Priority Scheduling Explained

Scheduling by Importance, Not Just Arrival or Length

Priority Scheduling assigns each process a priority value, and always runs the highest-priority process available - rather than the earliest arrival (FCFS) or the shortest job (SJF), both covered earlier in this section.

A Worked Example

Process   Burst Time   Priority (lower number = higher priority)
P1        10           3
P2        1            1
P3        2            4
P4        1            2
P5        5            5

Execution order (by priority): P2 -> P4 -> P1 -> P3 -> P5

Notice this ordering has nothing to do with each process's burst time directly - P2 runs first purely because it has the highest priority (lowest number), even though other processes might be shorter or have waited longer.

Where a Priority Value Actually Comes From

• Internally - based on factors like memory requirements or how many open files a process has.
• Externally - assigned by the system administrator, or based on how important a user or application is considered.

The Classic Weakness: Indefinite Starvation

A low-priority process can wait indefinitely if higher-priority processes keep arriving continuously - in the worst case, it may never run at all, a genuinely serious fairness problem for any long-running system.

Aging: The Standard Fix for Starvation

A waiting process's priority gradually increases the longer it waits,
until eventually even a low-priority process becomes high-priority
purely by having waited long enough.

Aging gradually increases a process's priority the longer it waits, guaranteeing that even an initially low-priority process will eventually become high-priority enough to run - directly solving the starvation problem without abandoning the core idea of priority-based scheduling. This exact technique is explored in more depth in the dedicated starvation and aging notes later in this section.