CPU scheduling. Kevin Marquet. September INSA Lyon / IF CITI Lab/INRIA Socrate

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1 CPU scheduling Kevin Marquet INSA Lyon / IF CITI Lab/INRIA Socrate September 2014

2 CPU-I/O burst CPU burst : process requires CPU execution I/O burst : process waits for I/O completion (CPU-IO burst) (burst duration) Generalisation : one scheduler per ressource

3 Role of scheduler CPU scheduler chooses next process When?

4 Role of scheduler CPU scheduler chooses next process When? 1. running waiting (e.g. I/O) 2. running ready (when?) 3. waiting ready (when?) 4. terminaison ( Absolutely necessary to schedule when a process terminates??)

5 Cooperative vs. Preemptive Cooperative scheduling: when scheduling takes place only in 1st and 4th cases Examples : Windows 3.x, old MacOS, Benefits? Drawbacks? Preemptive scheduling: otherwise Drawbacks? Advantages?

6 Cooperative vs. Preemptive Cooperative scheduling: when scheduling takes place only in 1st and 4th cases Examples : Windows 3.x, old MacOS, Benefits? Drawbacks? Preemptive scheduling: otherwise Drawbacks? Advantages? race conditions OS design consideration : Big kernel lock? Interrupt handling??)

7 Scheduling criteria CPU utilization between 0 and 100% Throughput For instance: number of processes that are completed per time unit. Which other measurement units for the throughput?? Turnaround time Time from user submission to completion: load into memory + waiting in queues + executing on the CPU + doing I/O Waiting Time amount of time spent in the ready queue Response Time Time from submission of a request until the first response it produces (Which kind of processes?) And also: Optimize the average time? Max? Min?

8 Scheduling policies Process Burst Time First-come, first-served P 1 24 (FCFS) P 2 3 P 3 3 Waiting times? Average waiting time?

9 Scheduling policies Process Burst Time First-come, first-served P 1 24 (FCFS) P 2 3 P 3 3 Waiting times? Average waiting time? Process Burst Time P 1 6 Shortest-job-first (SJF) P 2 8 P 3 7 P 4 3 Waiting times? Average waiting time? Preemptive SJF

10 Scheduling policies Priority scheduling SJF is a special case of Priority-scheduling One piority per process If priority are equal: FCFS Linux niceness Process Burst Time Priority P Exercise P P P P Average waiting time? starvation The other scheduling policies can they lead to starvation?

11 Scheduling policies Round-Robin Round-Robin (RR): Heavily depends on the Time Quantum (or Time Slice). Why?

12 Scheduling policies Round-Robin Round-Robin (RR): Heavily depends on the Time Quantum (or Time Slice). Why? Context switching takes ns on a Intel Sandy Bridge E Why does context switching takes so much time? Exercise : Process Burst Time P 1 10 P 2 10 P 3 10 Turnaround time: if time quantum = 1? if time quantum = 10? if time quantum = 15? In the general case, does the turnaround time increase with the context switch duration?

13 Scheduling policies Multi-level queues Groups of processes: interactive processes background processes Not one ready queue but one per group 2 possibilities then: One priority per queue, and each queue has priority over lower-level queues Time-slice among the queues. Example: the interactive queue gets 80% of the CPU time for RR.

14 Scheduling policies Multilevel feedback queues (Multilevel feedback queues)

15 Scheduling in Linux Now reading: Scheduling in Linux

16 Multiple processor scheduling Assymetric scheduling One processor schedules Symetric scheduling Each processor schedules Load-balancing Process migration Processor affinity

17 Real-time scheduling Nothing to do with performance latency Soft real-time: no guarantee Hard real-time: deadline 2 types of latencies: (Interrupt latency) (Dispatch latency) Preemptive kernel Priority scheduling: no guarantee

18 Real-time scheduling: periodic tasks Each task has: Processing time t Deadline d Period p Rate r = 1/p (Periodic task) Admission control

19 Real-time scheduling : RMA Rate Monotonic Analysis Static priorities with preemption The shorter the period, the higher the priority If a set of processes cannot be scheduled by this algorithm, it cannot be scheduled by any other algorithm that assigns static priorities (RMA) acceptability test (sufficient condition) if n processes: n t i n(2 1/n 1) p i i=1 lim n(2 1/n 1) = ln(2) = 0.69 in practice 0.88

20 Real-time scheduling : EDF Earlieast Deadline First Dynamic priorities with preemption The earlier the deadline, the higher the priority (EDF) For task where p=d, a sufficient condition is: Else, an additional condition is: i=1..n t i d i 1 i=1..n t i p i 1

21 We have not seen Memory : DMA, cache, allocation System calls Precedence constraints between tasks Priority inversion

W4118 Operating Systems. Instructor: Junfeng Yang

W4118 Operating Systems. Instructor: Junfeng Yang W4118 Operating Systems Instructor: Junfeng Yang Outline Introduction to scheduling Scheduling algorithms 1 Direction within course Until now: interrupts, processes, threads, synchronization Mostly mechanisms