Operating Systems - Process (CPU) scheduling

1. OperatingSystems- Process (CPU)scheduling Seehwan Yoo MSE.cis.dku

2. Review • Computer systems (hw) • Main components: CPU, mem, I/O devices • Inter-connect: bus • I/O device • interaction w/ external world • Comm. w/ cpu • Interrupt • Polling • Direct memory access • SW in computers • OS kernel, user applications • Dual-mode execution • For protection • System call – interface to OS • Process • Abstraction of processor (CPU) / machine • Execution context • Cf.) program • Memory layout • Address space • Process states • State transition • Time-sharing system • Implementation with timer interrupt

3. Process scheduling • Determine which process to run • Kernel maintains queues • Data structure! • Ready Q (runq) • Store processes in ready state • Device Q (ioq) • Store processes in waiting state (per-device) • Scheduler picks one of process to run during the next time slot • How?

4. Overall picture of scheduling

5. Scheduling • Diverse policies • Fair-share • First-In-First-Out • Shortest-Job-First • Good scheduler? • Evaluation criteria • CPU utilization • Throughput • Turn-around time • Waiting time • Response time

6. Scheduling Criteria • CPU utilization – keep the CPU as busy as possible • Throughput – # of processes that complete their execution per time unit • Turn-around time – amount of time tocomplete a particular process • Waiting time – amount of time a process has been waiting in the ready queue • Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)

7. Anti-break! • Waiting time of blue-process? • Response time of red-process? • Turn around-time of blue-process at time 3? • Throughput of the system? • CPU utilization? a b c d e f

8. First-In, First-Out • This is not a fair-share scheduler! • First-comer run until it is completed, or • Until it makes I/O request, or • Until it voluntarily yield CPU

9. FIFO (FCFS) example ProcessBurst Time P1 24 P2 3 P3 3 • Suppose that the processes arrive in the order: P1 , P2 , P3 (at time 0) • The Gantt Chart for the schedule is: • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17

10. FIFO (FCFS) another example! Suppose that the processes arrive in the order: P2 , P3 , P1 • The Gantt chart for the schedule is: • Waiting time for P1 = 6;P2 = 0; P3 = 3 • Average waiting time: (6 + 0 + 3)/3 = 3 • Much better than previous case • Convoy effect - short process behind long process • Consider one CPU-bound and many I/O-bound processes

11. Then, How about SJF? • Take the shortest job at first hand! ProcessArriva l TimeBurst Time P10.0 6 P2 2.0 8 P34.0 7 P45.0 3 • SJF scheduling chart • Average waiting time = (3 + 16 + 9 + 0) / 4 = 7

12. And How about SRTF?- Running scheduler at every time tick • Take the remaining time job at first hand! • Now we add the concepts of varying arrival times and preemption to the analysis ProcessAarriArrival TimeTBurst Time P10 8 P2 1 4 P32 9 P43 5 • SRTF Gantt Chart • Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msec

13. Forget about it… • Only exist in theory • Who knows the shortest job? • It is the most well-known NP-hard problem • Homework)survey halting problem in the Internet

14. But, an approximation can be made • From the last execution history • Commonly, α set to ½

15. Like this,

16. We can put them all together • Or stop it by now; • Break time

17. Fair-share schedulerRound-robin (RR) • Gives no priority to all process • Pick up one process in an equal probability • If all the processes have the same time quantum, then all the processes would utilize CPU time evenly • Implementation would be… • Randomly pick up a process in the run queue • When time quantum is expired, put it in the retired queue • When there is no process in the run queue,switch run queue with retired queue • In practice, • Pick the first process in the run queue • Run it until time quantum is expired • When time quantum is expired, put it in the run queue (tail)

18. RR property & example • Max (worst-case) response time? • Example ProcessBurst Time P1 24 P2 3 P3 3 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response • q should be large compared to context switch time • q usually 10ms to 100ms, context switch < 10 usec

19. Priority-based scheduling • No fair-share scheduler! • Give a priority to a process • Scheduling according to the priority • Can be fixed • Can be dynamically changed • Consider low-class citizens! • Low-priority process could be starved • Respect the seniors! • Apply aging • Increase priority when a process is in the low priority for a long time

20. Property of priority scheduling • Deterministic response time • Calculate it by hand! • Used in real-time systems • Rate-monotonic • Earliest-Deadline-First (EDF)

21. Example of priority scheduling ProcessA arri Burst TimeTPriority P1 10 3 P2 1 1 P32 4 P41 5 P5 5 2 • Priority scheduling Gantt Chart • Average waiting time = 8.2 msec

22. Re-think scheduling! • Basic ideas • Fair-share • First-in-First-out (FIFO, or queue) • Real-time • Do the right thing! • Does your application really want? • Compare • file copy (doing a lot of I/O operation) • Hanoii (scientific application for complex calculation)

23. Thing to know – when you schedule process • Workload characteristics • CPU-bound jobs (CPU-intensive) • I/O-bound jobs (I/O-intensive) • Giving long CPU time for I/O-bound jobs • Giving high priority for CPU-bound jobs • What actually happens in the scheduler is • Complex • Homework) survey the Linux’s Scheduler

24. A theory – Little’s Theorem • n = average queue length • W = average waiting time in queue • λ = average arrival rate into queue • Little’s law – in steady state, processes leaving queue must equal processes arriving, thus:n = λ x W • Valid for any scheduling algorithm and arrival distribution • For example, if on average 7 processes arrive per second, and normally 14 processes in queue, then average wait time per process = 2 seconds

25. Summary page • Various schedulers • FIFO, SJF, SRTF, RR, priority-based • Evaluation criteria • Throughput, …time(s) • Workload characteristics • I/O or CPU-bound tasks • Think about the scheduler operation when I/O is involved?