Priority scheduling, step by step

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Priority Scheduling

Each process is given a priority based on how important it is, and each time a process finishes, the one in line with the highest priority runs. Arrival order and run time are not considered. Here it runs as non-preemptive.

At a bank: when the person at the counter is done, the most important of the waiting customers is called first.

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0ms: P1 starts running

P1 is the only one in line, so it runs right away. Because this is non-preemptive, P1 runs to the end even if a higher-priority process arrives.

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1–3ms: P2, P3 and P4 join the line

P2 (priority 2) arrived at 1ms, P3 (priority 1) at 2ms and P4 (priority 4) at 3ms. P3 has a higher priority than P1, but it waits until P1 finishes.

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5ms: P3, with the highest priority, runs

P1 has finished, so the next one is picked. P3, with the highest priority (the smallest number), runs. Even though its run time of 8ms is the longest, priority comes first.

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13ms: P3 finishes and P2 runs

Of the remaining P2 (priority 2) and P4 (priority 4), P2 runs. P2 arrived at 1ms and waited 12ms, until 13ms.

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16ms: P2 finishes and P4 runs

P4, with the lowest priority, arrived at 3ms and waited 13ms, until 16ms. If higher-priority processes keep coming in, P4 can be pushed back indefinitely (starvation). That is why the priority of a process that has waited a long time is sometimes raised little by little (aging).

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18ms: all done

Waiting times are P1 0ms, P2 12ms, P3 3ms and P4 13ms, an average of 7ms. The important P3 was handled first, but the average is longer than FCFS (5.75ms). Priority scheduling can also be preemptive. In that case, when a higher-priority process arrives, the running process is stopped and the higher-priority one runs first.

Figure showing priority scheduling: each time a process finishes, the one with the highest priority in the ready queue is picked to run, and how long each process waited

CPU Scheduling at a glance

Figure showing priority scheduling: each time a process finishes, the one with the highest priority in the ready queue is picked to run, and how long each process waited