Operating Systems Processes Operating System Concepts 8 th

What is a process? n An instance of an application execution n Process is

What’s “in” a process? n A process consists of (at least): l l An

The OS’s process namespace n (Like most things, the particulars depend on the specific

Process State n As a process executes, it changes state l new: The process

Diagram of Process State Operating System Concepts – 8 th Edition 3. 6 Silberschatz,

Process States terminated running schedule wait for event preempt created ready blocked event done

State queues n The OS maintains a collection of queues that represent the state

Process Creation n Parent process create children processes, which, in turn create other processes,

A tree of processes on a typical Solaris Operating System Concepts – 8 th

Process Termination n Process executes last statement and asks the operating system to delete

CPU state=Register contents n Process Status Word (PSW) l exec. mode, last op. outcome,

The PCB n The PCB is a data structure with many, many fields: l

Address space n Text l Program code n Data l Predefined data (known in

Process Control Block (PCB) Information associated with each process n Process state n Program

Process Scheduling Queues n Job queue – set of all processes in the system

Ready Queue And Various I/O Device Queues Operating System Concepts – 8 th Edition

Representation of Process Scheduling Operating System Concepts – 8 th Edition 3. 18 Silberschatz,

Schedulers n Long-term scheduler (or job scheduler) – selects which processes should be brought

Context Switch n When CPU switches to another process, the system must save the

CPU Switch From Process to Process Operating System Concepts – 8 th Edition 3.

CPU Scheduler n Selects from among the processes in memory that are ready to

When to assign? n Pre-emptive vs. non-preemptive schedulers l Non-preemptive 4 l once you

Dispatcher n Dispatcher module gives control of the CPU to the process selected by

Scheduling Criteria n CPU utilization – keep the CPU as busy as possible n

First-Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P

Shortest-Job-First (SJF) Scheduling n Associate with each process the length of its next CPU

Example of SJF Process Arrival Time Burst Time P 1 0. 0 6 P

Round Robin (RR) n Each process gets a small unit of CPU time (time

Example of RR with Time Quantum = 4 Process Burst Time P 1 P

Time Quantum and Context Switch Time Operating System Concepts – 8 th Edition 3.

CPU SCHEDULING EXAMPLE DATA: Process 1 2 3 4 Operating System Concepts – 8

Example of RR with Time Quantum = 20 n Example: l Process P 1

Multilevel Queue n Ready queue is partitioned into separate queues: foreground (interactive) background (batch)

Multilevel Queue Scheduling Operating System Concepts – 8 th Edition 3. 35 Silberschatz, Galvin

Example of Multilevel Feedback Queue n Three queues: l Q 0 – RR with

Multilevel Feedback Queues Operating System Concepts – 8 th Edition 3. 37 Silberschatz, Galvin

Multiple-Processor Scheduling n CPU scheduling more complex when multiple CPUs are available n Homogeneous

Scheduling Metrics n Waiting Time: time the job is waiting in the ready queue

Scheduling Policy Goals/Criteria n Minimize Response Time l Minimize elapsed time to do an

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What is a process? n An instance of an application execution n Process is the most basic abstractions provided by OS l An isolated computation context for each application n Computation context l CPU state + address space + environment Operating System Concepts – 8 th Edition 3. 2 Silberschatz, Galvin and Gagne © 2009

What’s “in” a process? n A process consists of (at least): l l An address space, containing 4 the code (instructions) for the running program 4 the data for the running program (static data, heap data, stack) CPU state, consisting of 4 The program counter (PC), indicating the next instruction 4 The stack pointer 4 Other l general purpose register values A set of OS resources 4 open files, network connections, sound channels, … n In other words, it’s all the stuff you need to run the program l or to re-start it, if it’s interrupted at some point Operating System Concepts – 8 th Edition 3. 3 Silberschatz, Galvin and Gagne © 2009

The OS’s process namespace n (Like most things, the particulars depend on the specific OS, but the principles are general) n The name for a process is called a process ID (PID) l An integer n The PID namespace is global to the system l Only one process at a time has a particular PID n Operations that create processes return a PID l E. g. , fork() n Operations on processes take PIDs as an argument l E. g. , kill(), wait(), nice() Operating System Concepts – 8 th Edition 3. 4 Silberschatz, Galvin and Gagne © 2009

Process State n As a process executes, it changes state l new: The process is being created l running: Instructions are being executed l waiting: The process is waiting for some event to occur l ready: The process is waiting to be assigned to a processor l terminated: The process has finished execution Operating System Concepts – 8 th Edition 3. 5 Silberschatz, Galvin and Gagne © 2009

