self doubt

Question

consider a system using preemptive priority based scheduling with dynamically changing priorities. On its arrival a process is assigned a priority of zero and running process priority increases at the rate of 'β' and priority of the processes in the ready Q increases at the rate of 'α'. By dynamically changing the values of α and β one can achieve different scheduling disciplines among the processes . what discipline will be followed for the following conditions.

1.β>α>0

2.α<β<0

Answer 1

Assumption -  Larger priority numbers imply higher priority.

1. FCFS (First-come first-served)

All the processes in the ready queue has the same initial priority 0. Their priority increases at the same rate α(α>0).  Thus, the earlier the process enters the ready queue, the higher its priority will be.

Once the process with the highest priority (first-come compared to other process in the ready queue) is running, its priority increases at a higher rate (β>α>0) than the priorities of those processes in the ready queue. So no other process will preempt it and it will run to its completion.

2. LIFO (last-in-first-out)

All the processes in the ready queue has the same initial priority 0. Their priority decreases at the same rate α(α<0).  Thus, the earlier the process enters the ready queue, the lower its priority will be.

Once the process with the highest priority(last-come compared to other process in the ready queue) is running, no other process in the ready queue will preempt it since its priority decreases at a lower rate (α<β<0) than the priorities of those processes in the ready queue. But it will be preempted by any new coming process because the new process's priority is 0 which is the highest possible priority.