Module 6: CPU Scheduling - Simon Fraser University

Transcript Module 6: CPU Scheduling - Simon Fraser University

Chapter 5: CPU Scheduling

Chapter 5: Objectives

 Understand  Scheduling Criteria  Scheduling Algorithms  Multiple-Processor Scheduling

Operating System Concepts 5.2

Basic Concepts

 Maximum CPU utilization obtained with multiprogramming  CPU –I/O Burst Cycle  Process execution consists of a of CPU execution and I/O wait

cycle

 How long is the CPU burst?

Operating System Concepts 5.3

CPU Burst Distribution

  CPU bursts vary greatly from process to process and from computer to computer But, in general, they tend to have the following distribution (expo) Many short bursts Few long bursts

CPU Scheduler

   Selects

one

process from

ready queue

to run on CPU Scheduling can be 

Nonpreemptive

 Once a process is allocated the CPU, it does

not

leave unless:  1. it has to wait, e.g., for I/O request or for a child to terminate 2. it terminates

Preemptive

 OS can force (preempt) a process from CPU at

anytime

– Say, to allocate CPU to another higher-priority process Which is harder to implement? and why?  Preemptive is harder: Need to maintain 

consistency

of data shared between processes, and more importantly,  kernel data structures (e.g., I/O queues) – Think of a preemption while kernel is executing a sys call on behalf of a process (many OSs, wait for sys call to finish)

Dispatcher

  

Scheduler

: selects one process to run on CPU

Dispatcher

: allocates CPU to the selected process, which involves:  switching context  switching to user mode  jumping to the proper location (in the selected process) and restarting it

Dispatch latency

– time it takes for the dispatcher to stop one process and start another running 

How does scheduler select a process to run?

Scheduling Criteria

    

CPU utilization

– keep the CPU as busy as possible  Maximize

Throughput

– # of processes that complete their execution per time unit  Maximize

Turnaround time

– amount of time to execute a particular process (time from submission to termination)  Minimize

Waiting time

– amount of time a process has been waiting in the

ready queue

 Minimize

Response time

– amount of time it takes from when a request was submitted until the first response is produced, sharing environment)

not

output (for time  Minimize

Scheduling Algorithms

     First Come, First Served Shortest Job First Priority Round Robin Multilevel queues  Note: A process may have many CPU bursts, but in the examples we show only one for simplicity

5.8

First-Come, First-Served (FCFS) Scheduling

 Process Burst Time

P 1 P 2

24 3

P 3

3 Suppose that the processes arrive in the order:

P 1

The Gantt Chart for the schedule is: ,

P 2

P 3

P 1 P 2 P 3 27 30 0 24   Waiting time for

P 1

= 0;

P 2

= 24;

P 3

= 27 Average waiting time: (0 + 24 + 27)/3 = 17

FCFS Scheduling (Cont.)

Suppose that the processes arrive in the order 

P 2

P 3

P 1

3, 3, 24 The Gantt chart for the schedule is: P 2 P 3 P 1 0 3 6     Waiting time for

P 1 =

; P 2

= 0

; P 3 =

3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case Convoy effect: short process behind long process 30

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 Two schemes:  nonpreemptive – 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. This scheme is know as the Shortest-Remaining-Time-First (SRTF) SJF is optimal – gives minimum average waiting time for a given set of processes

Example of Non-Preemptive SJF

 Process

P 1 P 2 P 3 P 4

SJF (non-preemptive) Arrival Time 0.0

2.0

4.0

5.0

P 1 P 3 Burst Time 7 4 1 4 P 2 0 3 7 8  Average waiting time = (0 + 6 + 3 + 7)/4 = 4 12 P 4 16

Example of Preemptive SJF

 Process Arrival Time

P 1 P 2 P 3 P 4

SJF (preemptive, SRJF) 0.0

2.0

4.0

5.0

P 1 P 2 P 3 P 2 P 4  0 2 4 5 7 11 Average waiting time = (9 + 1 + 0 +2)/4 = 3 Burst Time 7 4 1 4 P 1 16

Determining Length of Next CPU Burst

  Can only estimate the length Can be done by using the length of previous CPU bursts, using exponential averaging

1.

t n



2.



