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Scheduling Example 3 (1)

Scheduling Example 3 (1). Assume: FIFO Job Scheduling 100 K Main Memory Processor Sharing Process Scheduling (Cont…). Scheduling Example 3 (2). Job Arrives Run Time Memory 1 10.0 0.3 10 2 10.2 0.5 60 3 10.4 0.1 50

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Scheduling Example 3 (1)

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  1. Scheduling Example 3 (1) • Assume: • FIFO Job Scheduling • 100 K Main Memory • Processor Sharing Process Scheduling • (Cont…)

  2. Scheduling Example 3 (2) Job Arrives Run Time Memory 1 10.0 0.3 10 2 10.2 0.5 60 3 10.4 0.1 50 4 10.5 0.4 10 5 10.8 0.1 30 HOLDQ Ready / Running

  3. Example 3 (Cont…) TimeEvent#JobsHeadwayMM FreeTime Left 10.0 1 A,S 90 1 0.3 10.2 2 A,S 1 0.2 30 1 0.1 2 0.5 10.4 1 F 2 0.1 40 2 0.4 3 A,H 10.5 4 A,S 1 0.1 30 2 0.3 4 0.4 10.8 5 A,S 2 0.15 0 2 0.15 4 0.25 5 0.1 11.1 5 F 3 0.1 30 2 0.05 4 0.15 11.2 2 F 2 0.05 90 4 0.1 3 S 40 3 0.1 11.4 3 F 2 0.1 50 4 F 100 Must be memory size Must be 10.0+ sum of all Ri

  4. T and W for Example 3 • Job Run Arrival Finish Ti Wi • 0.3 10.0 10.4 0.4 1.33 • 0.5 10.2 11.2 1.0 2.0 • 0.1 10.4 11.4 1.0 10.0 • 0.4 10.5 11.4 0.9 2.25 • 0.1 10.8 11.1 0.3 3.0 • ===== ===== • 3.6 18.58 • T = 0.72 • W = 3.716

  5. Scheduling Example 4 (1) • Assume: • FIFO Job Scheduling • 100 K Main Memory • Devices • Processor Sharing Process Scheduling • (Cont…)

  6. Scheduling Example 4 (2) Job Arrives Run Time Memory Devices 1 10.0 0.3 10 2 2 10.2 0.5 60 1 3 10.4 0.1 50 4 4 10.5 0.4 10 2 5 10.8 0.1 30 3 HOLDQ Ready / Running

  7. Example 4 (Cont…) TimeEvent#JobsHeadwayMM FreeDevicesTime Left 10.0 1 A,S 90 3 1 0.3 10.2 2 A,S 1 0.2 30 2 1 0.1 2 0.5 10.4 1 F 2 0.1 40 4 2 0.4 3 A,H 10.5 4 A,S 1 0.1 30 2 2 0.3 4 0.4 10.8 5 A,H 2 0.15 30 2 2 0.15 4 0.25 11.1 2 F 2 0.15 90 3 4 0.1 5 S 60 0 5 0.1 11.3 5 F 2 0.1 90 3 3 0.1 4 F 100 5 3 S 50 1 11.4 3 F 1 0.1 100

  8. T and W for Example 4 • Job Run Start Finish Ti Wi • 0.3 10.0 10.4 0.4 1.33 • 0.5 10.2 11.1 0.9 1.8 • 0.1 10.4 11.4 1.0 10.0 • 0.4 10.5 11.3 0.8 2.0 • 0.1 10.8 11.3 0.5 5.0 • ===== ===== • 3.6 20.13 • T = 0.72 • W = 4.026

  9. Scheduling Example 5 (1) • Assume: • FIFO Job Scheduling • 100 K Main Memory • Devices • Processor Sharing Process Scheduling • (Cont…)

  10. Scheduling Example 5 (2) Job Arrives Run Time Memory Tapes 1 1.0 0.5 30 2 2 1.2 1.0 50 1 3 1.3 1.5 50 1 4 1.4 2.0 20 2 5 1.7 0.5 30 3 6 2.1 1.0 30 2

  11. Example 5 (Cont…) (1) TimeEvent#JobsHeadwayMM FreeDevicesTime Left 1.0 1 A,S 70 3 1 0.5 1.2 2 A,S 1 0.2 20 2 1 0.3 2 1.0 1.3 3 A,H 2 0.05 20 2 1 0.25 2 0.95 1.4 4 A,S 2 0.05 0 0 1 0.2 2 0.9 4 2.0 1.7 5 A,H 3 0.1 0 0 1 0.1 2 0.8 4 1.9 2.0 1 F 3 0.1 30 2 2 0.7 4 1.8 2.1 6 A,S 2 0.05 0 0 2 0.65 4 1.75 6 1.0

