OS Assignment 5 - 2019121007

waitx

to implement waitx 3 fields ctime, rtime and etime are added in the proc structure.

• ctime is set during fork
• rtime is updated every time the clock timer interrupt is raised
• etime is set during exit

using these fields the running and waiting time of the process is calculated.

getpinfo

to implement getpinfo, a struct proc_stat is declared to store the results of the syscall.

the syscall simply copies all information from the proc struction to the proc_stat and returns it.

FCFS

fcfs is implemented as follows:

• the first runnable process is executed
• pre-emption is disabled (the process does not yield on clock tick)

PBS is implemented as followes:

• a field priority is defined in the proc stat
• a system call is defined to allow changing the priority
• the scheduler chooses the process with highest priority for execution
• the process is pre-empted after one tick

MLFQ:

• the proc stat is modified to add a current_queue field for each process.
• additionally, the proc stat contains a allocated field which indicates the time quantum allocated to each process
• the scheduler selects the process from the highest available queue and allocates it a time quantum (2 ^ lvl)
• when a clock tick occurs, we decrement the allocated field; if it becomes equal to 0, we pre-empt
• in case we start executing in the lowest priority queue, we finish all processes in the queue before returning to higher priority queues