State queues n The OS maintains a collection of queues that represent the state of all processes in the system l typically one queue for each state 4 e. g. , ready, waiting, … l each PCB is queued onto a state queue according to the current state of the process it represents l as a process changes state, its PCB is unlinked from one queue, and linked onto another n Once again, this is just as straightforward as it sounds! The PCBs are moved between queues, which are represented as linked lists. There is no magic! Operating System Concepts – 8 th Edition 3. 8 Silberschatz, Galvin and Gagne © 2009

Process Creation n Parent process create children processes, which, in turn create other processes, forming a tree of processes n Generally, process identified and managed via a process identifier (pid) n Resource sharing l Parent and children share all resources l Children share subset of parent’s resources l Parent and child share no resources n Execution l Parent and children execute concurrently l Parent waits until children terminate Operating System Concepts – 8 th Edition 3. 9 Silberschatz, Galvin and Gagne © 2009

Process Termination n Process executes last statement and asks the operating system to delete it (exit) l Process’ resources are de-allocated by operating system n Parent may terminate execution of children processes (abort) l Child has exceeded allocated resources l Task assigned to child is no longer required l If parent is exiting 4 Some operating system do not allow child to continue if its parent terminates – All children terminated - cascading termination Operating System Concepts – 8 th Edition 3. 11 Silberschatz, Galvin and Gagne © 2009

CPU state=Register contents n Process Status Word (PSW) l exec. mode, last op. outcome, interrupt level n Instruction Register (IR) l Current instruction being executed n Program counter (PC) n Stack pointer (SP) n General purpose registers Operating System Concepts – 8 th Edition 3. 12 Silberschatz, Galvin and Gagne © 2009

The PCB n The PCB is a data structure with many, many fields: l process ID (PID) l parent process ID l execution state l program counter, stack pointer, registers l address space info l UNIX user id, group id l scheduling priority l accounting info l pointers for state queues n In Linux: l defined in task_struct (include/linux/sched. h) l over 95 fields!!! Operating System Concepts – 8 th Edition 3. 13 Silberschatz, Galvin and Gagne © 2009

Address space n Text l Program code n Data l Predefined data (known in compile time) n Heap l Dynamically allocated data n Stack l Supporting Operating System Concepts – 8 th Edition function calls 3. 14 Silberschatz, Galvin and Gagne © 2009

Process Control Block (PCB) Information associated with each process n Process state n Program counter n CPU registers n CPU scheduling information n Memory-management information n Accounting information n I/O status information Operating System Concepts – 8 th Edition 3. 15 Silberschatz, Galvin and Gagne © 2009

Process Scheduling Queues n Job queue – set of all processes in the system n Ready queue – set of all processes residing in main memory, ready and waiting to execute n Device queues – set of processes waiting for an I/O device n Processes migrate among the various queues Operating System Concepts – 8 th Edition 3. 16 Silberschatz, Galvin and Gagne © 2009

Schedulers n Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue (seconds, minutes) (may be slow) n Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU (milliseconds) (must be fast) Operating System Concepts – 8 th Edition 3. 19 Silberschatz, Galvin and Gagne © 2009

Context Switch n 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 via a context switch n Context of a process represented in the PCB n Context-switch time is overhead; the system does no useful work while switching n Time dependent on hardware support Operating System Concepts – 8 th Edition 3. 20 Silberschatz, Galvin and Gagne © 2009

CPU Scheduler n Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them n CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates n Scheduling under 1 and 4 is nonpreemptive n All other scheduling is preemptive Operating System Concepts – 8 th Edition 3. 22 Silberschatz, Galvin and Gagne © 2009

When to assign? n Pre-emptive vs. non-preemptive schedulers l Non-preemptive 4 l once you give somebody the green light, they’ve got it until they relinquish it – an I/O operation – allocation of memory in a system without swapping Preemptive 4 you can re-visit a decision – – 4 setting the timer allows you to preempt the CPU from a thread even if it doesn’t relinquish it voluntarily in any modern system, if you mark a program as non-runnable, its memory resources will eventually be re-allocated to others Re-assignment always involves some overhead – Overhead doesn’t contribute to the goal of any scheduler n We’ll assume “work conserving” policies l Never leave a resource idle when someone wants it 4 Why even mention this? When might it be useful to do something else? Operating System Concepts – 8 th Edition 3. 23 Silberschatz, Galvin and Gagne © 2009

Dispatcher n Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: l switching context l switching to user mode l jumping to the proper location in the user program to restart that program n Dispatch latency – time it takes for the dispatcher to stop one process and start another running Operating System Concepts – 8 th Edition 3. 24 Silberschatz, Galvin and Gagne © 2009

