Review Operating System Manages all system resources ALU

Review: Operating System • Manages all system resources – ALU – Memory – I/O – Files • Objectives: – Security – Efficiency – Convenience

What is a process? • Any running program including its context and environment: – Register content – Memory content and address space – PC – State (example: waiting for I/O)

Process Management • Why process management? – There is only one (or a limited number of) CPU per computer, but most of the time many programs run at the “same” time – Multi user systems such as sales, debit or eureka (UNIX machines of Stern) – All modern computer support multitasking via time slicing – List of current processes • Task list on PC after CTR-Alt-Del • top command under UNIX

Process abstraction and context • Any process feels as if he was the only one running on the machine, he does not need to know anything about any other process (unless related) • But a processor has a limited amount of physical resources and every process on the machine has its own set of registers. Solution: save and restore hardware state on a context switch. Save the state in Process Control Block (PCB)

Process Control Block • All information that is associated with a process – Process state (new, ready, waiting …) – PC (Program Counter) how far got the execution – CPU Register content – Memory management information – I/O status – Some accounting (open files …) • All this information has to be updated whenever the process changes state: Overhead!

Historical Difference UNIX, DOS • Uniprogramming: one process at a time • Multiprogramming: multiple processes • DOS was originally uniprogramming – One key problem with DOS is that there is no memory protection - one program may write the memory of another program, causing weird bugs. • Unix plus all currently envisioned new operating systems allow system to separate out activities cleanly

Process Creation • Parent processes create child processes, which in turn create further child processes • Resource sharing – degree of sharing can vary from all to none • Execution – concurrently – parent waits for child to terminate – example in UNIX: call child with & option keeps the parent active

Process Termination • User logs off • Quit an application – User interaction – Time limit exceeded • Process finished: Normal completion • Error – Memory unavailable – Arithmetic error – Security error: Memory or file

Process Termination • • • I/O Failure Invalid instruction (you tried to execute data) Operating system intervention (e. g. deadlock) Parent terminates (shell logout) Parent requests termination(kill)

Process States • During Execution a process changes state – new: The process is being created • User calls a program • Some program starts another process – running: Instructions are executed – waiting: The process is waiting for an event to occur – ready: The process is waiting to be executed – terminated: The process has finished execution

Diagram of Process States new exit admitted terminated interrupt ready running scheduler I/O or event completion waiting I/O or event wait

Process States Example

Memory

CPU Switch from Process to Process P 0 executing OS P 1 Interrupt Save state in PCB 0 idle Load state from PCB 1 idle Interrupt executing Save state in PCB 1 Load state in PCB 0 executing idle

Interrupts • Two types: Hardware and Software • Hardware interrupts are generated by a physical device • Software generated by OS for instance after timeout of running process • Interrupt register/bus that communicates interrupt to the control unit of CPU • Different Levels – depending on the priority of the current process, lower level interrupts can be ignored – hardware have usually higher priority than software interrupts

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 during which the system does no useful work • Overhead depends on hardware support • Question: You have 6 big job to run on a 2 processor machine. What is the optimal strategy to start them?

Scheduling Objectives • Fairness – Give equal and fair access to all processes • Responsiveness – Discriminate between different classes of jobs • Efficiency – Maximize throughput – Minimize response time – Accommodate as many uses as possible

Process Scheduling Queues • Job queue - all processes in the system • Ready queue - all processes residing in main memory ready and waiting to execute • Device queue - set of processes waiting for an I/O device • Process migration between various queues

Two Queues

Appendix: UNIX Commands • ps: show all processes that were started from this shell – ps -u username : all processes started by user • top: show all processes running on the computer • kill id : Stop running process with id • nohup: make process immune to high level interrupts • nice +int process: change the priority of a process