===== Controlling Processes =====

  * 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//.

==== Viewing and debugging processes ====

    * 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:
<code>
               # 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
</code>

    * //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:
<code>
               # Shows all open files owned by jchung
               # (generally should run lsof as root)
               $ lsof | grep jchung | less
</code>

    * 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)

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===== Lab Activity =====

==== Strace ====

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.

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