Managing Processes

1
Managing Processes

• Chapter Six

2
The Linux Multitasking Model

• Process
• Program running on system
• Started from
• Command line
• Graphical desktop
• Kernel itself
• Another process
• Kernel maintains information about processes

3
The Linux Multitasking Model (continued)

• Linux is multitasking operating system
• Single microprocessor can really only perform one
  task at a time
• Time slice
• A few microseconds
• Allocated to each process by kernel
• Common to all operating systems that dont have
  multiple microprocessors

4
Creating Processes

• Kernel starts process called init
• When Linux first started
• Every process running on Linux is descendant of
  init process
• Process id (PID)
• Unique number identifying process within Linux
  kernel
• init PID 1

5
Creating Processes (continued)

• Fork
• Create new process in Linux
• Exit
• When process ends
• Information about process that kernel was
  maintaining discarded
• Resources that kernel had allocated to process
  released

6
Process States

• Running on run queue
• Sleeping waiting for a resource (e.g., CPU)
• Stopped (stopped from terminal, waiting on a
  pipe, completion of I/O)
• Zombie (child process whose parent did not free)

7
Starting Processes from the Shell

• When you start program
• Program takes control of command line
• Parent process pauses to wait for new process to
  finish
• Type ampersand () after name of command
• Shell forks new process without pausing itself
• Called placing process in background
• Can start another command immediately

8
Starting Processes from the Shell (continued)

• Job
• Process associated with shell
• jobs command
• Lists all jobs or processes running as
  descendants of current shell
• Also called child processes
• CtrlZ key combination
• Suspend job that shell is running

9
Starting Processes from the Shell (continued)

• CtrlZ stop program (suspended)
• bg command
• Run suspended program in background
• fg command
• Place suspended job in foreground
• Must specify
• Job number using
• Or PID

10
Starting Processes from the Shell (continued)

• Virtual console
• One type of login screen
• Start multiple text-based login sessions on same
  computer
• Assigned to function keys, typically F1 through
  F6
• Logged in using text-mode console instead of
  graphical login screen
• Start working in first virtual console
• AltF2
• Access second virtual console

11
Learning about Processes via the Command Line

• ps command
• Lists processes currently running on system
• Output
• PID
• Terminal that process is using for output
• CPU time that process has used so far
• Command that started process
• a and x options show
• Processes started by all users
• Processes started by system at boot time
• Other processes that have no controlling terminal

12
Learning about Processes via the Command Line
(continued)

• Daemon
• Background process
• Doesnt have any screen output but waits for
  certain system activity then acts on it
• ps command
• u option shows how each process is using system
• f option
• Displays relationship between different processes
  
• Shows which processes start other processes
• Processes presented in tree diagram

13
Learning about Processes via the Command Line
(continued)

• Administrators track how processes consume system
  resources, particularly
• CPU time
• Memory
• CPU field
• Compares amount of CPU time used by process with
  total time elapsed since previous computation of
  CPU field
• Form of percentage

14
Learning about Processes via the Command Line
(continued)

• CPU field
• Does not show average amount of CPU time used by
  process since it was started
• WCPU field
• Weighted to show 30-second average of percentage
  of CPU time used by process
• Helpful for showing overall usage pattern

15
Learning about Processes via the Command Line
(continued)

• TIME field
• Provides cumulative measure of amount of CPU time
  consumed by process
• /proc file system
• Collection of information about Linux kernel
• Accessed as if it were data files stored in
  subdirectories of /proc
• Need PID to access information
• Accessed using /proc/PID

16
Learning about Processes via the Command Line
(continued)

• top utility
• Displays list of running processes
• Arranged by how much CPU time each is using
• Process consuming greatest amount of CPU time
  shown at top of list
• Updated regularly
• Normally started without any options
• Cannot run in background
• k kill process
• r renice
• q - quit

17
Learning about Processes via the Command Line
(continued)
18
Controlling Processes

• kill command
• Controls any process
• Sends signals to processes
• -l option views all signals
• Examples
• kill -9 1482
• kill -SIGTERM 1482

19
Controlling Processes (continued)

• Signals
• Messages sent between processes
• About 30 different signals available
• Has name and number associated with it
• Software developer decides to which signals
  program responds
• SIGTERM (15) signal
• Requests that program end
• Almost all programs respond
• Software terminate

20
Controlling Processes (continued)

• SIGHUP(1) signal
• Restarts process with the same PID
• Can occur when editing a daemons configuration
  file
• SIGINT(2)
• Weak kill signal
• Same as CtrlC

21
Controlling Processes (continued)

• SIGKILL(9) signal
• Handled by Linux kernel
• Shuts down indicated process
• Unsaved data in program lost
• Rouge programs virtually never crash Linux kernel
• Use kill -9 3535 or kill SIGKILL 3535
• PID 3535

22
Controlling Processes (continued)

• killall command
• Sends signal to all processes started by given
  command
• Useful when program makes copies of itself faster
  than you can locate PIDs and use kill to shut
  them down
• killall gedit
• Each process assigned priority determines how
  much CPU time granted to process
• Normally all processes have same priority

23
Controlling Processes (continued)

• Nice level
• Another name for process priority
• Standard level is 0
• Highest level 20
• System administrator can make any process more or
  less nice
• User can raise nice level of process that he or
  she started and lower priority

