1. COT 4600 Operating Systems Spring 2011 Dan C. Marinescu Office: HEC 304 Office hours: Tu-Th 5:00-6:00 PM

2. Lecture 20 - Tuesday April 5, 2011 Last time: Tread coordination with a bounded buffer Today: Scheduling Multi-level memories Memory characterization Multilevel memories management using virtual memory Adding multi-level memory management to virtual memory Next Time: Page replacement algorithms Lecture 20

3. Scheduling • Basic concepts; scheduling objectives. • Scheduling policies • First-Come First-Serve (FCFS) • Shortest Job First (SJF) • Round Robin (RR) • Preemptive/non-preemptive scheduling • Priority scheduling • Priority inversion • Schedulers • CPU burst. Estimation of the CPU burst • Multi-level queues • Multi-level queues with feedback • Example: the UNIX scheduler Lecture 20

4. Scheduling – basic concepts • Scheduling  assigning jobs to machines. • A schedule S  a plan on how to process N jobs using one or machines. • Scheduling in the general case in a NP complete problem. • A job 1 <= j <- N is characterized by • CiS completion time of job j under schedule S • pi  processing time • ri release time; the time when the job is available for processing • di due time ; the time when the job should be completed. ui=0 if CiS <= diand ui=1 otherwise • Lj= CiS- di lateness • A schedule S is characterized by • The makespanCmax = max CiS • Average completion time Lecture 20

5. Scheduling objectives • Performance metrics: • CPU Utilization  Fraction of time CPU does useful work over total time • Throughput  Number of jobs finished per unit of time • Turnaround time  Time spent by a job in the system • Response time  Time to get the results • Waiting time  Time waiting to start processing • All these are random variables  we are interested in averages!! • The objectives - system managers (M) and users (U): • Maximize CPU utilization M • Maximize throughput  M • Minimize turnaround time  U • Minimize waiting time  U • Minimize response time  U Lecture 20

6. Scheduling policies • Burst time  time required by a thread to use the processor/core • Time slice/quantum  time a thread is allowed to use the processor/core • Preemptive scheduling  A thread could be forced to release the control of the processor • Policies • First-Come First-Serve  FCFS • Shortest Job First SJF • Round Robin  RR Lecture 20

7. First-Come First-Served (FCFS) P1 P2 P3 0 24 27 30 ThreadBurst Time P1 24 P2 3 P3 3 Processes arrive in the order: P1P2P3 Gantt Chart for the schedule: Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 Convoy effectshort process behind long process Lecture 20

8. The effect of the release time on FCFS scheduling P2 P3 P1 0 3 6 30 Now threads arrive in the order: P2P3P1 Gantt chart: Waiting time for P1 = 6;P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better!! Lecture 20

9. Shortest-Job-First (SJF) • Use the length of the next burst to schedule the thread/process with the shortest time. • SJF is optimal minimum average waiting time for a given set of threads/processes • Two schemes: • Non-preemptive  the thread/process cannot be preempted until completes its burst • Preemptive  if a new thread/process arrives with burst length less than remaining time of current executing process, preempt. known as Shortest-Remaining-Time-First (SRTF) Lecture 20

10. Example of non-preemptive SJF P1 P3 P2 P4 0 3 7 8 12 16 Thread Release timeBurst 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 Lecture 20

11. Example of Shortest-Remaining-Time-First (SRTF) (Preemptive SJF) P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Thread Release timeBurst time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 Shortest-Remaining-Time-First Average waiting time = (9 + 1 + 0 +2)/4 = 3 Lecture 20

12. Round Robin (RR) • Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the thread/process is preempted and added to the end of the ready queue. • If there are n threads/processes in the ready queue and the time quantum is q, then each thread/process gets 1/n of the processor time in chunks of at most q time units at once. No thread/process waits more than (n-1)q time units. • Performance • q large  FIFO • q small  q must be large with respect to context switch, otherwise overhead is too high Lecture 20

13. RR with time slice q = 20 P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 ThreadBurst Time P1 53 P2 17 P3 68 P4 24 Typically, higher average turnaround than SJF, but better response Lecture 20

14. Time slice (quantum) and context switch time Lecture 20

15. Turnaround time function of time quantum Lecture 20

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18. Priority scheduling • Each thread/process has a priority and the one with the highest priority (smallest integer  highest priority) is scheduled next. • Preemptive • Non-preemptive • SJF is a priority scheduling where priority is the predicted next CPU burst time • Problem  Starvation – low priority threads/processes may never execute • Solution to starvation  Aging – as time progresses increase the priority of the thread/process • Priority my be computed dynamically Lecture 20

19. Priority inversion • A lower priority thread/process prevents a higher priority one from running. • T3 has the highest priority, T1 has the lowest priority; T1 and T3 share a lock. • T1 acquires the lock, then it is suspended when T3 starts. • Eventually T3 requests the lock and it is suspended waiting for T1 to release the lock. • T2 has higher priority than T1 and runs; neither T3 nor T1 can run; T1 due to its low priority, T3 because it needs the lock help by T1. • Allow a low priority thread holding a lock to run with the higher priority of the thread which requests the lock Lecture 20