introductionto cpu scheduling -...

Real-Time Computing and Communications Lab.Hanyang University

Introduction to CPU Scheduling

Minsoo Ryu

Department of Computer Science and EngineeringHanyang University

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Process Scheduling1 XPage

Priority-Based Scheduling2 XPage

Proportional Share Scheduling3 XPage

Q & A4 XPage

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Process States

Three key states running: Instructions are being executed. waiting: The process is waiting for some event to occur

(I/O completion, signal reception). ready: The process is waiting to be assigned to a process.

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State Transitions

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Process Scheduling

Selects from among ready processes And allocates the CPU to one of them

Scheduling decisions may take place when a process: Switches from running to waiting state (e.g., I/O request) Switches from running to ready state (e.g., interrupt) Switches from waiting to ready (e.g., completion of I/O) Terminates

Scheduling only under 1 and 4 is nonpreemptive Otherwise, it is preemptive

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Scheduling Criteria

Performance objectives Maximize CPU utilization or throughput Minimize completion time, waiting time, or response time

Real-time constraints Satisfy deadlines

Proportional share constraints Provide CPU cycles proportional to weights

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Classification of Scheduling Policies

Scheduling mechanisms Priority-based scheduling Proportional share scheduling

Target processors Uniprocessor scheduling Multiprocessor scheduling

Real-Time Computing and Communications Lab.Hanyang University

Priority-Based Scheduling

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Priority-Based 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

Task A (priority 1)Task B (priority 2)Task C (priority 3)Task D (priority 4) time

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Priority-Based Scheduling Policies

Non-real-time policies FCFS (First-Come, First-Served) SJF (Shortest-Job-First) SRTF (Shortest-Remaining-Time-First)

Real-time policies RM (Rate Monotonic) EDF (Earliest Deadline First)

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FCFS (First-Come, First-Served)

Process Execution TimeP1 24P2 3P3 3

Suppose that processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17

P1 P2 P3

24 27 300

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SJF (Shortest-Job-First)

Process Arrival Time Execution TimeP1 0.0 7P2 2.0 4P3 4.0 1P4 5.0 4

Average waiting time = (0 + 6 + 3 + 7)/4 = 4

P1 P3 P2

73 160

P4

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SRTF (Shortest-Remaining-Time-First)

Process Arrival Time Execution TimeP1 0.0 7P2 2.0 4P3 4.0 1P4 5.0 4

Average waiting time = (9 + 1 + 0 +2)/4 = 3 SRTF is optimal in that it achieves minimum average

waiting time for a given set of processes

P1 P3P2

42 110

P4

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P2 P1

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Prediction of Execution Times

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Prediction of Execution Times

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i

it

CPU burst ( )it

“guess” ( )i6 4 6 4 13 13 13 …

10 8 6 6 5 9 11 12 …

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RM (Rate Monotonic)

Assumptions Processes have periods, worst-case execution times

(WCETs), and deadlines

Scheduling policy Give higher priorities to tasks with shorter periods Preemptive static priority scheduling

Optimality If a feasible static priority assignment exists for some

process set, the RM priority assignment is feasible for that process set

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RM (Rate Monotonic)

Consider the following tasks Process X : period = 20, WCET = 10, deadline = 20 Process Y : period = 30, WCET = 8, deadline = 30 Process Z : period = 40, WCET = 4, deadline = 40

Schedulability test CPU utilization: A set of m processes is schedulable if

• For large m,

10 20 30 40 50 60 70 80 900

1/(2 1)mU m 1

/m

i ii

U e p

1/(2 1) ln 2 0.69mm

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EDF (Earliest Deadline First)

Scheduling policy Give higher priorities to tasks with earlier absolute

deadlines Preemptive dynamic priority scheduling

Optimality If a feasible dynamic priority assignment exists for some

process set, the EDF priority assignment is feasible for that process set

Schedulability test A set of m processes is schedulable if and only if 1U

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EDF (Earliest Deadline First)