24
Controlling Processes (continued)

• Alter process priority
• nice command
• nice analyze -5
• renice command
• renice 10 1776
• 0 normal priority
• 19 lower priority
• -20 higher priority
• More chance at slices of time.
• Can issue nice and renice while top running

25
Controlling Processes (continued)
26
Using Graphical Process Management Tools

• Gkrellm
• View system status information on desktop
  continuously
• KDE System Guard utility
• Graphically displays process list
• Interact with list to rearrange or kill processes
• GNOME System Monitor
• Manage processes graphically

27
Using Graphical Process Management Tools
(continued)

• Manage Linux system effectively
• Keep CPU load monitor visible on Panel
• Check for processes that might need attention
• Reduce CPU load
• Raise nice level of numerous user processes
• Are not all competing at same level
• Add second microprocessor
• Move some tasks to different computer
• Add memory

28
Understanding Shared Libraries

• Library
• Contains prewritten functionality that any
  program can use
• Statically linked applications
• Include library functions in main program
• Dynamically linked applications
• Assume that needed library files are available on
  Linux system
• Use shared libraries
• Location /lib or /usr/lib
• ldd command
• Lists all libraries that program requires
• ldconfig
• Add library

29
Memory

• Physical memory (RAM)
• cat /proc/meminfo
• Virtual memory (swap space)
• More RAM always leads to better performance on
  busy system

30
Swap Space

• Used when system runs out of RAM for currently
  running programs
• Separate hard disk partition called swap
  partition
• Also called virtual memory
• Acts like extension of systems RAM
• Swapped-out process cannot run until swapped
  again
• Use free m or vmstat to monitor

31
Swap Space (continued)

• Thrashing
• Kernel spends so much time moving processes to
  and from swap space that kernel and processes bog
  down and work inefficiently

32
Understanding Paged Memory

• Information transferred to and from swap space in
  units known as pages
• Page of memory
• Block of 4 KB of RAM
• Kernel keeps track of which pages of memory are
  moved to swap space

33
Tracking Memory Usage

• free command
• Displays information about both RAM and virtual
  memory
• buffer
• Memory used by application for data storage
• Several fields of ps command output also provide
  information about memory usage

34
Viewing Virtual Memory Information

• vmstat command
• View detailed information about how swap space is
  being used
• Displayed information based on information
  averaged over time since system was started
• Can also run to be continuously updated
• Like top

35
Scheduling Processes

• at command
• Define one or more commands to be executed at
  some future time
• Command relies on background process called atd
• crontab command
• Define one or more commands to be executed
  repeatedly at intervals that you designate
• Relies on background process called crond

36
Automating One-Time Tasks

• Using at command enter commands
• Directly at command line
• at now 5 hours
• CtrlD to exit
• Or listed in file
• at -f ltfilenamegt lttime specificationgt
• atd daemon checks once per minute for any jobs
  that scheduled using at command
• Output of commands e-mailed to user

37
Automating One-Time Tasks (continued)
38
Automating One-Time Tasks (continued)

• tty command
• Sends output from command to terminal in which
  you are currently logged in
• batch command
• Similar to at
• Runs commands when system load average drops
  below 0.8 or 1.5.

39
Automating Recurring Tasks

• Use crontab command
• To execute commands at regular intervals
• Red Hat Linux
• /etc directory contains subdirectories named
  cron.Hourly, cron.Daily, cron.Weekly, and
  cron.Monthly
• Place file subdirectory
• Commands in file executed hourly, daily, weekly,
  or monthly

40
Automating Recurring Tasks (continued)

• Every Linux system should include /etc/crontab
  file
• Illustrates format of standard entry for crontab
  command
• crontab specification begins with five fields
  that identify time
• All five fields must match current time and date
  for command to execute
• Cannot enter cron job commands interactively

41
Managing Automated Tasks

• Daemons atd and crond started when Linux booted
• Commands submit using at or crontab stored in
  subdirectory of /var/spool
• atq and atrm commands
• Manage at jobs awaiting execution
• atq command lists all queued at jobs

42
Managing Automated Tasks (continued)

• atrm command
• Cancel command submitted using at
• Can also use -d option with at command
• Must include job number
• Use similar set of commands to manage cron jobs
• Options to crontab command

43
Review of at commands

• at l or atq
• List jobs submitted
• at d or atrm
• Delete a job specify job id
• Running process
• at c job id
• Shows contents of job

44
Controlling Access to at and crontab

• Default settings allow any user to submit
  commands using either at or crontab
• Can restrict access to only certain users
• /etc/at.allow and /etc/at.deny
• /etc/cron.allow and /etc/cron.deny
• One user name per line
• Allow list overrides deny list

45
cron command

• cron daemon is running in the background
• crontab e
• Eidt crontab table with nano
• crontab l
• List crontab contents

46
cron command

• cron parameters
• 1 2 3 4 5 command
• 1 minutes past hour (0-59)
• 2 hour (0-23)
• 3 day of month (1-31)
• 4 month of year (1-12)
• 5 day of week
• 0 or 7 Sunday, 1 Monday, 6 Saturday

47
cron command

• 10,30,50 10,30,50 minutes
• - every possible option

48
System cron table

• Only root may edit
• /etc/crontab or /etc/cron.d
• Run-parts executes all files in a particular
  directory
• Copy scripts to /etc/cron.daily or use a symbloic
  like to shell
• Runs daily, weekly, or monthly