CPU Scheduling Algorithms: Efficiency and Optimality

1. Operating SystemsCSE 411 CPU Management Sept. 18 2006 - Lecture 6 Instructor: Bhuvan Urgaonkar

2. Hmm .. Who should I pick to run? Running OS (scheduler) Ready Lock Waiting Disk

3. P1 P2 P3 0 24 27 30 First-Come, First-Served Scheduling(FCFS) Process Run Time P1 24 P2 3 P3 3 • Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17

4. P2 P3 P1 0 3 6 30 FCFS Scheduling (Cont.) 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

5. Shortest-Job-First (SJF) Scheduling • Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time • SJF is optimal for avg. waiting time – gives minimum average waiting time for a given set of processes • In class: Compute average waiting time for the previous example with SJF • Prove it (Homework 1, Will be handed out next Friday)

6. Architecture-dependent part of the Scheduler: Dispatcher • Dispatcher module gives control of the CPU to the process selected by the scheduler; this involves: • switching context • switching to user mode • jumping to the proper location in the user program to restart that program • Dispatch latency – time it takes for the dispatcher to stop one process and start another running • Also called the Context Switch time.

7. Costs/Overheads of a Context Switch • Direct/apparent • Time spent doing the switch described in the last slide • Fixed (more or less) • Indirect/hidden costs • Cache pollution • Effect of TLB pollution (will study this when we get to Virtual Memory Management) • Workload dependent

8. Example from Linux 2.6.x asmlinkage void __sched schedule(void) { [ . . . ] prepare_arch_switch(rq, next); prev = context_switch(rq, prev, next); barrier(); finish_task_switch(prev); [ . . . ] } task_t * context_switch(runqueue_t *rq, task_t *prev, task_t *next) { struct mm_struct *mm = next->mm; struct mm_struct *oldmm = prev->active_mm; /* Here we just switch the register state and the stack. */ switch_to(prev, next, prev); return prev; } #define switch_to(prev,next,last) \ asm volatile(SAVE_CONTEXT \ "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* saveRSP */ \ "movq %P[threadrsp](%[next]),%%rsp\n\t" /* restore RSP */ \ "call __switch_to\n\t" \ ".globl thread_return\n" \ "thread_return:\n\t" \ "movq %%gs:%P[pda_pcurrent],%%rsi\n\t" \ "movq %P[thread_info](%%rsi),%%r8\n\t" \ LOCK "btr %[tif_fork],%P[ti_flags](%%r8)\n\t" \ "movq %%rax,%%rdi\n\t" \ "jc ret_from_fork\n\t" \ RESTORE_CONTEXT \ : "=a" (last) \ : [next] "S" (next), [prev] "D" (prev), \ [threadrsp] "i" (offsetof(struct task_struct, thread.rsp)), \ [ti_flags] "i" (offsetof(struct thread_info, flags)),\ [tif_fork] "i" (TIF_FORK), \ [thread_info] "i" (offsetof(struct task_struct, thread_info)), \ [pda_pcurrent] "i" (offsetof(struct x8664_pda, pcurrent)) \ : "memory", "cc" __EXTRA_CLOBBER)

9. When is the scheduler invoked? • CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates • Scheduling only under 1 and 4: nonpreemptive scheduling • E.g., FCFS and SJF • All other scheduling is preemptive

10. Why Pre-emption is Necessary • To improve CPU utilization • Most processes are not ready at all times during their lifetimes • E.g., think of a text editor waiting for input from the keyboard • Also improves I/O utilization • To improve responsiveness • Many processes would prefer “slow but steady progress” over “long wait followed by fast process” • Most modern CPU schedulers are pre-emptive

11. SJF: Variations on the theme • Non-preemptive: once CPU given to the process it cannot be • preempted until completes its CPU burst - the SJF we already saw • Preemptive: if a new process arrives with CPU length less • than remaining time of current executing process, preempt. • This scheme is know as Shortest-Remaining-Time-First (SRTF) • Also called Shortest Remaining Processing Time (SRPT) • Why SJF/SRTF may not be practical • CPU requirement of a process rarely known in advance

12. Choosing the Right Scheduling Algorithm/Scheduling Criteria • CPU utilization – keep the CPU as busy as possible • Throughput – # of processes that complete their execution per time unit • Turnaround time – amount of time to execute 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) • Fairness

13. Round Robin (RR) • Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. • If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units. • Performance • q large => FCFS • q small => q must be large with respect to context switch, otherwise overhead is too high

14. P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 Example of RR with Time Quantum = 20 ProcessCPU Time P1 53 P2 17 P3 68 P4 24 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response

15. Time Quantum and Context Switch Time

16. Turnaround Time Varies With Time Quantum

17. Priority-based Scheduling • Associate with each process a quantity called itsCPU priority • At each scheduling instant • Pick the ready process with the highest CPU priority • Update (usually decrement) the priority of the process last running • Priority = Time since arrival => FCFS • Priority = 1/Size => SJF • Priority = 1/Remaining Time => SRPT • Priority = Time since last run => Round-robin (RR) • UNIX variants • Priority values are positive integers with upper bounds • Decreased every quantum • Fairness, avoid starvation • Increased if the process was waiting, more wait => larger increase • To make interactive processes more responsive • Problems • Hard to analyze theoretically, so hard to give any guarantees • May unfairly reward blocking processes

18. Multilevel Queue • Ready queue is partitioned into separate queues:foreground (interactive)background (batch) • Each queue has its own scheduling algorithm • foreground – RR • background – FCFS • Scheduling must be done between the queues • Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation. • 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 FCFS

19. Multilevel Queue Scheduling

20. Multilevel Feedback Queue • A process can move between the various queues; aging can be implemented this way • Multilevel-feedback-queue scheduler defined by the following parameters: • number of queues • scheduling algorithms for each queue • method used to determine when to upgrade a process • method used to determine when to demote a process • method used to determine which queue a process will enter when that process needs service

21. Example of Multilevel Feedback Queue • Three queues: • Q0 – RR with time quantum 8 milliseconds • Q1 – RR time quantum 16 milliseconds • Q2 – FCFS • Scheduling • A new job enters queue Q0which is servedFCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. • At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.

22. Multilevel Feedback Queues