Fundamental Concepts, Chapter 2

• You should read chapters 1 and 2.
• The Kernel
  • Frequently in /boot
  • vm -virtual memory
  • linu - linux
  • z - compressed
  • version information
• The boot loader, based on /boot/grub.conf
  • Accesses this file system
  • Uncompresses the kernel
  • Loads it into memory
  • Jumps to the starting address of the kernel
• He lists things a kernel does
  • Process scheduling
    • A process is a program that has been loaded for execution.
      • Ready to run
      • Running
      • Waiting for a resource
      • Suspended
    • What is Multitasking?
    • What does preemptive mean?
    • Linux is preemptive multitasking
    • A quantum is the maximum length of time a process is allowed to run without interruption.
      • But it may be interrupted sooner if it needs resources.
  • Memory Management
  • File System
    • Linux/Unix treats everything as a file.
    • EVERYTHING
  • Process creation, maintenance and termination
  • Device access
    • Linux treats everything as a file.
  • Networking
    • Linux treats everything as a file.
• Most CPS have two modes
  • Normal mode, where most commands are executed
  • Kernel/Supervisor mode where protected commands are executed
    • IE: halt, interacting with devices
  • Generally access to supervisor mode is through a system call
  • The kernel uses this mode to protect processes from each other and to protect the system from harmful actions by users.
• A process' life
  • The process is spawned (or forked) and it begins executing
  • It really has no knowledge of where it is
    • all references to memory and hard drive are through the kernel
  • It has no, or very little knowledge of other processes
    • Unless an interprocess communication mechanism is used.
  • At some point it will do one of the following
    • Exit and the story is over
    • Make a system call or use up it's quanta or have an extraordinary event
      • In this case, it makes a system call of some form
      • And the kernel begins to execute code on behalf of the process
      • At the end of the kernel code, the scheduler is called
      • And then the kernel code returns, but perhaps not to the same process.
      • 
      • The big thing is as a process working with other processes, has no knowledge of
        • Where it will be interrupted (will it execute the next instruction or not?)
        • What process executes next.
      • MUCH more detail is provided in section 3.1 for those detail hungry people.
• Processes
  • Are created with a system call fork
    • where a parent process creates a child process.
    • The child is mostly identical to the parent
  • A process can call exec to run a different program.
  • Eventually the process will call exit to exit.
  • Each process is provided with memory
    • Text, data, heap, stack, ... as we discuss in architecture.
  • Each process has a unique process id
  • Processes have credentials (the id of the user that owns the process, plus other ids it might assume)
  • More information is stored too.
  • There is a special process called init or systemd
    • The change from init to systemd is new (last 5 years)
    • A decent article.
    • This is a daemon process, or a process that
      • Is running in the background, disassociated from a terminal
      • Is most likely performing some task on behalf of the kernel
      • Runs long term (look at ps on mirkwood)
    • This process is the first started and normally has pid 1
    • And parent id (ppid) 0.
    • It has some special tasks
      • It is responsible for spawning and respawing most other daemons
      • It ultimately becomes the parent of orphan processes.
      • It usually can not be killed
        • If this happens, the system normally shuts down.
      • It generally doesn't receive signals, unless it wants them.
      • It usually runs as root.
  • Non-processes or kernel-threads
    • Programs that show up with commands as [cmd] from ps are kernel threads.
    • These are processes started directly by the kernel
    • Or the kernel thread daemon (kthreadd)
    • They are an equivalent to a daemon
    • You probably can't kill, stop, suspend, or do anything else to them.
    • And you probably don't want to.
• ps
  • This is a unix/linux command
  • It shows a snapshot of the current processes
  • ps -ef to see all processes
  • ps -ef | more to really see all processes
  • ps -ef | grep pts/15 to see all processes associated with pseudo terminal 15
  • There are many, many command line arguments, see man ps
  • ps -ef --forest is cool
• top
  • top is an interactive display of processes
  • man top.