CS204:Scheduling

Jiasi ChenLectures:MWF12:10-1pminWCH139

http://www.cs.ucr.edu/~jiasi/teaching/cs204_spring16/

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Overview

• Whatisscheduling?• Roundrobin• Weighted roundrobin

• Generalizedprocessorsharing(GPS)• ImplementationsofGPS

• Deficitroundrobin• Weighted fairqueuing

• RatecontrolwithGPS:Leakybucket• Paperdiscussion

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Q:Howshouldacommonresourcebesharedbetweenmultipleusers?

Whatisscheduling?

• Sendpacketsfrommultiplehosts• Requiredfeatures

• Fair• Easytoimplement• Scalable• Work-conserving

• Desirablefeatures• Admission control• QoS fordifferenttypesoftraffic

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H1

H2

H3

Whose datagetstransmitted?

WhatisNOTscheduling?

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H1

H2

H3

centralizedscheduling

random access

Uplink?Downlink?

Toyexample

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Sender A:5-bit packets

Sender C:2-bit packets

Sender B:7-bit packets

Totalbandwidth =12bits/s

Fairness

• Howmuchofthelinkdoeseachuserget?• Onemeasureoffairness:max-minfair

• Maximizetheminimum rateacrossallusers

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Sender A:5-bit packets

Sender C:2-bit packets

Sender B:7-bit packets

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5

2

4

6

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Max-min fair NOTmax-minfair

Roundrobin

• Choose1packetfromeachsubqueue

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Afterone round570

Aftertworounds1052

Weightedroundrobin

• Choosecertain#packetsfromeachsubqueue• #ofpackets sent=weight/packetsize

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5

5

2

Weights5/5=1à 7

5/7à 5

2/2=1à 7

#packets sent7packets *5bits/packet=35bits

5packets *7bits/packet=35bits

7packets *2bits/packet=14bits

Numberofbitssent

Overview

• Whatisscheduling?• Roundrobin• Weighted roundrobin

• Generalizedprocessorsharing(GPS)• ImplementationsofGPS

• Deficitroundrobin• Weighted fairqueuing

• RatecontrolwithGPS:Leakybucket• Paperdiscussion

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Q:Howshouldacommonresourcebesharedbetweenmultipleusers?

Bit-by-bitRoundRobin

• Notimplementable

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Afterone round552

Aftertworounds10104

Generalizedprocessorsharing

• Sendinfinitesimalamountofbitseachround• Idealizedversionofbit-by-bitroundrobin• Notimplementable,butachievesperfect fairness

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2/12

5/125/12

Howtoimplement this?

Overview

• Whatisscheduling?• FIFO• Roundrobin• Weighted roundrobin

• Generalizedprocessorsharing(GPS)• ImplementationsofGPS

• Deficitroundrobin• Weighted fairqueuing

• RatecontrolwithGPS:Leakybucket• Paperdiscussion

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Q:Howshouldacommonresourcebesharedbetweenmultipleusers?

• SeewhichpacketswouldfinishfirstunderGPS

• Sendpackets inthatorder

Weightedfairqueuing

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timeP1, P1P1P2 P2,P2P3,P3

Afterone round552

Aftertworounds10104

O(log(n)) complexityNeed tosortthelistof finishing times

• Problem:whatifnewuserentersthesystem?

Weightedfairqueuing(2)

14time

P1, P1P1P2P1,P2

P1 P1, P1P2

time

P2,P2P3,P3

P3P2,

Finishing timesaredifferent,need tobeupdated!

• Solution:use“virtualtime”=#ofbit-by-bitroundrobinrounds

Weightedfairqueuing(3)

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P1 =5P1 =2P1=7P2=10P2 =6

P1 P1, P1P2

time

P2,P2P3,P3

GPS:

virtual time1

Bit-by-bit RR:

2345678910

• Let’scalculatethevirtualfinishingtimewiththenewuser

Weightedfairqueuing(4)

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virtual time

P1 =5P1 =2P1=7P2=10P2 =6

P1 P1, P1time

P2P3 P1,P2P3 P2,

GPS:

Bit-by-bit RR:

1234567891011

Weightedfairqueuing(5)

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P1 =5P1 =2P1=7P2=10P2 =6

• Evenifnewuserjoins,the“virtualtime”staysthesame

virtual time1110987654321

Bit-by-bit RR:

virtual time10987654321

Bit-by-bit RR: P1 =5P1 =2P1=7P2=10P2 =6

Thevirtualfinishing timesstayedthesame!

Note:BBRRvirtualtimeisshownhere forease ofexposition;WFQvirtualtimeisslightlydifferent.See Parekh-Gallager fordetails.

Deficitroundrobin

• Thatseemsconfusing…isthereasimplerway?

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Q =5

Q =5

Q =2

• DefineaquantumQforeachsubqueue• DefineacounterCforeachsubqueue

• Howmanybitsallowedtobesenteachturn

Deficitroundrobin

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Afterone round1002

Aftertworounds1374

Q =5

Q =5

Q =2

O(1), butless accurate

D =Q =5SendD =0

D =Q =5CannotsendD =5

D =Q =2SendD =0

D =Q= 5SendD =0

D =D +Q =10SendD =3

D =Q =2SendD =0

D =Q =5SendD =0

Overview

• Whatisscheduling?• FIFO• Roundrobin• Weighted roundrobin

• Generalizedprocessorsharing(GPS)• ImplementationsofGPS

• Deficitroundrobin• Weighted fairqueuing

• RatecontrolwithGPS:Leakybucket• Paperdiscussion

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Q:Howshouldacommonresourcebesharedbetweenmultipleusers?

PerformanceguaranteeswithGPS

• GPSimplementationsgiveusanaveragethroughput

• Whataboutdelayguarantee?

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LeakyBucket

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Tokens arrivewithrateak

Bucketcanhold Bk tokens

Packetscanonly leaveifaccompanied by atoken

Ifthebuffer isfull, howmanypackets canleaveduring timeinterval T?

GPS+LeakyBucket

• Maxdelayexperiencedbyabit:

• Why?1. AtmostBk packetswaiting inGPS

scheduler2. Delay=(Bk pkts)/(ρk pkts/s)

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GPS scheduler

Kleaky buckets

Sources

• AnIntroduction toComputer Networks,PeterDordalhttp://intronetworks.cs.luc.edu/current/html/queuing.html

• A.ParekhandR.Gallager,“AGeneralizedProcessorSharingApproachtoFlowControlinIntegratedServicesNetworks:TheSingle-NodeCase”,ToN 1993.

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