Working with the Unix OS

Introduction to kernel
Every process is composed of:
- code area executable portion of a process
- data area used by process to contain static data
- stack area used by process to store temporary data
- user area holds housekeeping info about process
- page tables used by memory management system
Every process has its own user area created in the kernel's data region and only accessible by the kernel.
- how process should react to each signal
- process's open file descriptors
- how much CPU time used
Process Table created in the kernel's data region and only accessible by the kernel.
- PID and parent PID
- real and effective user id and group id
- state (running, runnable, sleeping, suspended, idle, zombie)
- location of its code, data, stack, user areas
- list of pending signals
The Scheduler
- responsible for sharing CPU time
- maintains a multi-level priority queue
- a linked list of runnable processes
- allocate CPU time in proportion to importance
- CPU time allocated in "time quantums" 1/10 sec.
! Scheduling Rules
Every second, calculate the priorities of all runnable processes and organize them into several priority queues.
Every 1/10 sec, the scheduler selects the highest priority process. If the process is still running at end of time
quantum, it is placed at the end of its priority queue.
92