1 / 40

Operating Systems Scheduling

Operating Systems Scheduling. Scheduling. Short term scheduler (CPU Scheduler) Whenever the CPU becomes idle, a process must be selected for execution The Process is selected from the Ready queue Ready queue is not necessarily a FIFO queue It can be Priority based A Tree

shana-logan
Download Presentation

Operating Systems Scheduling

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Operating SystemsScheduling

  2. Scheduling • Short term scheduler (CPU Scheduler) • Whenever the CPU becomes idle, a process must be selected for execution • The Process is selected from the Ready queue • Ready queue is not necessarily a FIFO queue • It can be • Priority based • A Tree • Unordered linked list etc

  3. When to select a new process to Run 2. Interrupt occurs, move from Running to Ready • Four circumstances Dispatch Admit Release New Ready Running Exit Time-out 1. Wait for I/O/ waitpid()/ P()/ Acquire() etc 3. Event I/O Completion/ exit(0) / V() / Release() Event occurs Event wait Blocked 4. A Process terminates

  4. Non Preemptive Scheduling • Only the case 1 and 4 • Must select a new process, if any, from the Ready Queue Dispatch Release Admit New Ready Running Exit Time-out 1. Wait for I/O/ waitpid()/ P()/ Acquire() etc Event occurs Event wait Blocked 4. A Process terminates

  5. Non Preemptive Scheduling • Once the CPU has been allocated to a process • The process keeps it until • It Terminates • Or has to wait for: • I/O • Mutex • Child process • Semaphore • Conditional Variables etc • There is no way, to get the CPU back, FORCEFULLY

  6. Preemptive Scheduling 2. Interrupt occurs, move from Running to Ready • All four cases, 1,2,3 and 4 Dispatch Release Admit New Ready Running Exit Time-out 1. Wait for I/O/ waitpid()/ P()/ Acquire() etc 3. Event I/O Completion/ exit(0) / V() / Release() Event occurs Event wait Blocked 4. A Process terminates

  7. Preemptive Scheduling 2. Interrupt occurs, move from Running to Ready • All four cases, 1,2,3 and 4 Dispatch Release Admit New Ready Running Exit Time-out In case of 2 and 3, there is a choice Whether to continue, with the same process or select a new one from the ready queue 3. Event I/O Completion/ exit(0) / V() / Release() Event occurs Event wait Blocked

  8. Scheduling Issues • Fairness • Don’t starve process • Priorities • Most important first • Deadlines • Task X must be done by time t • Optimization • Throughput, response time • Reality - No universal scheduling policy • Many models

  9. Optimization Criteria • CPU Utilization • Keep the CPU as busy as possible • May range from 0% to 100% • Throughput • Number of processes completed per unit time • E.g. long processes • 1 process / hr • Short processes • 10 processes / hr

  10. Turnaround Time How long it take to execute a Process Turnaround = Completion_Time – Submission_Time Turnaround = Wait_TimeGetIntoMemory + Wait_TimeReadyQueue + Wait_TimeBlockQueue + CPU_Execution_Time Optimization Criteria

  11. Optimization Criteria • Scheduling Algorithm does not effect the waiting time in Block Queue • It only effect the Waiting Time in the Ready Queue • Waiting Time • Sum of the periods spent waiting in the Ready Queue

  12. Optimization Criteria • Turnaround Time is not a good criteria for Interactive Systems • A process may • Produce “Some” output • Computes new results, while previous results are output to the user • Response Time • Response_Time = First_Response_Start_Time – Submission_Time

  13. Optimization Criteria - Summary • We would like to Maximize • CPU Utilization • Throughput • And Minimize • Turnaround Time • Waiting Time • Response Time

  14. Scheduling Algorithms • First come, First serve • Shortest Job First • Priority Scheduling • Round-Robin Scheduling • Multi-level Queue Scheduling • Multi-level Feed back queue Scheduling

  15. First come, First serve • Simplest scheduling algorithm: • Run jobs in order that they arrive • Uni-programming: • Run until done • Multi-programming: • Run until done or Blocks on I/O • Nonpreemptive • A Process keeps CPU until done or I/O • Advantage: • Simplicity

  16. First come, First serve • Disadvantage • Wait time depends on arrival order • Unfair to later jobs • (worst case: long job arrives first) • Three jobs (times: A=100, B=1, C=2) arrive in the order A, B, C B C cpu A time 100 101 103 = 67 = (0 + 100 + 101) / 3 Average Waiting Time

  17. B C A time 1 3 103 First come, First serve • Now if they arrive in the order B, C, A cpu = (0 + 1 + 3) / 3 = 1.33 Average Waiting Time

  18. FCFS Convoy effect • A CPU bound job will hold CPU until • Terminates • Or it causes an I/O burst • Rare occurrence, since the thread is CPU-bound • Long periods where no I/O requests issued, and CPU held • Result: • Poor I/O device utilization

  19. FCFS Convoy effect : Example • One CPU bound job, many I/O bound • CPU bound runs • I/O jobs blocked in ready queue • I/O devices idle • CPU bound blocks • I/O bound job(s) run, quickly block on I/O • CPU bound runs again • I/O of the I/O bound jobs completes • CPU bound still runs while I/O devices idle (continues…)

