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Process Management. Operating Systems Lecture 3 , 2 7 March 2003 Mr. Greg Vogl Uganda Martyrs University. Overview. Monolithic kernel vs. microkernel (lecture 1) Program, Process, Thread Interrupts Process States Process Scheduling Processes in UNIX Processes in Windows.

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Process management

Process Management

Operating Systems

Lecture 3, 27 March 2003

Mr. Greg Vogl

Uganda Martyrs University


Overview
Overview

  • Monolithic kernel vs. microkernel (lecture 1)

  • Program, Process, Thread

  • Interrupts

  • Process States

  • Process Scheduling

  • Processes in UNIX

  • Processes in Windows

Operating Systems: Process Management


1 monolithic kernel
1. Monolithic Kernel

Operating Systems: Process Management


Microkernel
Microkernel

Operating Systems: Process Management


2 program
2. Program

  • Series of commands

  • Instructions only (usually not data)

  • Stored on disk, copied into memory

  • Can be in use by many users and processes at the same time

  • Also called software

    • Includes shell scripts, batch files

Operating Systems: Process Management


Process
Process

  • Variables point to instructions + data

  • Loaded in memory

    • context contains all process state info.

    • stored in Process Control Block

  • One user and one program per process

  • Also called task

    • (or job for batch processes)

Operating Systems: Process Management


Process context
Process Context

  • program counter

    • register indicating next program instruction to run

  • other registers

    • accumulator, index/address, status, general

  • stack of subroutine return addresses

    • subroutines call other subroutines

  • values of local and global variables

    • pointers to open data files

  • user and terminal number (in multi-user OS)

Operating Systems: Process Management


Examples of processes
Examples of Processes

  • User Processes

    • Shells

    • Text editors, databases

    • Background jobs (end with & in UNIX)

  • System Processes

    • Memory management, process scheduling

    • daemons (system background processes)

      • Mail and print servers

Operating Systems: Process Management


Thread
Thread

  • Also called sub- or lightweight process

  • Little private memory; memory is shared

  • Subdivides work of the process

  • Threads are managed by the process

  • Reduces high overhead for creating processes and context switching

  • Java was designed to write thread-based programs

Operating Systems: Process Management


Thread components
Thread Components

  • At minimum, every thread has its own

    • program counter

    • stack

  • Program text shared with other threads

  • Each procedure has a frame to hold its local variables

  • Heap of objects is shared by all threads

Operating Systems: Process Management


Uses of threads
Uses of Threads

  • Servers (database, mail, print, etc.)

    • one thread per client request

  • Network server

    • one thread per connection

  • Time-sharing

    • one thread per user

  • Real-time factory control

    • one thread per device

Operating Systems: Process Management


3 interrupt
3. Interrupt

  • Signal hardwareCPU requesting services

    • CPU postpones its work to handle interrupt

  • Interrupt handler routines are part of OS code

    • OS switches modes (usersupervisor)

  • Interrupts are prioritised

    • Stack used to store multiple interrupt levels

  • Programs can mask (ignore) some interrupts

    • Others unmaskable (to avoid losing data)

Operating Systems: Process Management


Examples of interrupts
Examples of Interrupts

  • Key pressed

  • Disk or other I/O task finished

  • System clock

Operating Systems: Process Management


Software interrupts
Software Interrupts

  • Also called traps or exceptions

  • Processor interrupted by programs

  • Examples

    • Division by 0 error

    • Bus error

    • Array out of bounds

    • Buffer overflow

Operating Systems: Process Management


Context switching
Context Switching

  • Preserve state of current process

  • Start or re-start another process

  • Performed by dispatcher (OS component)

  • When to change context?

    • Overhead cost (takes CPU time)

    • Processes prioritised by properties

Operating Systems: Process Management


4 process states
4. Process States

  • Runnable

    • Running (only one per processor)

    • Ready (waiting for its turn to run)

  • Blocked

    • Explicit e.g. wait() until a child terminates

    • Implicit e.g. read()

    • Blocked by another process

  • New (not yet allowed to wait its turn)

  • Suspended (e.g. swapped to virtual memory)

  • Terminated (finished but still using resources)

Operating Systems: Process Management


Process state diagram ritchie p 62
Process State DiagramRitchie p. 62

entry (HLS)

I/O completion

ready

blocked

(LLS)dispatch

timeout

I/O wait

(MLS)suspend

running

resume

suspend

resume

suspend

termination

blocked suspended

ready suspended

I/O completion

Operating Systems: Process Management


5 scheduling
5. Scheduling

  • Assign each process time to use CPU

    • Determine sequence (order), timing (when)

    • Conflicting objectives  need compromise

  • Scheduling levels

    • High: whether to admit a new process

    • Medium: suspend/resume a process

    • Low: dispatch (run) a ready process

Operating Systems: Process Management


Scheduling objectives
Scheduling Objectives

  • Maximise throughput

  • Give all users a “fair” (not equal) chance

  • Provide tolerable performance

    • Response time for on-line user

    • Turnaround time for batch users

  • Degrade performance gracefully

    • OK to be slow but avoid complete collapse

  • Be consistent, predictable over time

Operating Systems: Process Management


Scheduling criteria
Scheduling Criteria

  • Priority which may be either/both:

    • Assigned to job by user

    • Determined by properties of the job

  • Class of job (real-time > on-line > batch)

  • Resources needed (CPU time, memory)

  • I/O or CPU bound (the aim is a balance)

  • Resources already used

  • Time already waited

Operating Systems: Process Management


Types of scheduling policies
Types of Scheduling Policies

  • Preemptive (Used by most OS today)