 1

3.



, 0 actual lenght of

n th

CPU burst



predicted value for the next CPU burst

  

1 4.

Define :



 1  

t n

  1    



Operating System Concepts 5.14

Exponential Averaging

 If we expand the formula, we get: 

+1 =



-



) t

-1 +



(

1 -



)

t

+ …

+ (

1 -



)



 Since both  and (1  ) are less than or equal to 1, each successive term has less weight than its predecessor  Examples:   = 0 ==>  n+1 value) =  n ==> Last CPU burst does not count (transient   =1 ==>  n+1 = 

n ==> Only last CPU burst counts (history is stale)

Prediction CPU Burst Lengths: Expo Average

 Assume  = 0.5,  0 = 10

Operating System Concepts 5.16

Priority Scheduling

      A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer  highest priority)  Preemptive  nonpreemptive 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 the priority of the process Aging: increase the priority of a process as it waits in the system

Round Robin (RR)

   Each process gets a small unit of CPU time (

time quantum

), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.

If there are

processes in the ready queue and the time quantum is

, then each process gets 1/

of the CPU time in chunks of at most

time units at once. No process waits more than (

-1)

time units.

Performance 

large  FCFS 

small 

must be large with respect to context switch, otherwise overhead is too high

Example of RR with Time Quantum = 20

 Process

P 1 P 2 P 3 P 4

The Gantt chart is: Burst Time 53 17 68 24 0 P 1 20 P 2 37 P 3 57 P 4 77 P 1 P 3 97 117 P 4 P 1 P 3 P 3 121 134 154 162  Typically, higher average turnaround than SJF, but better response

Time Quantum and Context Switch Time

 Smaller q  more responsive but more context switches (overhead)

Operating System Concepts 5.20

Turnaround Time Varies With The Time Quantum

  Turnaround time varies with quantum, then stabilizes Rule of thumb for good performance:  80% of CPU bursts should be shorter than time quantum

Operating System Concepts 5.21

Multilevel Queue

   Ready queue is partitioned into separate queues:  foreground (interactive)  background (batch) Each queue has its own scheduling algorithm  foreground – RR  background – FCFS Scheduling must be done between the queues   Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation.

Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; e.g.,  80% to foreground in RR  20% to background in FCFS

Multilevel Queue Scheduling

Operating System Concepts 5.23

Multilevel Feedback Queue

  A process

can move

implemented this way between various queues; aging can be Multilevel-feedback-queue scheduler defined by the following parameters:  number of queues  scheduling algorithms for each queue  method used to determine when to upgrade a process  method used to determine when to demote a process  method used to determine which queue a process will enter when that process needs service

Example of Multilevel Feedback Queue

  Three queues:   

0 – RR with time quantum 8 milliseconds

1 – RR time quantum 16 milliseconds

2 – FCFS Scheduling   A new job enters queue

Q 0

which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue

1 .

1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue

Multilevel Feedback Queues

 Notes:  Short processes get served faster (higher prio)  more responsive  Long processes (CPU bound) sink to bottom  served FCFS  more throughput

Operating System Concepts 5.26

Multiple-Processor Scheduling

  Multiple processors ==> divide load among them  More complex than single CPU scheduling How to divide load?

 Asymmetric multiprocessor  One master processor does the scheduling for others  Symmetric multiprocessor (SMP)  Each processor runs its own scheduler  One common ready queue for all processors, or one ready queue for each  Win XP, Linux, Solaris, Mac OS X support SMP

SMP Issues

   Processor affinity   When a process runs on a processor, some data is cached in that processor’s cache A process migrates to another processor ==>  Cache of new processor has to be

re-populated

 Cache of old processor has to be

invalidated

  ==> performance penalty Load balancing  One processor has too much load and another is idle Balance load using 

Push migration

: A specific task periodically checks load on all processors and evenly distributes it by moving (pushing) tasks 

Pull migration

: Idle processor pulls a waiting task from a busy processor  Some systems (e.g., Linux) implement both Tradeoff between load balancing and processor affinity: what would you do?

