Job Scheduling. CS170 Fall 2018

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1 Job Scheduling CS170 Fall 2018

2 What to Learn? Algorithms of job scheduling, which maximizes CPU utilization obtained with multiprogramming Select from ready processes and allocates the CPU to one of them Useful scheduling algorithms for general resource management Performance evaluation criteria

3 CPU Scheduling blocked Life-cycle (states) of a process or thread Active processes/threads transit from Ready queue to Running to various waiting queues. Question: How is the OS to select from each queue Obvious queue to worry about is ready queue Others can be scheduled as well, however Scheduling: deciding which processes/threads are given access to resources

4 Scheduling Applications Job scheduling, resource scheduling, request scheduling. For example, web server scheduling to handle high traffic

5 I/O and CPU Burst Cycle of a Process Process execution consists of a cycle of CPU execution and I/O wait Time

6 Histogram of CPU-burst Times Short CPU bursts are often dominating

7 Time Scheduling happens among CPU/IO bursts P0 P1 P2

8 Process State Change vs CPU Scheduling CPU scheduling decisions may take place when a process: 1. Switches from running to waiting (blocked) state 2. Switches from running to ready state 3. Switches from waiting/blocked to ready 4. Terminates Preemptive Scheduling Take the processor away from one process (job) and give it to another State change from running to ready Non-preemptive scheduling A process runs continuously until it is blocked or terminates

9 Scheduling Terminology Task/Job Process, thread. User request: e.g., mouse click, web request, shell command, CPU utilization keep the CPU as busy as possible Throughput # of processes that complete their execution per time unit Waiting time amount of time a process has been waiting in the ready queue Turnaround time amount of time to execute a particular process Completion time arrival time = Waiting time + job size + overhead Minimize turnaround time minimize waiting time Response time amount of time it takes from when a request was submitted until the first response is produced, not output (for timesharing environment)

10 Scheduling Algorithm Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time Five preemptive or nonpreemptive algorithms are considered FIFO SJF (Shortest Job First) Priority scheduling Round robin Multi feedback queue scheduling

11 First-In-First-Out (FIFO): also called FCFS First-Come, First-Served Scheduling Process Burst Time P 1 24 P 2 3 P 3 3 Processes arrive in the order: P 1, P 2, P 3 at time 0. The Gantt Chart for the schedule is: P 1 P 2 P 3 0 Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17 Turnaround time P 1 = 24; P 2 = 27; P 3 = 30. Average turnaround time= ( )/3=27 = 17 (Average waiting time) + 10 (average job size)

12 FIFO Scheduling (Cont.) Suppose that the processes arrive in the order: P 2, P 3, P 1 The Gantt chart for the schedule is: Arrive Time P 1 2 P 2 0 P 3 1 P 2 P 3 P Waiting time for P 1 = 6-2=4; P 2 = 0 ; P 3 = 3-1=2 Average waiting time: ( )/3 = 2 Summary: Schedule tasks in the order they arrive Continue running them until they complete or give up CPU On what workloads is FIFO particularly bad?

13 FIFO vs. SJF Suppose we have five tasks arrive one right after each other, but the first one is much longer than the others

14 Shortest-Job-First (SJF) Scheduling Always do the task that has the shortest remaining amount of work to do Often called Shortest Remaining Time First (SRTF) SJF is optimal gives minimum average waiting time for a given set of processes The difficulty is knowing the length of the next CPU request. What is its weakness? Starvation low priority processes may never execute

15 Example of SJF Process Arrival Time Burst Time P P P P SJF scheduling chart P 4 P 1 P 3 P Average waiting time = ( ) / 4 = 7

16 Predicting Length of Next CPU Burst Why? SJF scheduling requires size information Predict by using the length of previous CPU bursts, using exponential averaging New prediction is weighted average of previous prediction and actual time. t = + - ( 1 ). a t a t n + 1 n n t n = actual length of n t n + 1 = predicted value for the next CPU a, 0 a 1 th CPU burst burst

17 Prediction of the Length of the Next CPU Burst Guess Actual α=0.5 Initial guess 10. Time 0 actual: 6 Time 1 prediction: (6+10)/2=8 Time 1 actual: 4 Time 2 prediction: (4+8)/2=6

18 Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer º highest priority) SJF is a priority scheduling where priority is the predicted next CPU burst time Problem º Starvation low priority processes may never execute Solution Aging as time progresses, decrease the priority of the long-running process Round robin

19 Scheduling Algorithm Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time Five preemptive or nonpreemptive algorithms are considered FIFO SJF (Shortest Job First) Priority scheduling Round robin Multi-feedback queue scheduling

20 Round Robin (RR) Scheduling Each process gets a small unit of CPU time (time quantum), usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. Performance large quantum Þ FIFO small quantum Þ context switch overhead is too high No starvation.