    • OS stops one process to run another

  • Non-preemptive

    • process runs until termination or I/O wait

  • Cooperative (Used by Windows 3.11)

    • Programs must voluntarily give up CPU

    • Not managed by OS; trusts programmers

    • If a process hangs the whole PC hangs 

Operating Systems: Process Management


Scheduling policies
Scheduling Policies

  • First come first served (FCFS/FIFO)

  • Shortest job first (SJF)

  • Shortest remaining time (SRT)

  • Highest response ratio next

  • Round robin (RR)

  • Multi-level feedback queues (MFQ)

Operating Systems: Process Management


First come first served
First come first served

  • Also called first in first out (FIFO)

  • Process waiting longest is first in queue

  • Favours long jobs

    • high run-time/wait-time ratio

  • Favours CPU-bound jobs

    • I/O devices underused

Operating Systems: Process Management


Shortest job first next
Shortest job first (next)

  • Run job with shortest estimated run time

  • Long jobs may be delayed indefinitely

  • JCL commands can specify run time

Operating Systems: Process Management


Shortest remaining time
Shortest remaining time

  • Run job w/ shortest est. remaining time

  • Highly favours short jobs

  • Long jobs will be delayed indefinitely

  • Requires estimating total run time

  • Requires measuring elapsed run time

Operating Systems: Process Management


Highest response ratio next
Highest response ratio next

  • P = (time waiting + run time) / run time

  • Priority based on two factors

  • Favours shorter jobs

  • Guarantees a job cannot be starved

Operating Systems: Process Management


Round robin
Round Robin

  • Each process given a set time slice

  • Pre-emption at end of time quantum

    • Hardware timer generates interrupts

  • After running, go to back of queue

  • Used in most interactive operating systems

  • How large should each time slice be?

    • Small  high context switch overhead

    • Large  user response time is reduced

    • In practice, about 10-20 ms

Operating Systems: Process Management


Multi level feedback queues
Multi-level feedback queues

  • Separate queues for different priorities

    • New process  level 1 FIFO (highest)

    • After timeout  level 2 FIFO, etc.

    • Lowest level is Round Robin

  • Adapts to past process behaviours

    • high CPU usage  reduced access level

  • Long processes may starve

    • if wait time is long, promote/increase slice

Operating Systems: Process Management


6 unix process creation
6. UNIX process creation

  • PID 0: sched (process scheduler)

  • PID 1: init (ancestor of other processes)

  • daemons (automatic, for sys. admin.)

  • getty (one per terminal)

  • login (lets users log in)

  • shell (lets users run programs)

  • user-initiated processes (run from shell)

Operating Systems: Process Management


Unix fork
UNIX fork()

  • fork() produces new “child” process

  • Child is exact copy of parent

    • Starts in same program, at same place

    • Exact copy of memory space

      • (globals, stack, heap object)

  • Both calls to fork() return a value

    • Parent fork() returns process ID of child

    • Child fork() returns 0

Operating Systems: Process Management


Unix exec
UNIX exec()

  • Execute (load, run) another program

  • Overwrite calling process in memory

  • Uses same PID; not a new process

  • Used after fork() to start new process

  • Parent waits for child process to finish

  • Run in background to not hold up shell

    • myprogram &

Operating Systems: Process Management


Unix scheduling
UNIX scheduling

  • Varies with the flavour of UNIX

    • Often dynamic priority, round robin, MFQ

  • Processes have number priority values

    • 0=highest, 60=lowest, 20=initial/default

  • Users can reduce priority using nice

    • nice -10 myprog # makes priority 30

  • Only superuser can increase priority

    • nice --10 myprog # makes priority 10

Operating Systems: Process Management


Unix ps
UNIX ps

  • Command to display process status

  • ps # list PIDs, TTYs, CPU time, name

  • ps -a # all terminal-created processes

  • ps -e # everything (incl. daemons)

  • ps -f # full list (more details)

  • ps -l # long list (ppid, priority, size, nice)

Operating Systems: Process Management


Other unix commands
Other UNIX commands

  • top – process list, continuously updated

  • jobs – list jobs running in current shell

  • bg, fg – send jobs to back/foreground

  • at – run batch job at specified time(s)

  • kill – send signal to terminate process

  • sleep – pause for some seconds

Operating Systems: Process Management


7 processes in windows
7. Processes in Windows

  • Every process treated as thread

  • Process can create additional threads

  • Scheduler operates over all threads

  • Each process has virtual address space

  • No parent-child process relationship

  • Environment copied to new processes

Operating Systems: Process Management


Dynamic link libraries dlls
Dynamic Link Libraries (DLLs)

  • Executable code routines

  • Linked into application only at run time

  • Can be shared by several programs

Operating Systems: Process Management


Starting programs
Starting Programs

  • Type program name (maybe full path)

    • MS-DOS prompt, run dialog box, Explorer

  • Double-click icon

    • Desktop, Explorer, My Computer

  • Click shortcut

    • Start or Programs menu, QuickLaunch

  • Start when the computer starts

    • autoexec.bat, config.sys, Startup menu

Operating Systems: Process Management


Managing tasks
Managing Tasks

  • Ways to close programs

    • Ctrl-C; File, Exit; Close button (X); Alt-F4

  • Press Ctrl-Alt-Del to view task manager

    • End Task: stop non-responding program

    • Ctrl-Alt-Del again to shut down/restart

Operating Systems: Process Management


Winme system information
WinMe System Information

  • Loaded modules (exe, dll, etc.)

    • Name, version, size, file date, mfg., path

  • Running tasks

    • Name, path, PID, priority, version, size

  • Startup programs

    • Program, command, user name, location

  • Print jobs

Operating Systems: Process Management


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