Scheduling Criteria n CPU utilization – keep the CPU as busy as possible n Throughput – # of processes that complete their execution per time unit n Turnaround time – amount of time to execute a particular process n Waiting time – amount of time a process has been waiting in the ready queue n Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) Operating System Concepts – 8 th Edition 3. 25 Silberschatz, Galvin and Gagne © 2009

First-Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P 3 3 n Suppose that the processes arrive in the order: P 1 , P 2 , P 3 The Gantt Chart for the schedule is: P 1 P 2 0 24 P 3 27 30 n Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 n Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts – 8 th Edition 3. 26 Silberschatz, Galvin and Gagne © 2009

Shortest-Job-First (SJF) Scheduling n Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time n SJF is optimal – gives minimum average waiting time for a given set of processes l The difficulty is knowing the length of the next CPU request Operating System Concepts – 8 th Edition 3. 27 Silberschatz, Galvin and Gagne © 2009

Example of SJF Process Arrival Time Burst Time P 1 0. 0 6 P 2 0. 0 8 P 3 0. 0 7 P 4 0. 0 3 n SJF scheduling chart P 4 0 P 3 P 1 3 9 P 2 16 24 n Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 Operating System Concepts – 8 th Edition 3. 28 Silberschatz, Galvin and Gagne © 2009

Round Robin (RR) n Each process gets a small unit of CPU time (time quantum), usually 10 -100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. n If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units. n Performance l q large FIFO l q small q must be large with respect to context switch, otherwise overhead is too high Operating System Concepts – 8 th Edition 3. 29 Silberschatz, Galvin and Gagne © 2009

Example of RR with Time Quantum = 4 Process Burst Time P 1 P 2 P 3 24 3 3 n The Gantt chart is: P 1 0 P 2 4 P 3 7 P 1 10 P 1 14 P 1 18 22 P 1 26 P 1 30 n Typically, higher average turnaround than SJF, but better response Operating System Concepts – 8 th Edition 3. 30 Silberschatz, Galvin and Gagne © 2009

Example of RR with Time Quantum = 20 n Example: l Process P 1 P 2 P 3 P 4 Burst Time 53 8 68 24 Remaining Time 53 8 68 24 The Gantt chart is: Operating System Concepts – 8 th Edition 3. 33 Silberschatz, Galvin and Gagne © 2009

Multilevel Queue n Ready queue is partitioned into separate queues: foreground (interactive) background (batch) n Each queue has its own scheduling algorithm l foreground – RR l background – FCFS n Scheduling must be done between the queues l Fixed priority scheduling; (i. e. , serve all from foreground then from background). Possibility of starvation. l Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i. e. , 80% to foreground in RR l 20% to background in FCFS Operating System Concepts – 8 th Edition 3. 34 Silberschatz, Galvin and Gagne © 2009

Example of Multilevel Feedback Queue n Three queues: l Q 0 – RR with time quantum 8 milliseconds l Q 1 – RR time quantum 16 milliseconds l Q 2 – FCFS n Scheduling l A new job enters queue Q 0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. l At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2. Operating System Concepts – 8 th Edition 3. 36 Silberschatz, Galvin and Gagne © 2009

Multiple-Processor Scheduling n CPU scheduling more complex when multiple CPUs are available n Homogeneous processors within a multiprocessor n Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing n Symmetric multiprocessing (SMP) – each processor is self-scheduling, all processes in common ready queue, or each has its own private queue of ready processes n Processor affinity – process has affinity for processor on which it is currently running l soft affinity l hard affinity Operating System Concepts – 8 th Edition 3. 38 Silberschatz, Galvin and Gagne © 2009

Scheduling Metrics n Waiting Time: time the job is waiting in the ready queue l Time between job’s arrival in the ready queue and launching the job n Service (Execution) Time: time the job is running n Response (Completion) Time: l l Time between job’s arrival in the ready queue and job’s completion Response time is what the user sees: 4 Time to echo a keystroke in editor 4 Time to compile a program Response Time = Waiting Time + Service Time n Throughput: number of jobs completed per unit of time l Throughput related to response time, but not same thing: 4 Minimizing response time will lead to more context switching than if you only maximized throughput Operating System Concepts – 8 th Edition 3. 39 Silberschatz, Galvin and Gagne © 2009

Scheduling Policy Goals/Criteria n Minimize Response Time l Minimize elapsed time to do an operation (or job) n Maximize Throughput l Two parts to maximizing throughput 4 Minimize overhead (for example, context-switching) 4 Efficient use of resources (CPU, disk, memory, etc) n Fairness Share CPU among users in some equitable way l Fairness is not minimizing average response time: l 4 Better average response time by making system less fair Operating System Concepts – 8 th Edition 3. 40 Silberschatz, Galvin and Gagne © 2009