Process Control

Components of a process

• Represents a running program
• An address space
• A set of data structures within the kernel
  • Address space map
  • Current status of the process
  • Execution priority of the process
  • Information about resources
  • Information about files and open ports
  • Signal mask
  • Owner
  • …

Identifiers

• PID: process ID number
• PPID: parent PID - how is a process spawn?
• UID: user identifier of the person who created the process
• EUID: effective user identifier - what resources the process has access to at any given moment
• GID: group identifier of the person who created the process
• EGID: effective group identifier

Life cycle of a process

• When system boot, the first process is created (init or systemd) with process ID of 1
• All other processes are created through fork - fork() creates a copy of the parent process. - In the copy (child), fork() returns 0. - In the original (parent), fork() returns the new PID of the child process.

Signals

• Process-level interrupt requests
• Approximately thirty signals
  • Are sent as means of communication
  • Are sent by terminal driver to kill/interrupt/suspend processes using Ctrl-C or Ctrl-Z
  • Are sent by kill to terminal processes
  • Are sent by kernel when faults happen (e.g., divided by 0)
  • Are sent by kernel to notify the process of interesting events (I/O data is available, child process is dead …)

Important signals

• Signal description
• HUP
• INT
• QUIT
• KILL
• BUS
• SEGV
• TERM
• STOP
• TSTP
• CONT
• WINCH
• USR1
• USR2

Monitoring processes: ps

• ps: Snapshot of current processes
• ssh to sulu and run:

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ps aux
 

• USER, PID, %CPU, %MEM
• VSZ: Virtual size of the process
• RSS: Resident set size (number of pages)
• TTY: control terminal ID
• STAT: current process status (Process State Codes from manual)
• TIME: CPU time process consumed
• COMMAND: command and arguments.

Other monitoring commands

• top
• htop

Hands-on

• Spend 10-15 minutes to read and learn how to use tmux
• This is to be done on CloudLab
• strace: check what a process is doing
  • Create a tmux session with two horitonzal panel.
  • Run top in the first screen
  • In the second screen
  • Identify the process ID of top
  • Run strace on this process ID:

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sudo strace -p <top_process_ID>
 

Runaway processes

• System slows down!
• Use ps and top (htop) to identify those that soak up CPU and memory
• Check filesystem disk usage: df -h
• Check directory usage: du -h

Periodic processes

• The cron daemon allows the execution of commands on a predetermined schedule.
• Run the followings

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crontab -e
 

• Select an editor (recommend nano)
• Type * * * * * echo $(/bin/date) >> /users/<your_username>/cron.log into the file
• Save and quit nano (we did this before)
• Wait for a few minutes, then check the content of cron.log
• Common usage:
  • Sending mail
  • Cleaning up a file system
  • Rotating a log file
  • Running batch jobs
  • Backing up and mirroring
• A legacy!!!