Operating Systems

1. Operating Systems Prof. Navneet Goyal Department of Computer Science & Information Systems BITS, Pilani

2. Topics for Today • Process Scheduling • Criteria & Objectives • Types of Scheduling • Long term • Medium term • Short term • Few simple scheduling algorithms

3. The Players • Queues • Processes • Scheduler • Scheduling Algorithms

4. Queues • Queue at the cash counter of a supermarket • Queue at the ticket counter of a movie theatre • Queue at the railway reservation counter • Queue at toll gates • Ready queue in memory STARVATION!

5. Processes • I/O bound • CPU bound • Processes alternate between CPU & I/O • Process execution begins with a CPU burst • Process terminates with a final CPU burst (system call to terminate execution) • Characteristic of CPU burst times

6. CPU Bursts Figure taken from text book

7. Scheduler • Long term • Controls the degree of multiprogramming • Minutes between creation of a new process and the next • Should select a good mix of I/O and CPU bound processes to maximize utilization • Medium term • Swapping • Short term (CPU scheduler) • Fast • Typically executes every 100 ms • Typically takes 10 ms to decide which process to schedule

8. CPU Scheduler • Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them • CPU scheduling decisions may take place when a process: 1. Switches from running to blocked state 2. Switches from running to ready state (interrupt) 3. Switches from blocked to ready (completion of I/O) 4. Terminates • Scheduling under 1 and 4 is non-preemptive whereas, 2 & 3 is preemptive

9. Preemptive & Non-preemptive Scheduling • SO what is Preemptive & Non-preemptive scheduling? • Non-preemptive • Once CPU is allocated to a process, it holds it till it • Terminates (exits) • Blocks itself to wait for I/O Used by Windows 3.x

10. Preemptive & Non-preemptive Scheduling • Preemptive • Currently running process may be interrupted and moved to the ready state by the OS • Windows 95 introduced preemptive scheduling • Used in all subsequent versions of windows • Mac OS X uses it

11. Criteria & Objectives • CPU utilization – keep the CPU as busy as possible (40% – lightly loaded, 90% – heavily loaded) • Throughput – # of processes that complete their execution per time unit • Turnaround time – amount of time to execute a particular process (total time spent on the system) • Waiting time – amount of time a process has been waiting in the ready queue • Response time – In an interactive system, TAT may not be the best criterion. amount of time it takes from when a request was submitted until the first response is produced, not the time it takes to output the response

12. Optimization Criteria • Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time

13. Types of Scheduling • CPU scheduling • I/O scheduling • decision as to which process’s pending request shall be handled by an available I/O device

14. Short-Term Scheduling • Known as the dispatcher • Executes most frequently • Invoked when an event occurs • Clock interrupts • I/O interrupts • Operating system calls

15. Process Scheduling Example

16. 5 0 10 15 20 First-Come-First-Served (FCFS) • Each process joins the Ready queue • When the current process ceases to execute, the oldest process in the Ready queue is selected 1 2 3 4 5

17. First-Come-First-Served (FCFS) • Short jobs suffer penalty (waiting time) • Favors CPU-bound processes over I/O bound processes • I/O processes have to wait until CPU-bound process completes • May result in inefficient use of both the processor & I/O devices • Reasons?Convoy effect!

18. First-Come-First-Served (FCFS) • Not an attractive alternative for single processor systems. Must be combined with priority scheme

19. Round Robin • Reduces the penalty that short jobs suffer with FCFS • Uses clock based preemption • A ready job is selected from the queue on a FCFS basis • Also called as ‘Time Slicing’

20. 5 0 10 15 20 Round-Robin (q=1) 1 2 3 4 5

21. Round-Robin (q=4) • DO IT YOURSELF!!

22. Round-Robin • Main design issue – length of time quantum • If q very short: + short processes will move very quickly - more interrupt handling & context switching • q should be greater than time required for a typical interaction (fig) • Degenerates to FCFS in the limiting case (q is very large)

23. Time Quantum for Round Robin • must be substantially larger than the time required to handle the clock interrupt and dispatching • should be larger then the typical interaction (but not much more to avoid penalizing I/O bound processes)

24. Round-Robin • Drawback: Favors CPU-bound processes • A I/O bound process uses the CPU for a time less than the time quantum and then is blocked waiting for I/O • A CPU-bound process run for all its time slice and is put back into the ready queue (thus getting in front of blocked processes) • Example (q=4) A: 1-I/O-1-I/O-1-I/O-1 B: 4-I/O-4-I/O-4 C: 4-I/O-4-I/O-4 D: 4-I/O-4-I/O-4-I/O-4 • Virtual Round Robin (VRR) avoids this unfairness

25. Virtual Round-Robin (VRR) • What is VRR? • When a running process is timed out, it joins the ready queue (FCFS queue) • When a process is blocked, it joins an I/O queue • This is normal! • New queue: Auxiliary FCFS queue • Processes are moved to this queue after being released from an I/O block • When dispatching decision is made, processes in the auxiliary queue are given preference over those in main ready queue • Figure

