Process Scheduling Course Notes in Operating Systems 1 (OPESYS1) Justin David Pineda

Transcription

1 Process Scheduling Course Notes in Operating Systems 1 (OPESYS1) Justin David Pineda Faculty, Asia Pacific College November 2015 Introduction On the first half of the term, we discussed the conceptual terms and functions of the operating system such as process states, threads, synchronization to name a few. The second half of the term is dedicated for the simulation on how an operating system works. Review During the discussion of processes, we have defined it as a program in execution. It has, in simplest forms, three states: Ready state, Running state and the Blocked state. For the purposes of process scheduling, we will limit ourselves with the three states and not anymore use the expanded one. All processes reside in the Ready state, particularly in the ready queue prior to its execution. Also, only one process is executed at a time. Context switching happens when a process is switched from the ready to the running state or vice versa. We also learned that all processes in the ready state reside in the RAM while a running process is found in the processor. Terms 1. Arrival Time (AT) Time when the process reaches the Ready Queue (RQ). 2. Burst Time (BT) Total execution or service time of a process. 3. Turnaround Time (TAT) Total time from the time a process arrives in the RQ until it finishes its execution. 4. Waiting Time (WT) Total wait time before the process gets executed. 5. Time Quantum (TQ) Time given to a process to execute before switching to the next process. 6. Response Ratio (RR2) Value computed to determine the balance between processes based on AT and BT 7. Start Time (ST) Time when process starts executing. 8. Completed Time (CT) Time when process finishes executing. Requirements By default, you are required to show the following in all algorithms that will be discussed: 1. BT (10%) Summation BT or Sigma BT is the total BT of all processes in the Process Table. 2. Gantt Chart (60%) This shows the order of the execution of processes with its time. 3. Performance Table (30%) Show the TAT and WT of the processes. Page 1 of 6

2 Note: The Gantt Chart is the most important requirement. A correct Gantt Chart is a prerequisite of a correct performance table. Example Process Table For all algorithms, we will be using this process table: Process AT BT P P2 3 6 P3 4 4 P4 5 1 BT 21 Note: BT is always the same regardless of the algorithm used. Algorithms There are 5 algorithms that will be discussed: First Come First Serve (FCFS), Shortest Job First- Non Preemptive (SJF-NP), Shortest Job First- Preemptive (SJF-P), Round Robin (RR) and Highest Response Ratio Next (HRRN). First Come First Serve (FCFS) Description Advantages Disadvantages Exceptions Arrival Time (AT) First process to arrive gets executed first. Simple to implement First process to arrive does not mean it is an important process. Short important process that arrive in a later time will not be executed until the early process gets finished. If the early process is very long, a problem arises. If two or more processes have the same AT, choose the first given in the question. IDLE P1 P2 P3 P Total: Page 2 of 6

3 Shortest Job First Non Preemptive (SJF-NP) Description Advantages Disadvantages Exceptions Burst Time (BT) If two or more processes are in the RQ, choose the one that has the lowest BT and execute until completion. Efficient Starvation Long processes are always the last to be executed. Hypothetical Unless you are executing batch files, you cannot predict the BT of a process. If two or more process have the same BT, choose the first given in the question. IDLE P1 P4 P3 P Total: Shortest Job First Preemptive (SJF-P) SJF-P has same specifications with SJF-NP with additional features. Burst Time (BT) Description If two or more processes are in the RQ, choose the one that has the lowest BT and execute until completion. If a process enters the ready queue while another process is in execution, compare the BT of both processes. If the BT of the former is lower than the latter, PREEMPT. Otherwise, the process in execution will continue to execute. Advantages Efficient Disadvantages Starvation Long processes are always the last to be executed. Hypothetical Unless you are executing batch files, you cannot predict the BT of a process. Exceptions If two or more process have the same BT, choose the first given in the question. If during execution, another process enters the RQ with the same BT, don t preempt anymore. Give priority to the process in execution. Page 3 of 6

4 IDLE P1 P2 P3 P4 P3 P2 P Total: Round Robin (RR) Arrival Time (AT), order in Ready Queue (RQ) & Time Quantum (TQ) Description The first process in the RQ is executed based on TQ. Preemptive Advantages Gives equal time share to each process. Disadvantages If TQ is very small, there will be a lot of context switching. If TQ is very big, it will become FCFS. Exceptions If two or more process have the same AT, choose the first given in the question. Additional Show the order of processes in the RQ. Requirement This has a 10% weight in grading and the Gantt chart becomes 50%. Note: Even if it is the only process remaining, you need to show the execution based on TQ. DO NOT RAILROAD THE EXECUTION! TQ = 4 RQ = P1, P2, P3, P4, P1, P2, P1 IDLE P1 P2 P3 P4 P1 P2 P Total: Page 4 of 6

5 Highest Response Ratio Next (HRRN) Response Ratio (RR2) Description The goal of the algorithm is to balance FCFS and SJF algorithms. The algorithm takes into consideration the AT and BT at the same time. RR2 is computed as (W+S) / S where W or (X AT) is Waiting Time so far and S is the Service Time or Burst Time (BT). Advantages Better than SJF No Starvation Disadvantages Too much complexity Exceptions If two or more processes have the same RR2, choose the first given in the question. Additional Show all RR2 computations. Requirement This has a 10% weight in grading and the Gantt chart becomes 50%. Process AT BT t=12 t=13 P P P P IDLE P1 P4 P3 P Total: Caveats 1. There are no negative values when you do the Performance Table. If you get a negative answer, either your subtraction or Gantt chart is wrong. 2. The end time and BT for a Process Table given will always be the same regardless of the algorithm given. Only the TAT and WT will be different. 3. The BT is not always equal to the end time in the Gantt chart. This will differ if the Gantt chart has idle time. Page 5 of 6

6 4. For preemptive algorithms, always look for the last CT when the process finishes executing. Otherwise, it will yield to a negative result. 5. Always check the AT. A lot of students get confused that they look for a lower BT in SJF but the process has not arrived yet. It will yield to a negative result! 6. The basis of your performance table is the Gantt chart. Write the processes based on the order in the Gantt chart, NOT based on the Process Table. References Agarwal, A. (2015, November 22). Anshul Agarwal's Tutorial. Retrieved from YouTube Channel: Gagne, G., Galvin, P. B., & Silverschatz, A. (2005). Operating System Principles (7th Ed). NJ: John WIley & Sons Inc.. Krzyzanowski, P. (2015, November 22). Operating Systems CS416. Retrieved from Pau at Rutgers: Course Notes softcopy at Anshul Agarwal s YouTube tutorial Paul at Rutgers course notes NOTHING FOLLOWS Page 6 of 6