Announcement. Midterm Exam is on Monday June 8th. Office hours on Friday 2-3 pm. Exam Material will be posted on Piazza.

Transcription

1 Announcement Midterm Exam is on Monday June 8th. Office hours on Friday 2-3 pm. Exam Material will be posted on Piazza.

2 Operating Systems: Internals and Design Principles Chapter 9 Uniprocessor Scheduling Seventh Edition By William Stallings Modified by Rana Forsati for CSE 410

3 Learning Objective Identify different goals of process scheduling algorithms Identify different types of scheduling and their role Given a set of processes, evaluate them to identify the next process to run Identify effect of different scheduling algorithms on CPU bound/i/o bound processes

4 Scheduling and Process State Transitions

5 Types of Scheduling Broken down into three separate functions: long term scheduling medium term scheduling short term scheduling

6 Long-Term Scheduling Determines when to create a new process and which programs are admitted next to the system for processing Once admitted a user program becomes a process and is added to the queue for short term scheduler Controls the degree of multiprogramming The more processes that are created, the smaller the percentage of time that each process can be executed

7 Medium-Term Scheduling Part of the swapping function Swapping-in decisions are based on the need to manage the degree of multiprogramming Considers the memory requirements of the swapped-out processes

8 Short-Term Scheduling Known as the dispatcher The main objective of short-term scheduling is to allocate CPU time to processes Executes most frequently [minimum overhead?] Invoked when an event occurs that may lead to the blocking of the current process or that may provide an opportunity to preempt a currently running process in favor of another - Clock interrupts I/O interrupts Operating system calls Short-term success criteria?

9 Short Term Scheduling Algorithms 9

10 Aim of Short-term-Scheduling Assign processes to be executed by the processor in a way that meets system (performance) objectives: Response time: amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) Minimize response time Throughput: # of processes that complete their execution per time unit Maximize throughput Processor efficiency: keep the CPU as busy as possible Maximize CPU utilization

11 Short-Term Scheduling Dispatcher OR short-term scheduler Short-Term Scheduling: Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them

12 Scheduling Objectives The scheduling function should: Share time fairly among processes Prevent starvation of processes Have low overhead Prioritize processes when necessary [flexible]

13 Scheduling Policies First-Come-First-Served (FCFS) Round-Robin (RR) Shortest-Process-Next (SPN) Shortest-Remaining Time (SRT) Highest Response Ratio Next (HRRN)

14 Selection Function Determines which process, among ready processes, is selected next for execution This function may be based on priority, resource requirements, or the execution characteristics of the process: time spent in system so far, waiting time spent in execution so far total service time required by the process [generally, this quantity must be estimated or supplied by the user]

15 Decision Mode Two categories: Nonpreemptive Once a process is in the running state, it will continue to execute until it terminates OR it blocks itself for I/O or some OS service Preemptive Currently running process may be interrupted and moved to the Ready state by the operating system. a new process arrives interrupt occurs periodically based on a clock interrupt

16 Process Scheduling Example Whatʼs the execution order?

17 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 A B C D E What are the potential consequences of this algorithm?

18 First-Come-First-Served (FCFS) Favors long processes: performs much better for long processes than short ones A short process may have to wait a very long time before it can execute Favors CPU-bound processes (mostly uses the processor) I/O processes have to wait until CPU-bound process completes

19 First-Come-First-Served (FCFS)

20 First-Come-First-Served (FCFS) Process Service Time (ms) P 1 24 P 2 3 P 3 3 Suppose that the processes arrive in the order: P 1, P 2, P 3 P 1 P 2 P 3 0 Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17 ms

21 First-Come-First-Served (FCFS) Suppose that the processes arrive in the order P 2, P 3, P 1 P 2 P 3 P Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 ms Much better than previous case Convoy effect: short process behind long process

22 Round Robin Clock interrupt is generated at periodic intervals When an interrupt occurs, the currently running process is placed in the ready queue Next ready job is selected This technique is also known as time slicing,because each process is given a slice of time before being preempted. Principal design issue is the length of the time quantum, or slice, to be used [too large >?, too short >?]

23 Round Robin Each process gets a small unit of CPU time known as time slicing (time quantum). After this time has elapsed, the process is preempted and added to the end of the Ready queue. Process Service Time P 1 24 P 2 3 P 3 3 P 1 P 2 P 3 P 1 P 1 P 1 P 1 P 1 An amount of time is determined that allows each process to use the processor for that length of time q=

24 Round Robin q = 4 A B C D E Whatʼs the execution order?

25 Round Robin, q = 4

26 Round Robin, q =1

27 Round Robin

28 Finish time of each process? a) Round Robin q=30 b) Round Robin q=10 Example

29 Example Solution A B C D E

30 Shortest Process Next (SPN) Another approach to reducing the bias in favor of long processes inherent in FCFS Associate with each process the length of its next CPU service. Use these lengths to schedule the process with the shortest time Process with the shortest expected processing time is selected next A short process jumps ahead of longer processes Nonpreemptive policy

31 Shortest Process Next (SPN)- Issues The difficulty is knowing or estimating the length of the required processing time of each process. A risk with SPN is the possibility of starvation for longer processes, as long as there is a steady supply of shorter processes. It still is not desirable for a time-sharing or transaction processing environment because of the lack of preemption.