26. Queuing for Virtual Round Robin

27. Virtual Round-Robin • Process selected from the auxiliary queue runs for q-q1 time • A process dispatched from the auxiliary queue runs no longer than the basic time quantum minus the time spent running since it was selected from the ready queue • This is done to avoid monopolization by I/O bound processes

28. Shortest Process Next (SPN or SJF) • Kind of a priority scheduling • Gives priority to a process with smallest next CPU burst • Decision mode: non preemptive • I/O bound processes will be picked first • In case of a tie between 2 processes, break the tie using FCFS • Ideally should be called as “shortest-next-CPU-burst” algorithm • Because scheduling depends on length of the next CPU burst rather than on its total length

29. 5 0 10 15 20 Shortest Process Next (SPN or SJF) 1 2 3 4 5

30. Shortest Process Next • Predictability of longer processes is reduced • Possibility of starvation for longer processes (bank example) • Reduces bias in favor of longer processes as compared to FCFS • Reduces the average waiting time • If estimated time for process not correct, the operating system may abort it • Provably optimal in terms of Av. Waiting Time

31. 5 0 10 15 20 Shortest Remaining Time(Preemptive SJF) 1 2 3 4 5

32. SJF • How to estimate the length of the next CPU burst? • Approximate SJF! • Predict the value of the next CPU burst • Philosophy: next CPU burst will be similar in length to the previous ones • Exponential Average of the measured lengths of previous CPU bursts

33. Examples of Exponential Averaging •  =0 • n+1 = n • Recent history does not count •  =1 • n+1 =  tn • Only the actual last CPU burst counts • If we expand the formula, we get: n+1 =  tn+(1 - ) tn-1+ … +(1 -  )j tn-j+ … +(1 -  )n +1 0 • Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor

34. Normalized Turnaround Time • TAT (Tr) does not tell us anything about the breakup between the service time (Ts) and the waiting time • Normalized TAT (NTAT) = Tr / Ts • Indicates the relative delay experienced by a process • Minimum possible value for NTAT is • Increasing value corresponds to decreasing level of service 1.0

35. Normalized Turnaround Time • TAT = 1(wt)+1(st) = 2 • NTAT = TAT/st = 2/1 = 2 • TAT = 1(wt)+9(st) = 10 • NTAT = 10/9 = 1.1 (good service) • By looking at TAT, we get no idea about the quality of service!

36. Highest Response Ratio Next (HRRN) • NTAT is a better metric than TAT • For each individual process we would like to reduce this ratio • Minimize its average value for all processes • Consider Response Ratio (RR) RR = (w+s)/s, where w = time spent waiting for processor s = expected service time • Min (RR) = 1.0 (when w=0)

37. Highest Response Ratio Next (HRRN) • When current process completes or is blocked, choose the ready process with max. value of R • HRRN is attractive as it incorporates age of a process (in terms of w) • Shorter jobs are favored (small s) • Aging without service (increase in w) will increase R • Like SJF & SRT, HRRN also needs to estimate expected service time • DO the running example with HRRN

38. 5 0 10 15 20 HRRN Example 1 2 3 4 5

39. Priority Scheduling • SJF is a special case of a general priority scheduling algorithm • Priority associated with each process • CPU allocated to the process with highest priority • Tie broken using FCFS • SJF: the lower the next CPU burst, the higher is the priority

40. Priority Scheduling • Preemptive • Non-preemptive

41. Priority Scheduling • Starvation is a major problem • A steady stream of higher priority processes can prevent a lower priority process from ever getting the CPU • May get the CPU at on odd hour when the system is lightly loaded • System crashes and loses all unfinished low priority processes • When IBM 7094 was shut down in 1973 at MIT, a low priority was found which was submitted in 1967!!!

42. Priority Scheduling • Solution – Aging • Gradually increasing the priority of a process that has been waiting in the system for long • After every t time units, increase the priority by 1

43. Priority Scheduling • Static Priorities • Priority of a process never changes • Dynamic Priorities • Priority of a process changes with time (aging)

44. Multi-level queue Scheduling • Single queue – processes with different priorities in a single queue • Dispatcher needs to search the highest priority process

45. Multi-level queue Scheduling • Static Priorities

46. Multi-level Feedback Queue Scheduling • If service time is not known, we can not use any of the following : • SJF or SPN • SRT • HRRN • Cannot focus on the time remaining to execute • Focus on time spent on processor • Another way of penalizing processes that have been running longer and giving preference to shorter jobs

47. Multi-level Feedback Queue Scheduling • Quantum based preemption • Dynamic priority mechanism • With each quantum based preemption, a process is demoted to the next lower-priority queue • A short process will complete quickly without slipping very far down the hierarchy of ready queues • A longer process will gradually drift down

48. Feedback Scheduling • Newer shorter processes are favored over older, longer processes • In each ready queue, except the lowest priority queue, FIFO queue is used • Lowest priority queue is treated in RR fashion • Scheme is called as multi-level feedback scheme

49. 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.

50. Multilevel Feedback Queues