  12. Example 5 (Cont…) (2) TimeEvent#JobsHeadwayMM FreeDevicesTime Left 4.05 2 F 3 0.65 50 1 4 1.1 3 S 0 0 6 0.35 3 1.5 5.1 6 F 3 0.35 30 2 4 0.75 3 1.15 6.6 4 F 2 0.75 50 4 3 0.4 5 S 20 1 5 0.5 7.4 3 F 2 0.4 70 2 5 0.1 7.5 5 F 1 0.1 100 5

  13. T and W for Example 5 • Job Run Arrival Finish Ti Wi • 0.5 1.0 2.0 1.0 2.0 • 1.0 1.2 4.05 2.85 2.85 • 1.5 1.3 7.4 6.1 4.06 • 2.0 1.4 6.6 5.2 2.6 • 0.5 1.7 7.5 5.8 11.6 • 2.1 2.1 5.1 3.0 3.0 • ===== ===== • 23.95 26.11 • T = 3.99 • W = 4.35

  14. Job Scheduling Also known as FCFS: first come, first served Scheduling based on arrival time Non-preemptible discipline Fair – no job is given preferential treatment, and every arriving job eventually runs Whey is this policy bad for interactive users Run Hold Ready Wait

  15. Process Management A multiprogramming OS must interleave the execution of multiple jobs The OS must decide when each process gets to use each resource (CPU,disk, etc.) The scheduler decides “when”

  16. Time Quantums How does the OS interleave execution of many processes on the CPU? Ans: Time slicing Quantum: Amount of time given to jobs when time sharing

  17. Process States • Running: • Process currently has CPU • Ready: • Process could use CPU, if CPU were free • Blocked: • Process is waiting for some event to occur: • I/O Completion • Buffer available from another process • Data arrives from network • (More states will be discussed later.)

  18. Process State Transitions Running Dispatch Block Timeout Ready Blocked Wakeup Is there a maximum number of processes in any state?

  19. Suspend and Resume Wakeup Blocked Ready Timeout Dispatch Running Suspend Resume Suspend Resume Suspend Suspended Ready Suspended blocked Event (e.g., I/O) completion

  20. Process Control Block (PCB) • One PCB per process, containing all information about that process • Process Identifier (pid) • Parent’s pid • State (e.g., Running, Blocked in ReadyQ) • Priority • Time at which its execution started • Amount of CPU time consumed so far • Copy of all register contents when process was last suspended • Main memory used by process (e.g., base and bound registers, page table pointer) • Accounting information • Room for pointers to PCB into a queue • File descriptor table

  21. Context Switch A Context Switch occurs when a process exchange is made between the ready and run queues: Must: Save the state of the running process AND Restore the state of the ready process Run Ready

  22. What happens on a Context Switch? (1) • Hardware: • Resets Program Counter (PC to that of the interrupt handler (IH), which is an address in the OS kernel • Switches from user to supervisor mode • Kernel: • Copies A’s state from CPU registers to A’s PCB • Sets A’s state to Ready or Blocked • Inserts A’s PCB on ReadyQ or BlockedQ • Scheduler selects a new process to run (B), based on its scheduling discipline • (Contd…)

  23. What happens on a Context Switch? (2) • 4. Kernel: • Sets B’s state to Running • Copies B’s state from B’s PCB to CPU registers • Kernel transfers control to B and thereby switches from kernel back to user context • What machine instructions can achieve number 5?

  24. Preemption • Two classes of scheduling disciplines • Premptive • Scheduler takes CPU away from running job and gives it to another job • Preempt upon arrival of higher priority job • Non-premptive

  25. Process Scheduling Algorithms (1) • 1. Round Robin (RR) • Each process runs either until • Its time quantum expires or • It blocks to perform I/O • 2. Processor Sharing (PS) • Limit of RR as time quantum goes to zero (Like giving each CPU cycle to a different process, in round robin fashion) • N processes scheduled by PS = each job runs on dedicated N-fold slower CPU. • Thus, READY = RUNNING

  26. Process Scheduling Algorithms (2) • 3. Priority • Each process is statically assigned a priority; run high before low priority • 4. Dynamic Priority • Same as #3, except priority level of each process can change dynamically • 5. Inverse of the Remainder of the Quantum • Position in ready queue is determined by the amount of time remaining in the time slice (e.g., if ¾ of time slice is left, the job moves ¼ ahead in the ready queue)

  27. Process Scheduling Algorithms (3) • 6. Multiple-Level Feedback Variant on the Round Robin • Current processes are forced to wait until new jobs “catch up” in time, then all RR • 7. System Balance • Balance system between I/O bound and CPU bound jobs • 8. Preference to Interactive jobs • Interactive jobs have higher priority

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