Kernel

• The kernel is the core of the operating system.
• Taken from (http://harish-kayarohanam.blogspot.com/2011/12/ltrace-and-strace.html)
• Users interact with the kernel mostly via the shell
• Applications are started by the shell (mostly)
• The shell and all applications interact with the kernel via system and library calls.
• Our book lists the following as activities performed by the kernel
  • Process scheduling: Linux uses preemptive multitasking
    • Multitasking : multiple processes appear to run at the same time.
    • Each is given a share of time on the cpu
    • But a process can be interrupted to allow another process to run.
    • And this occurs across multiple processors
    • This is done by a portion of the kernel called the scheduler
    • This is a 1 minute statement about several weeks in OS and perhaps a lifetime of study.
  • Memory Management
    • Each process has a portion of allocated memory
    • The kernel makes sure that one process does not have access to the memory of another process.
    • Unless, of course, they want to share memory.
    • And that kernel memory is protected.
    • It also takes care of where data is stored
      • Real memory
      • Hard drive
      • And if part of this memory is shared. (libc for example)
  • The file system
    • The kernel is responsible for the actual layout of the files (bits) on the disk.
  • Devices
    • All device interaction is handled through the kernel.
  • API
• There are generally two modes in which the cpu operates
  • User mode : access to memory is restricted.
  • Kernel mode, or supervisor mode: access is unrestricted.
  • Some instructions are only permitted to be executed in kernel mode (halt)
  • A system call is required to extract information from kernel space to user space.
• Looking for the kernel
  • Tools:
    • ps shows the running processes on the system.
    • top is an interactive tool to do the same.
  • Each of these shows a snapshot of what processes are running on the system.
  • Some important fields:
    • PID or process id, unique to each process
    • User, who started the process
    • command: the command line argument (probably) used to start the process.
  • pid 1 is the system spawner
    • It is not the kernel, it is the first process started by the kernel when it goes multiuser
    • It was called init but seems to have become systemd
  • Processes with commands in brackets are most likely something started by the kernel (These are manifestations of the kernel)
  • And low level pids are reserved for system processes.
  • kthreadd for example
    • It is apparently difficult to start a thread in kernel space
    • So this daemon is implemented to allow for the kernel to start threads
    • Anything with a ppid of 2 is probably part of the kernel.
  • So we see parts of the kernel, but we don't really see the kernel.
• That is because it somewhat doesn't exist.
• At least not as a process.
• But more on that later.