Simple Scheduler

• A Simple scheduler runs NCPU number of processes and after TSLICE milliseconds, it stops the execution of the running processes and runs the next NCPU processes. This follows round-robin scheduling policy.
• This is repeated till all the processes terminates.
• NCPU and TSLICE are taken as command line parameters along with shell

./shell <NCPU> <TSLICE>

• Timer interrupts are generated to raise SIGALRM signal after every TSLICE milliseconds, which signals the scheduler to switch the processes.
• Use of signals such as SIGSTOP and SIGCONT is done to pause and resume the child processes.
• The Simple Scheduler is a daemon, that is it is a background process that runs simulataneously along with the shell. It starts scheduling only when SIGCONT is passed to it.

Priority queues

submit ./a.out <PRIORITY>

Apart from unbiased round-robin, the order in which the processes executes also depends on the priority assigned to it, which is the third command line parameter of the submit command. The range available is 1-4 and the default priority is 1. Four ready queues are created to classify the processes according to their priorities and run round-robin on them accordingly. The highest priority is assigned to 1 and the lowest to 4.

Heuristics followed

• It follows non - preemptive priority scheduling, that is, if a process of higher priority arrives before the process of lower priority completes its TSLICE, it will wait and will start executing only in the next TSLICE.
• Processes of different priorities cannot run together. First, all the processes of higher priority will run and then lower priority. The number of CPUs available has no role in this.