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CPU Scheduling

CPU Scheduling. CPU Scheduling. Process execution a cycle of CPU execution and I/O wait figure Preemptive Non-Preemptive. The four circumstance. 1. When a process switches from the running state to the waiting state 2. When a process switches from the running state to the ready state

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CPU Scheduling

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  1. CPU Scheduling SunMoon University

  2. CPU Scheduling • Process execution • a cycle of CPU execution and I/O wait • figure • Preemptive • Non-Preemptive SunMoon University

  3. The four circumstance 1. When a process switches from the running state to the waiting state 2. When a process switches from the running state to the ready state 3. When a process switches from the waiting state to the ready state 4. When a process terminates SunMoon University

  4. scheduler • Select one of the processes in the ready queue • Short-term scheduling SunMoon University

  5. Dispatcher • gives the control of the CPU to the process selected by the scheduler • functions • switching context • switching to user mode • jumping to the proper location in the user program to restart that program • Dispatch latency : • the time it takes for dispatcher to stop one process and start another running. SunMoon University

  6. Scheduling Criteria • CPU utilization • % of time device is busy • Throughput • the number of jobs completed per unit time • Turnaround time • including I/O • Waiting time • Response time SunMoon University

  7. Scheduling Goal • fair among all processes • max. the CPU utilization and throughput • min. the turnaround / waiting / response time • optimize the average measure • minimize the max. response time • consider the worst case • interactive systems • minimize the variance in the response time SunMoon University

  8. Scheduling Algorithm • consider only one CPU burst per process • execution units • ex) 10 milliseconds • our measure of compare • average waiting time SunMoon University

  9. FiFO Scheduling(1) • First-come First-served • Easily managed with a FiFO queue • Fair • The average waiting time is often quite long • 129 pages(Fifth Edition) • One CPU-bound process and many I/O bound processes • I/O devices idle or CPU sits ilde • Non-preemptive • Troublesome for time sharing system SunMoon University

  10. CPU-Bound Pr. I/O-Bound Pr. main ( ) { int sum,j=0; while( ){ sum += j; j++; } } main( ) { FILE *fp; int j, buf[512]; fp = open(“tmp.dat”, “r+”); while( ){ j = read( fp, buf, sizeof(buf)); sum( buf, 256 ); wirte( fp, buf, 1024 ); } } FiFO Scheduling(2) SunMoon University

  11. P1 P2 P3 프로세스 실행시간 P1p2p3 24 0 24 27 30 3 3 P1 P2 P3 0 3 6 30 평균 반환 시간 : (24+27+30)/3 = 27평균 대기 시간 : (0+24+27)/3 = 17 FiFO Scheduling(3) 예 작업이 1,2,3의 순서  간트 도표(Gantt chart) 작업이 P2, P3, P1의 순서  간트 도표 평균 반환 시간 : (3+6+30)/3 = 13평균 대기 시간 : (6+0+3)/3 = 3 SunMoon University

  12. SJF Scheduling(1) • Shortest-Job-First Scheduling • Assign the CPU to the process that has the smallest next CPU burst • If the same burst, FCFS • p 130 ~ 132 • Minimize the average waiting time • Difficulty with the SJF • knowing the length of the next CPU request • figure 5.3 : p 132 • Non_preemptive SunMoon University

  13. 프로세스 실행시간 P1p2p3p4 6 평균 대기 시간 : (3+0+16+9)/4 = 7평균 반환 시간 : (3+9+16+24) / 4 = 13 3 8 7 평균 대기시간 최소 : 최적 알고리즘 SJF Scheduling(2) SunMoon University

  14. SJF Scheduling(3) • 짧은 작업을 긴 작업의 앞에 놓음  긴 작업의 대기시간 증가 • 짧은 작업의 대기시간을 줄여 평균 대기시간 감소 • SJF는 작업 스케줄링에서 많이 사용되는 기법 • 단기 스케줄링 단계에서 다음 프로세스의 프로세서 버스트 시간을 예상할 수가 없기 때문에 하드웨어 구성 어려움 - SJF 스케줄링의 근사치 사용하여 해결 SunMoon University

  15. SJF Scheduling(4) • Preemptive version • shortest-remaining time first • shorter CPU burst of new arrival process than what is left of currently executing process • p 133 SunMoon University

  16. 프로세스 도착시간 실행시간 P1p2p3p4 0 8 4 1 9 2 3 5 P1 P3 P2 P4 12 0 8 17 26 SJF Scheduling(5) 평균반환시간 : 선점 : ((17-0)+(5-1)+(26-2)+(10-3))/4 = 13 비선점 :((8-0)+(12-1)+(17-3)+(26-2))/4 = 14.25평균 대기시간 : 선점 : ((10-0)+(1-1)+(17-2)+(5-3))/4 = 6.75 비선점 :((0+(8-1)+(12-3)+ (17-2))/4 = 7.75 비선점 SunMoon University

