manage resources for them? memory tables – I/O tables – file tables – process tables – These tables are maintained by the OS How does it know what and how to create? – Initialization and configuration Process control structures What is the physical manifestation of a process? a program or set of programs – a set of data locations for variables and constants – memory to hold programs and data – stack for procedure calls, parameter passing – PCB - Process Control Block entity that defines a process to the OS – collection of process attributes: process identification, processor state and process control information. – OS core, kernel or supervisor Execution modes: system or user modes OS core or kernel Process management process creation and termination process scheduling and dispatching process switching process synchronization and support for inter-process communication management of PCBs – Memory management – I/O management – Support functions – Is the Operating System a process? 1. No, separate P1 P2 P3 . . . . Pn Kernel P 1 P 2 P3 Pn .... 2.No, subroutines 3.Yes OS OS OS OS Process switching U1 U2 U3 .... Un O1 O2 Process switching O3 .... On Processes and Threads Process is a unit of: resource ownership – dispatching – Threads only an unit of dispatching – inside a process – saves time to create, switch and terminate – A good example: a file server process each request as a thread – the process spawn the threads –