  20. Round robin (RR) • Solution to job monopolizing CPU? • Interrupt it. • Run job for some “time slice,” • When time is up, or it blocks • It moves to back of a FIFO queue • Advantage: • Fair allocation of CPU across jobs • Low average waiting time when job lengths vary 1 2 3 4 5 103 CPU B C C A A A time What is avg completion time? = (103 + 2 + 5) / 3

  21. 1 2 3 4 5 199 200 CPU B B B B A A A A A Round Robin’s Disadvantage • Good for Varying sized jobs • But what about same-sized jobs? • Assume 2 jobs of time =100 each: time • Avg completion time? • (200 + 200) / 2 = 200 • How does this compare with FCFS for same two jobs? • (100 + 200) / 2 = 150

  22. RR Time slice tradeoffs • Performance depends on length of the time slice • Context switching isn’t a free operation. • If timeslice time is set too high (attempting to amortize context switch cost) • You get FCFS. • i.e. Processes will finish or block before their slice is up anyway • If it’s set too low you’re spending all of your time context switching between threads.

  23. Priority scheduling • Not all jobs equal • So: rank them. • Each process has a priority • Run highest priority ready job in system • Priorities can be static or dynamic or both • Among the Processes of equal priority • Round robin • FCFS

  24. Priority scheduling • Priority scheduling can be Preemptive or Non-Preemptive • When a process arrives and enters the Ready Queue • Its priority is compared with the currently Running Process • If Higher • Preemptive Scheduling • Run the New Thread • Non-Preemptive Scheduling • Continue running the Current Thread

  25. Priority scheduling • High priority always runs over low priority. • Starvation • A low Priority process may indefinitely wait for the CPU • Solution: Aging • Gradually increase the Priority of processes that wait in the system for a long time. • Which type of processes should be given Higher Priority: • I/O Bound??? • CPU Bound??? • In order to keep I/O busy increase priority for jobs that often block on I/O

  26. Shortest Job First (SJF) • Consider 4 jobs, a, b, c, d, run in lexical order a a+b a+b+c a+b+c+d A B C CPU D time • The first (a) finishes at time a • The second (b) finishes at time a+b • The third (c) finishes at time a+b+c • The fourth (d) finishes at time a+b+c+d • Therefore average completion = (a + ( a + b) + (a +b+c) + (a + b + c + d))/4 = (4a+3b+2c+d)/4 • Minimizing this requires a <= b <= c <= d. • or Shortest Job First

  27. Shortest Job First (SJF) • Run whatever job has smallest next CPU burst • Can be pre-emptive or non-pre-emptive • Example: same jobs (given jobs A, B, C) 1 3 103 cpu B C A time Average completion = (1+3+103) / 3 = ~35

  28. P1 P3 P2 P4 0 3 7 8 12 16 Example of Non-Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (non-preemptive) Average waiting time = (0 + 6 + 3 + 7)/4 = 4

  29. P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Example of Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (preemptive) Average waiting time = (9 + 1 + 0 + 2)/4 = 3

  30. 10ms 1ms10ms1ms10ms1ms …. blocked blocked blocked blocked 100ms 1ms 100ms 1ms P1 P1 P2 P2 I/O idle I/O busy SJF vs. RR • Two processes P1, P2 P1 P2 • RR with 100ms time slice: I/O idle ... • I/O idle ~90% • SJF Offers better I/O utilization

  31. Shortest Job First • The most important issue in SJF • Accuracy in estimation of Job length

  32. Multilevel Queue Scheduling • Sometimes processes are classified into groups • One classification can be: • Foreground (or Interactive) processes • Background (or batch) processes • Different response time requirement • => Different scheduling requirements • Foreground processes usually have higher priorities

  33. Multilevel Queue Scheduling (MQS) • Partition the Ready queue into a number of queues • Processes are permanently assigned to one of the queues • Each queue may have its own scheduling algorithm • In addition, there must be scheduling between the queues

  34. Multilevel Queue Scheduling • Example: FCFS Foreground Processes Priority Scheduling RR Background Processes

  35. Multilevel Queue Scheduling • Example: System Processes Interactive Processes Interactive editing Processes Batch Processes Student Processes

  36. System Processes Interactive Processes Interactive editing Processes Batch Processes Student Processes Multilevel Queue Scheduling • Each queue may have absolute priority over the other queue • Alternatively, Time slice between the queues • Time slots can be equal • Or • 80% time for Foreground processes • 20% time for Background processes

  37. Multilevel Feedback Queue Scheduling • In Multilevel Queue a process is permanently assigned to a queue • The queue to which a process should belong is decided statically • Multilevel Feedback Queue Scheduling: • A Process may move between the Queues • Aging can be implemented this way.

  38. Multilevel Feedback Queue Scheduling • Multilevel-feedback-queue scheduler defined by: • Number of queues • Scheduling algorithms for each queue • Method used to select when upgrade process • Method used to select when demote process • Method used to determine which queue a process will enter when that process needs service

  39. Multilevel Feedback Queue Scheduling • Example • If a process used too much CPU time, then move it to a lower-priority queue • If a process waits too long in a lower priority queue, then move it to a higher priority queue

  40. Multilevel Feedback Queue Scheduling • Example: Three queues: • Q0 – RR time quantum 8 milliseconds • Q1 – RR time quantum 16 milliseconds • Q2 – FCFS • Scheduling • A new job enters queue Q0served by RR. • Then job receives 8 milliseconds. • If not finished in 8 milliseconds, moved to Q1. • At Q1 job served by RR. • Then receives 16 milliseconds. • If not complete, preempted and moved to Q2.

More Related