Main Algorithms of a kernel…

Timer Interrupts

• There is one hardware timer that generates interrupts every 10ms and all the software timer synchronizes with it.
• Usually the timer stored in the variable jiffies.

unsigned long volatile jiffies;

The variable jiffies is modified by the timer interrupt every 10ms and hence it is declared as volatile.

volatile struct timeval xtime:

This is the actual time which again modified by the timer interrupt

Other functions of timer interrupt like

do_timer();

updates the jiffies

timer_bh();

updates the timer and processing of the timer related functions

update_process_time();

collects data for the scheduler and decides whether it has to be scheduled.

 

The Scheduler

schedule () is function declared in kernel/sched.c

The actions of the scheduler is given below, once the schedule() function is called,

• Upcoming software interrupts are processed (so interrupts are given higher priority over the other entities in the system)
• process with highest priority determined (if two tasks has equal priority, then the OS will determine which task to be executed first.)
• real time process takes over normal ones. (Real time processes area associated with deadlines, whereas the normal ones doesn’t have deadlines, this factor is determined by the rt_priority of the schedule structure)
• new process becomes current process. (whenever a process is getting scheduled by the scheduler, then it will become the current process)