32 Shortest Process Next (SPN) SPN: Process Arrival Time Service Time P P P P P 4 P P 3 1 P Average waiting time = ( ) / 4 = 7 ms

33 Shortest Process Next (SPN) A B C D E Whatʼs the execution order?

34 Shortest Process Next (SPN)

35 Shortest Process Next (SPN)

36 Shortest Process Next (SPN)- Service Estimation One difficulty is the need to know, or at least estimate, the required processing time of each process (Predictability of longer processes is reduced) Possibility of starvation for longer processes If estimated time for process not correct, the operating system may abort it Can be done by using the length of previous CPU services, using exponential averaging

37 Shortest Remaining Time (SRT) Preemptive version of SPN Scheduler always chooses the process that has the shortest expected remaining processing time While a process A is running, if a new process B comes whose length is shorter than the remaining time of process A, then B preempts A and B is started to run. Must estimate processing time A B C D E Whatʼs the execution order?

38 Shortest Remaining Time (SRT) Should give superior turnaround time performance to SPN because a short job is given immediate preference to a running longer job Risk of starvation of longer processes

39 Shortest Remaining Time (SRT) SRT: Process Arrival Time Burst Time P P P P P 1 P 2 P 4 P 1 P Average waiting time = ( ) / 4 = 6.5 ms

40 Highest Response Ratio Next (HRRN) Chooses next process with the greatest value of response ratio Attractive because it accounts for the age of the process While shorter jobs are favored, aging without service increases the ratio so that a longer process will eventually get past competing shorter jobs

41 Highest Response Ratio Next (HRRN)

42 The algorithms with CPU-I/O cycle The previous examples included processes with just a single CPU service. Each process execution is a CPU-I/O cycle First, it executes on CPU Then, it waits for I/O After completion of I/O, executes on CPU again and so on. It ends with execution on CPU. Example <3, 2, 1, 4, 5> CPU for 3 time units then I/O for 2, We will make the following assumptions while considering these algorithms with I/O services as well. In this class, we will assume that all I/O is concurrent.

43 The algorithms with CPU-I/O cycle FCFS: A process that finishes its I/O enters at the end of the ready queue. No other change required. Round Robin: Same as above We will assume the process enters the ready queue as soon as its I/O is finished. If a process finishes its CPU service, another process can start executing immediately. No history is maintained E.g., if the time quantum is 10 and the process CPU service is 4 then the process will release the CPU after time 4. After this process releases the CPU, another process can start right away and will get time quantum of 10 When the first process comes back again, it will get the entire time quantum of 10.

44 The algorithms with CPU-I/O cycle SPN and SRT When a process is being considered, we will focus on the next CPU service to decide who runs first HRRN We will consider total service time and total waiting time in deciding the process to be chosen

45 First-Come-First-Served (FCFS) Consider a system where there are 3 processes, A, B, and C. Their details are as follows A: Arrival time = 0, CPU-I/O cycle = <4,3,2,6,6> B: Arrival time = 2, CPU-I/O cycle = <2,5,4,5,4> C: Arrival time = 4, CPU I/O cycle = <5,2,3,3,5> Show the execution with FCFS:

46 Feedback Scheduling Penalize jobs that have been running longer Donʼt know remaining time process needs to execute Preemptive with time quantum Demoted to the next lower-priority queue With each queue (except the lowest priority queue), FCFS Lowest-priority queue: RR

47 Traditional UNIX Scheduling Multilevel feedback using round robin within each of the priority queues Priorities are recomputed once per second Base priority divides all processes into fixed bands of priority levels Adjustment factor used to keep process in its assigned band (decreasing order of priority): Swapper Block I/O device control File manipulation Character I/O device control User processes

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49 Summary The operating system must make three types of scheduling decisions with respect to the execution of processes: Long-term determines when new processes are admitted to the system Medium-term part of the swapping function and determines when a program is brought into main memory so that it may be executed Short-term determines which ready process will be executed next by the processor From a user s point of view, response time is generally the most important characteristic of a system; from a system point of view, throughput or processor utilization is important Algorithms: FCFS, Round Robin, SPN, SRT, HRRN, Feedback