Lecture 004

This lecture covers OSC Chapter 3 (but not 3.5, 3.6 "POSIX Shared Memory", and it's fine to skip)

Pseudo machine life cycle kernel states kernel state memory layout

Birth

fork(): we copy the following, which is expensive

• memory (text, data, stack): copy all

• register: copy all but one (parent learns child's process ID, child gets 0)

• io (file descriptor, stdin/stdout/stderr): copy all, also copy references to open-file state

• other: timer state, current directory, umask

execve(): replace stack, heap, register, data, code, and some other kernel tracked process variables (like timer). This usually run by user after fork().

spawn()/posix_spawn(): fill out a large form (no need to copy, efficient)

Plan9 rfork()/linux clone(): specify which things is shared vs copied.

School

execve():

• kernel transfer argv[], envp[] specified by execve() to top of new stack

• hand-crafts stack frame for main

• set registers (stack pointer, program counter)

scheduler states:

• running: can be either user mode or kernel mode

• blocked: can only be in kernel mode

  • waiting IO / user wants sleeping / waiting for exit of another process / message

• runnable: can only be in kernel mode

• forking: obsolete, used to cache program to swap disk when run out of memory during fork

• zombie: process called exit() but parent hasn't noticed yet

Death

Signals

exit(int reason)

• SIGSEGV: hardware exception

• SIGXCPU: software exception ("if I use too much CPU, execute me")

SIGUSR convention:

• SIGUSR1: increase logging verbosity

• SIGUSR2: decrease logging verbosity

SIGDANGER: Kernel hope program to give up memory.

• garbage collector can use, but never because their job is to arrange memory efficiently but keep all avaliable memory.

• db2: if you have clean page in some memory region, you can return them back to kernel

Cleanups

Release resources:

• close() files

  • (TCP socket can take up to 2 minutes)
  • (Solaris OS: disk offline can take forever, because Solaris don't tell user disk offline, just block IO)

• release memory

Accounting: record resource usage in file

Zombie: process state reduced to exit code, util parent calls wait(), process control block (PCB) deleted from kernel

process control block (PCB):

• kernel datastructure in process table, which is in kernel memory space

• stores everything about process

  • pointer to CPU registers
  • process number, parent process number
  • countdown timer, file descriptors
  • memory segment info (user memory segment list, kernel stack reference)
  • scheduler info (linked list slop, priority, "sleep channel")

Summary

Minimum Kernel consists of:

• getpid()

• fork()

• exec()

• wait()

• exit()