Joe Chung
Monmouth U. Homepage

• process priorities
  • normal user process priorities range from 0-19
    • called “niceness” of a process, where 0 is normal and 19 is nicest
    • root and user can control priority of processes
  • root process priorities range from -20 to 19.
  • See man nice

• view most resource intensive processes with top

• control priorities with nice / renice commands
  • use nice to start processes with a certain priority
  • use renice to set priority of already-running process

• finding processes
  • pgrep, ps aux | grep [string]

• terminating processes (UNIX)
  • kill PID
  • pkill [process name pattern]
  • using signals (most commonly used signals in UNIX kill command)
    • kill -HUP PID or kill -1 PID forces a hangup, causing some processes to reload and re-read their configuration
    • kill -KILL PID or kill -9 PID causes immediate, unclean termination of process
      • May use if normal kill PID or pkill [process name pattern] does not work
        • normal, default signal is SIGTERM
    • See output of kill -L.

• strace, dtrace, SystemTap
  • Install the strace package on your Linux systems if you don't have it.
  • strace is a tool for tracing system calls and signals.
    • Intercepts and records the system calls made by a running process.
    • Can print a record of each system call, its arguments, and its return value.
    • Often useful in instances where a program freezes or otherwise fails to work and offers few obvious clues as to the problem.
    • Examples:

               # Shows system calls made by running 'hostname'
               $ strace hostname


               # Shows what files are opened when running 'who'
               $ strace -e open who


               # Attach to an already running process; useful for watching
               # daemons in action:               
               $ ps aux | grep ssh
               root      1243  0.0  0.1   5488   968 ?        Ss   Mar12   0:00 /usr/sbin/sshd
               ...
               # Attach strace to the running /usr/sbin/sshd process by its PID (1243),
               # and watch the output as someone logs in via ssh:
               $ strace -p 1243

• lsof
  • a command meaning “list open files”, which is used in many UNIX-like systems to report a list of all open files and the processes that opened them.
  • Open files in the system include disk files, pipes, network sockets and devices opened by all processes.
  • One use for this command is for when a disk cannot be unmounted because unknown files are in use.
  • Example:

               # Shows all open files owned by jchung
               # (generally should run lsof as root)
               $ lsof | grep jchung | less

• Controlling process output verbosity
  • Well-written programs and daemons include options to include extra information in output to facilitate debugging and performance analysis.
  • Example: ssh -v (verbose mode)
    • ssh -vv, ssh -vvv (extra verbose modes)

On your Linux virtual machines, what do you see when you

 $ ping <your Linux system hostname>   # example: $ ping richardson

?

Run strace on your Linux virtual machines to investigate how ping goes about finding an IP address associated with your system hostname to send ping packets to.

Force ping to send just one ping packet, and force strace to send its output to a file called strace.out, so that you can look at strace output offline.