Transcript Operating Systems: Process Management

Process Management
Operating Systems
Lecture 3, 27 March 2003
Mr. Greg Vogl
Uganda Martyrs University
Overview
1.
2.
3.
4.
5.
6.
7.
Monolithic kernel vs. microkernel (lecture 1)
Program, Process, Thread
Interrupts
Process States
Process Scheduling
Processes in UNIX
Processes in Windows
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1. Monolithic Kernel
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Microkernel
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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
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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)
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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)
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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)

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Mail and print servers
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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 threadbased programs
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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
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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
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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)
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Examples of Interrupts
Key pressed
Disk or other I/O task finished
System clock
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Software Interrupts
Also called traps or exceptions
Processor interrupted by programs
Examples
Division by 0 error
 Bus error
 Array out of bounds
 Buffer overflow

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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

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4. Process States
Runnable


Running (only one per processor)
Ready (waiting for its turn to run)
Blocked
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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)
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Process State Diagram
Ritchie p. 62
I/O completion
running
termination
resume
suspend
ready
suspended
blocked
suspend
resume
(MLS)
ready
blocked
suspended
I/O completion
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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
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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
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Degrade performance gracefully

OK to be slow but avoid complete collapse
Be consistent, predictable over time
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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
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Types of Scheduling Policies
Preemptive (Used by most OS today)
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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 

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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)
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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
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Shortest job first (next)
Run job with shortest estimated run time
Long jobs may be delayed indefinitely
JCL commands can specify run time
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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
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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
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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?


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Small  high context switch overhead
Large  user response time is reduced
In practice, about 10-20 ms
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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
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Adapts to past process behaviours
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high CPU usage  reduced access level
Long processes may starve
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if wait time is long, promote/increase slice
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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)
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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
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(globals, stack, heap object)
Both calls to fork() return a value
Parent fork() returns process ID of child
 Child fork() returns 0
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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 &
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UNIX scheduling
Varies with the flavour of UNIX

Often dynamic priority, round robin, MFQ
Processes have number priority values
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0=highest, 60=lowest, 20=initial/default
Users can reduce priority using nice
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nice -10 myprog
# makes priority 30
Only superuser can increase priority
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nice --10 myprog
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# makes priority 10
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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)
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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
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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
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Dynamic Link Libraries (DLLs)
Executable code routines
Linked into application only at run time
Can be shared by several programs
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Starting Programs
Type program name (maybe full path)

MS-DOS prompt, run dialog box, Explorer
Double-click icon

Desktop, Explorer, My Computer
Click shortcut
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Start or Programs menu, QuickLaunch
Start when the computer starts

autoexec.bat, config.sys, Startup menu
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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

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WinMe System Information
Loaded modules (exe, dll, etc.)

Name, version, size, file date, mfg., path
Running tasks
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Name, path, PID, priority, version, size
Startup programs
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Program, command, user name, location
Print jobs
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