 May be, invoke load balancer when imbalance exceeds a threshold

Real-time Scheduling

  Hard-real time systems  A task must be finished within a deadline  Ex: Control of spacecraft Soft-real time systems  A task is given higher priority over others  Ex: Multimedia systems

Operating System Concepts 5.29

Operating System Examples

 Windows XP scheduling  Linux scheduling

Operating System Concepts 5.30

Windows XP Scheduler

    Priority-based, preemptive scheduler  The highest-priority thread will always run 32 levels of priorities, each has a separate queue Scheduler traverses queues from highest to lowest till it finds a thread that is ready to run Priorities are divided into classes, each has several

relative

priorities

Operating System Concepts 5.31

Windows XP Scheduler (cont’d)

   

Real-time class (fixed):

Levels 16 to 31

Other classes (variable):

Levels 1 to 15  Priority may change (decrease or increase) Priority decreases  After thread’s quantum time runs out  but never goes below the base (normal) value of its class  Limit CPU consumption of CPU-bound threads Priority increases  After a thread is released from a wait operation  Bigger increase if thread was waiting for mouse or keyboard  Moderate increase if it was waiting for disk  Also, active window gets a priority boost  Yield good response time

Linux Scheduler

 Priority-based, preemptive scheduler with two separate ranges  Real-time: 0 to 99   Nice: 100 to 140 (from -20 to 19) Higher priority tasks get larger quanta (unlike Win XP, Solaris)

Operating System Concepts 5.33

Linux Scheduler (cont’d)

     A task is initially assigned a time slice (quantum) Runqueue has two arrays:

active

and

expired

A runnable task is eligible for CPU if it still has time left in its time slice If the time slice runs out, the task is moved to the expired array When there are no tasks in the active array, the expired array becomes the active array and vice versa (change of pointers)

Operating System Concepts 5.34

Algorithm Evaluation

    Deterministic modeling  Takes a particular predetermined workload and defines the performance of each algorithm for that workload  Not general Queuing models  Use queuing theory to analyze algorithms  Many (unrealistic) assumptions to facilitate analysis Simulation  Build a simulator and test with  synthetic workload (e.g., generated randomly), or  Traces collected from running systems Implementation  Code it up and test!

Summary

       Process execution: cycle of CPU bursts and I/O bursts  CPU bursts lengths: many short bursts, and few long ones Scheduler selects one process from ready queue  Dispatcher performs the switching Scheduling criteria (usually conflicting)  CPU utilization, waiting time, response time, throughput, … Scheduling Algorithms  FCFS, SJF, Priority, RR, Multilevel Queues, … Multiprocessor Scheduling  Processor affinity vs. load balancing Evaluation of Algorithms  Modeling, simulation, implementation Examples  Win XP, Linux

Module 6: CPU Scheduling - Simon Fraser University

Transcript Module 6: CPU Scheduling - Simon Fraser University

Chapter 5: CPU Scheduling

Chapter 5: Objectives

Basic Concepts

CPU Burst Distribution

CPU Scheduler

Dispatcher

Scheduling Criteria

Scheduling Algorithms

First-Come, First-Served (FCFS) Scheduling

FCFS Scheduling (Cont.)

Shortest-Job-First (SJF) Scheduling

Example of Non-Preemptive SJF

Example of Preemptive SJF

Determining Length of Next CPU Burst

1.

2.

3.

, 0 actual lenght of

CPU burst

predicted value for the next CPU burst

1 4.

Define :

Exponential Averaging

-

) t

(

)

t

+ (

)

Prediction CPU Burst Lengths: Expo Average

Priority Scheduling

Round Robin (RR)

Example of RR with Time Quantum = 20

Time Quantum and Context Switch Time

Turnaround Time Varies With The Time Quantum

Multilevel Queue

Multilevel Queue Scheduling

Multilevel Feedback Queue

Example of Multilevel Feedback Queue

Multilevel Feedback Queues

Multiple-Processor Scheduling

SMP Issues

Real-time Scheduling

Operating System Examples

Windows XP Scheduler

Windows XP Scheduler (cont’d)

Linux Scheduler

Linux Scheduler (cont’d)

Algorithm Evaluation

Summary

Directory