Overview

1. Overview • Resource Sharing & Inter-Process Interactions • Issues and Problems (Deadlocks, CS, ME…) • Task Orderings (Scheduling …) • Algorithmic Solutions (Races, ME…) • Program Level Solutions (Semaphores, Monitors)

2. Deadlocks  Starvation • How to allocate a resource to have progress… • Let’s get a task running!!! E.g. shortest job first • Works great for multiple short jobs in a system • May cause a long job to be postponed indefinitely with shorter ones jumping ahead … even though this is “waiting” and not blocked • Alternates? • First-come, first-serve (FIFO) policy, Round Robin Slotting • Shortest job first, highest priority first, earliest deadline first, random … Resource Allocation: Efficient? Fair? Optimal?

3. OS Scheduling Criteria (Optimizations) • CPU utilization: keep CPU as busy as possible (maximize) • Nice but the CPU better be doing something useful! Remember livelock? • Throughput: # of processes completed/time unit (maximize) • Turnaround time: average amount of time to execute a particular process (minimize) • Waiting time: processwait time in ready queue (minimize) • Response time (minimize): amount of time it takes from when a request was submitted until the first response (not output) is produced. [result output time = RT deadline] • Optimize all  plus want “Fair & Efficient” solutions!

4. 1. Task/Process/Thread Types • Non pre-emptive (NP): An ongoing task cannot be displaced • Pre-emptive: Ongoing tasks can be switched in/out as needed

5. 2. Sequences of CPU And I/O Bursts Apps typically stay CPU or I/O-bound – Hybrids generally for interactive apps

6. Scheduling Variants (Processes/Threads) • First Come, First Served (FCFS) • Shortest Job First (SJF) • Round Robin (RR) • Priority Based (PB)

7. A: First-Come, First-Served (FCFS) Scheduling (Non-Preemptive) P1 P2 P3 0 24 27 30 ProcessLength (CPU Burst Time) P1 24 P2 3 P3 3 • Processes arrive (& get executed) in their arrival order: P1 , P2 , P3 Gantt Chart for the schedule (single queue of ready processes: NP): • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17

8. FCFS Scheduling (NP: Non-Preemptive) P2 P3 P1 0 3 6 30 Suppose that the processes arrive in a different order as: P2 , P3 , P1 [3,3,24] • Gantt chart for the schedule: • Waiting time for P1 = 6;P2 = 0; P3 = 3 • Average waiting time: (6 + 0 + 3)/3 = 3 • Variability of wait time (from 17 to 3!) – control? response prediction? • Convoy effect: clustershort processes behind long process (starvation possible!)

9. Scheduling in Batch Systems where we can actually see and re-order FCFS? Shortest job first (SJF) scheduling?

10. B: 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 Options: • non-preemptive – once CPU given to the process it cannot be preempted until it completes its CPU burst • preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. Variation: Shortest-Remaining-Time-First (SRTF)

11. Example of Non-Preemptive SJF P1 P3 P2 P4 0 3 7 8 12 16 Process Arrival TimeBurst Time P10.00 7 P2 0.0+ 4 P3 0.0+ 1 P4 0.0+ 4 • SJF (non-preemptive) P1,then P3 then P2, P4(FCFS across equals) • Average waiting time = (0 + 8 + 7 + 12)/4 = 6.75 (fixed arrival) • Av. waiting = (0 + [8-2] + [7-4] + [12-5])/4 = 4.00 (staggered arrival) 0.0 2.0 4.0 5.0

12. 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 Options: • non-preemptive – once CPU given to the process it cannot be preempted until it completes its CPU burst • preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. Variation: Shortest-Remaining-Time-First (SRTF)

13. Preemptive SJF(Shortest Remaining Time First: Reassess SRT on Arrivals) P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (preemptive) • Average waiting time = P1:[0, (11-2)]; P2:[0, (5-4)]; P3: 0; P4: (7-5) • Average waiting time = (9 + 1 + 0 +2)/4 = 3 …[6.75;4.00] • Dynamic scheduling (SRT) calculations + context switches cost factor! P2:2, P4:4, P1:5 P1: 5 left P2: 2 left

14. Determining Length of Next CPU Burst Can only estimate the task length (simple for batch jobs) Use the length of previous CPU bursts (history logs & ageing) (0) direct fn. of history (1) direct fn. of most recent run τ1 = ½ to + ½ τ0<add + simple left shift (divide by 2!)> τ2 = ½ t1 + ½ τ1 = ½ t1 + ½ ( ½ t0 + ½ τ0) = ½ t1 + ¼ t0 + ¼ τ0 τ3 = ½ t2 + ½ τ2 = ½ t2 + ¼ t1 + 1/8 t0 + 1/8 τ0

15. 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 Options: • non-preemptive – once CPU given to the process it cannot be preempted until completes its CPU burst • preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. Shortest-Remaining-Time-First (SRTF) • SJF is optimal – gives minimum average waiting time for a given set of processes • SJF can still lead to starvation: if new short jobs keep coming in and getting considered in the “shortest” ranking, a “long” job can potentially starve!