  17. Priority Scheduling(1) • Allocate CPU to the highest priority process • SJF : a special case of the general priority scheduling • equal-priority processes : FCFS order SunMoon University

  18. Priority Scheduling(2) • Priority • is defined either internally or externally • Static priority • do not change through out execution time • easy to implement • low-overhead • Dynamic priority • response to change • more complex • greater overhead  the increased responsiveness of the system SunMoon University

  19. Priority Scheduling(3) • Purchased priority • willing to pay a extra charge • P. 134 • Non_preemptive or preemptive • when a new process arrives at the ready queue, • new process priority > running process • Problem • indefinite blocking or starvation • unfair  aging SunMoon University

  20. 프로세스 버스트시간 우선순위 3 P1p2p3p4p5 10 1 1 2 3 15 42 Priority Scheduling(4) 평균 대기시간 : 8.2 SunMoon University

  21. Deadline scheduling • Schedule the certain jobs to be completed by a specific time or deadline • high value if delivered on time • worthless if delivered later than the deadline • Very complex • Real time systems • control system • etc. SunMoon University

  22. HRN scheduling • Highest Response-Ratio Next • Brinch Hansen(1971)에 의해 개발 • Non-preemptive • Compensated version of SJF • longer jobs given favored treatment • the priority of each job is a function • P = (waiting_time + service_time) /service_time SunMoon University

  23. 준비 큐 종료 프로세서 선점 Round-Robin Scheduling(1) • Time sharing system • FCFS + time_quantum or time_slice • a time quantum : 10~100 ms • Ready queue : circular queue • Preempted SunMoon University

  24. Round-Robin Scheduling(2) • Maximum waiting time • “n” processes in the ready queue • time quantum : q • (n – 1) * q • The effect of context switching • time quantum vs. the number of context switching • figure 5.4/5.5 (p. 136 ~137) SunMoon University

  25. 반환 시간 = 작업 완료 시간 - 도착 시간 평균 반환시간 : (20+7+24+22)/4 =18.25 평균 대기시간 : (12+3+15+17) =47÷4=11.75 프로세스 도착시간 실행시간 P1p2p3p4 0 8 규정 시간량 : 4 P1 P2 P3 P4 P1 P3 P4 P3 4 1 9 2 3 5 0 4 8 12 16 20 25 26 24 Round-Robin Scheduling(3) SunMoon University

  26. Multi_level queue scheduling(1) • For situation in which processes are easily classified into different groups • foreground( interactive ) processes • background( batch ) processes • Permanently assigned to a queues on entry to the systems • low scheduling overhead • inflexible SunMoon University

  27. 최고의 우선순위 시스템 프로세스 대화형 프로세스 대화형 편집 프로세스 일괄 프로세스 학생의 프로세스 최저의 우선순위 Multi_level queue scheduling(2) 큐는 프로세서 시간 일정량 받아, 자기 큐에 있는 프로세스들을 스케줄링 • 전면 작업 큐 :순환 할당 스케줄링 위하여 프로세서 시간의 80% 할당 • 후면 작업 큐: 선입선처리(FCFS)방식으로 프로세서 시간의 20% 할당 SunMoon University

  28. Multi_level queue scheduling(3) • Figure 5.6(p. 138) • each queue has the absolute priority over lower_priority queue • different time slice between the queue SunMoon University

  29. Multi_level Feedback Queue Scheduling(1) • allows a process to move between queues • separate processes with different CPU_burst characteristics • I/O bound process • CPU bound process • aging SunMoon University

  30. 할당량 = 8 할당량 = 16 Multi_level Feedback Queue Scheduling(2) • 프로세서 버스트가 8 이하인 모든 프로세스에게 최고 우선순위 부여 • 8 ~ 24 프로세스들은 더 짧은 프로세스보다는 낮은 우선 순위 받음 • 긴 프로세스들은 큐 2로 • 큐 0과 큐 1에 프로세서 주기의 여유 생기면 FCFS 방식으로 처리 SunMoon University

  31. Multi_level Feedback Queue Scheduling(3) • Parameters of multi_level feedback q. • the number of queues • the scheduling algorithm for each q. • the method used to determine • when to upgrade a process to a higher priority queue. • when to demote a process to a lower priority queue. • Which queue a process will enter when that process needs service. SunMoon University

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