Consider the following tasks Process X : period = 20, WCET = 10, deadline = 20 Process Y : period = 30, WCET = 10, deadline = 30 Process Z : period = 40, WCET = 5, deadline = 40

10 20 30 40 50 60 70 80 900

10 20 30 40 50 60 70 80 900

deadline miss of Task Z

EDF schedule

RM schedule

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Non-schedulable Behavior

Consider the following tasks Process X : period = 20, WCET = 10, deadline = 20 Process Y : period = 30, WCET = 8, deadline = 30 Process Z : period = 40, WCET = 15, deadline = 40

RM schedule10 20 30 40 50 60 70 80 900

10 8 10 8 10 10 108

deadlinemiss

deadlinemiss

10 20 30 40 50 60 70 80 900

deadlinemiss

EDF schedule

10 8 15 10 8 10 15 10 8

deadlinemiss

deadlinemiss

deadlinemiss

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Proportional Share Scheduling

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Proportional Share Scheduling

Basic concept A weight value is associated with each process The CPU is allocated to the process in proportion to its

weight

Two contexts Fair queueing (in the context of communication networks)

• Packet scheduling Proportional share (in the context of operating systems)

• Process scheduling

time

Task A (weight 25.0%)

Task B (weight 12.5%)Task C (weight 50.0%)Task D (weight 12.5%)

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Scheduling Algorithms

Network scheduling PGPS (= WFQ), Demers et al., 1989 Virtual Clock, Lixia Zhang, 1990 SCFQ, Golestani, 1994 SFQ, Goyal et al., 1996 WF2Q, Bennett et al., 1996

CPU scheduling Lottery and Stride, Waldspurger, 1995 Hierarchical SFQ, Goyal et al., 1996 BVT, Duda et al., 1999 VTRR, Nieh et al., 2001

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GPS (Generalized Processor Sharing) A GPS server is defined by (Kleinrock, 1976)

: the amount of session traffic served in an interval

: weight of session

Packets of all sessions are served simultaneously Idealized fluid-flow system or bit-by-bit weighted round-

robin

),( 21 ttWi ],( 21 tt

Njrr

ttWttW

j

i

j

i ...,,2,1,),(),(

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ir i

i

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PGPS (Packet-by-Packet GPS)

PGPS is an approximation to GPS Fair queueing on a packet-by-packet basis Also known as WFQ (Weighted Fair Queueing)

Algorithm : the time at which packet will depart (finish

service) under GPS Serve packets in increasing order of

pF p

pF

GPS PGPS

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Fairness Bounds of PGPS

Bound on lag

• : the time at which packet departs under PGPS • : the time at which packet will depart under GPS • : the maximum packet length• If and , then the lag is 1 ms

Bound on difference of services received

rLFF pp

maxˆ

pF̂pF

Gbpsr 1 KbL 1max

max),0(ˆ),0( LWW ii

pp

maxL

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Worst-case Fair WFQ

WF2Q only considers Packets that have started receiving service under GPS

GPS(fluid-flow)

WFQ

WF2Q

WFQ satisfies only

notmax2121 ),(ˆ),( LttWttW pp

max2121 |),(ˆ),(| LttWttW pp

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Lottery and Stride Scheduling

Tickets: relative resource rights Task has tickets A total of tickets

Lottery scheduling: probabilistic algorithm Use random number generator to select a winning ticket Task is probabilistically guaranteed a rate of

Stride scheduling: deterministic algorithm “Stride” is inversely proportional to tickets Task with minimum “pass” value is selected and its pass

is advanced by its stride

imiM

i /ip m M

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Stride Scheduling

Task : tickets = 3, stride = 2 Task : tickets = 2, stride = 3 Task : tickets = 1, stride = 6

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1pass

time 0 1 2 3 4 5 6 7 8 9 10 11 12 13

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‘s pass value

2 ‘s pass value

3 ‘s pass value

Ties are broken arbitrarily1. Smallest value is chosen2. Pass is advanced by stride 3

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