21 Round Robin

22 Example of RR with Time Quantum = 4 3 processes ready at time 0 The Gantt chart is: Process Burst Time P 1 24 P 2 3 P 3 3 P 1 P 2 P 3 P 1 P 1 P 1 P 1 P Typically, longer average turnaround time than SJF, but better response

23 Overhead during Job Switches Process context switch: CPU switch from one process to another process Code executed in kernel above is overhead Overhead sets minimum practical switching time Less overhead with SMT/Hyperthreading, but contention for resources instead Anthony D. Joseph UCB CS162

24 Time Quantum and Context Switch Overhead

25 The Numbers Context switch in Linux: 3-4 µsecs (Current Intel i7 & E5). Some surprises: Thread switching only slightly faster than process switching (100 ns). But switching across cores about 2x more expensive than within-core switching. Context switch time increases sharply with the size of the working set*, and can increase 100x or more. The working set is the subset of memory used by the process in a time window. Moral: Context switching depends mostly on cache limits and the process or thread s hunger for memory. Anthony D. Joseph UCB CS162

26 Round Robin vs. FIFO Everybody finishes very Late with overhead. Longer turnaround. But faster response. Relatively fair

27 Multi-level Feedback Queue (MFQ) Scheduling Optimization Goals: Responsiveness, especially for interactive/high priority jobs Less overhead Fairness (among equal priority tasks). No startvation Not perfect at any of them: FIFO, SJF, RR MFQ addresses this Used in Linux, Windows

28 Multilevel Feedback Queue (MFQ) Maintain multiple job queues. Each queue receives a fixed percentage of system resource. A process can move between the various queues, representing priority aging so that high priority jobs will gradually lose their priority. Jobs FCFS = FIFO

29 Multilevel Queue Scheduling: Example with 2 queues Ready queue is partitioned into separate queues: foreground (interactive) background (batch) Foreground queue Each queue has its own scheduling algorithm: foreground RR background FIFO (FCFS) Background queue Time slice each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR 20% to background in FIFO

30 Multilevel Feedback Queue: Example with 3 queues Q 0 RR with quantum 8 ms Q 1 RR time quantum 16 ms Q 2 FIFO Scheduling A new job enters queue Q 0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2.

31 Windows Scheduling Classes and priorities Real time priority class: Static priorities (priorities donot change) Priority values: from 16 to 32 Variable class: variable priorities (e.g. 1-16) If a process has used up its quantum, lower its priority If a process waits for an I/O event, raise its priority Priority-driven scheduler For real-time class, do round robin within each priority For variable class, do multiple queues

32 Linux Scheduling Linux uses two process-scheduling algorithms: A time-sharing algorithm for fair preemptive scheduling between multiple processes. A real-time algorithm for tasks where absolute priorities are more important than fairness Time sharing is prioritized credit-based Process with most credits is scheduled next Credit subtracted when timer interrupt occurs. Long running jobs lose credits. When credit = 0, another process chosen When all processes have credit = 0, recrediting occurs based on priority and history. I/O event will raise the priority -- fast response time when ready.

33 Example of Linux Priorities and Time-slice length

34 Thread Scheduling Scheduling user-level threads within a process Known as process-contention scope (PCS) Kernel thread scheduled onto available CPU is system-contention scope (SCS) competition among all threads in system Pthread API during thread creation PTHREAD_SCOPE_PROCESS schedules threads using PCS scheduling PTHREAD_SCOPE_SYSTEM schedules threads using SCS scheduling.

35 Example of Pthread Scheduling API #include <pthread.h> #include <stdio.h> #define NUM THREADS 5 int main(int argc, char *argv[]) { int i; pthread t tid[num THREADS]; pthread attr t attr; /* get the default attributes */ pthread attr init(&attr); /* set the scheduling algorithm to PROCESS or SYSTEM */ pthread attr setscope(&attr, PTHREAD_SCOPE_SYSTEM); /* set the scheduling policy - FIFO, RT, or OTHER */ pthread attr setschedpolicy(&attr, SCHED OTHER); /* create the threads */ for (i = 0; i < NUM THREADS; i++) pthread create(&tid[i],&attr,runner,null);

36 Example of Pthread Scheduling API /* now join on each thread */ for (i = 0; i < NUM THREADS; i++) pthread join(tid[i], NULL); } /* Each thread will begin control in this function */ void *runner(void *param) { } printf("i am a thread\n"); pthread exit(0);

37 Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Each processor is self-scheduling. Each has its own private queue of ready processes Or all processes in common ready queue Synchronization with a lock Affinity scheduling Once a process/thread is scheduled on a processor, it is rescheduled to the same processor next round, maximizing cache reuse

38 Summary Scheduling: selecting a process from the ready queue and allocating the CPU to it FIFO (FCFS) Scheduling: Pros: Simple Cons: Short jobs get stuck behind long ones Round-Robin Scheduling: Pros: Better for short jobs (+). Relatively fair Cons: weak when jobs are same length. No prioritization. Longer average turnaround. More context switch overhead.

39 Summary (cont d) Shortest Job First (SJF)/Shortest Remaining Time First (SRTF): Pros: Optimal (average response time) Cons: Hard to predict future, Unfair Multi-Level Feedback Scheduling: Fairness while having different priorities. Everybody makes progress. Automatic promotion/demotion of process priority in order to approximate SJF/SRTF Responsiveness, especially for interactive/high priority jobs Less context switch overhead with different quantum. Windows/Linux scheduling Multi-processor scheduling

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