16. Variations • SJF  EDF (Earliest Deadline First)

17. Onwards to Preemptive Scheduling

18. C: *Round Robin (RR/TDMA): Fixed Slots • Each process gets a fixed slice of CPU time (time quantum: q), 10-100ms • After this time has elapsed, the process is “forcefully” 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 too large  FIFO (poor response time & poor CPU utilization) • q toosmall  q must be large with respect to context switch ovhd., else context switching overhead reduces efficiency c a c a a b b b ReadyQ

19. Example of RR with Time Quantum = 20Rudimentary Static RR! P1 P2 P3 P4 P1 P3 P4 P1 40 60 80 100 120 140 160 180 200 0 20 ProcessBurst Time P1 53 P2 17 P3 68 P4 24 37 173 144

20. Better Dynamic RR with Quantum = 20Dynamic Q: if a process finishes in less than 20, then the next slot starts earlier! P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 ProcessBurst Time P1 53 P2 17 P3 68 P4 24 • Typically, higher average turnaround than SJF, but better response • Turnaround time – amount of time to execute a particular process (minimize) • Response time (min) – amount of time it takes from request submission until first response • Scheduling is static though calculating/estimation “right” length of quantum is crucial!!! • Possible to have quanta of varying lengths!!! (dynamic scheduling costs ~ SJF)

21. D: *Priority Scheduling (RR  All tasks have same priority) • A priority number (integer) is associated with each process • The CPU is allocated to process with the highest priority (For Unix BSD: smallest integer  highest priority) • Preemptive (priority and order changes possible at runtime) • Non-preemptive (initial execution order chosen by static priorities) • SJF is a priority scheduling where priority is the predicted next CPU burst time • Problem Starvation: low priority processes may never execute • Solution  Aging: as time progresses increase priority of process

22. Ageing and Priority • Priority can be static or dynamic (as a function of wait-time & usage-time for fairness etc: Unix BSD, Linux) Note: In BSD  Large # =‘s Low Priority! P_user-pri = P_USER + P_cpu/4 + (2 * P_nice) • processusingCPU: P_cpu increaseswith time  P_user-pri goes up(ie lower priority) • processwaiting: P_cpukeepsdecreasing  P_user-prikeeps going down(ie higher priority)

23. Priority Scheduling (Solaris, Windows, Linux) • Kernel process: generally non-preemptive • User processes: mostly pre-emptable (FCFS within a priority level) Higher Priority: Small quanta Lower Priority: Bigger quanta

24. Which Scheduling Policy to Use? • OS schedulers implement *all* policies • The nature of specific applications (computing, I/O, multimedia etc) and the mixture of applications (at kernel- level, at user-level) determines the specific scheduling policy or groups of policies to use! • Kernel and user-level schedulers can be the same or more generally discrete at each level.

25. Thread Scheduling • Local Scheduling – How the threads library decides which thread to put onto an available LWP • Global Scheduling – How the kernel decides which kernel thread to run next

26. Thread Scheduling (User Scheduler) Possible scheduling of user-level threads • 50-msec process quantum; threads run 5 msec/CPU burst • simple process level context switch; thread block  process blocks • split level scheduling (kernel + dedicated application/thread type specific)

27. Thread Scheduling (Kernel Scheduler) Possible scheduling of kernel-level threads • 50-msec process quantum; threads run 5 msec/CPU burst • flexible…costly full context switch for threads • monolithic kernel scheduler also avoids thread blocks (on I/O etc)

28. OS Examples • All work with dynamically • altering priorities & time slices • All have pre-emptive tasking • Solaris scheduling • Windows scheduling • Linux scheduling

29. Solaris 2 (& Linux) Scheduling Higher the priority, shorter the time slice

30. Solaris Dispatch Table TQE RFS New dynamically altered priorities …and time slots (msec) CPU-tasks mid-priority, Interactive: higher priority * Lowest priority 0 (diff. from BSD) is given largest quantum TQE: new (reduced) priority for tasks that have used quanta without blocking (CPU Intensive Tasks) ; Solaris 9+ : No TQE (fixed priority & CPU shares) RFS: (raised) priority of awakened tasks (responsiveness for I/O Tasks)

31. Windows: Priority Based, Preemptive(Fixed & Variable Components) priority classes relative priority within a priority class (High # =‘s High Priority; diff. from BSD) • Fixed Priority Class (RT) will always run when invoked • Thread runs till quanta finished; priority lowered; task coming in from “wait” • gets priority raised – only within its priority class!

32. Linux Scheduling (0=highest) • Two algorithms: time-sharing and real-time • Time-sharing • Prioritized credit-based – process with most credits is scheduled next • Credit subtracted when timer interrupt occurs • When credit = 0, another process chosen • When all processes have credit = 0, re-crediting occurs • Based on factors including priority and history • Real-time • Soft real-time • Two classes • FCFS and RR • Highest priority process always runs first