QoS-aware Scientific Application Scheduling Algorithm in Cloud Environment
Scheduling for QoS Management
description
Transcript of Scheduling for QoS Management
Scheduling for QoS Management
Engineering Internet QoS 2
Outline
What is Queue Management and Scheduling?
Goals of schedulingFairness (Conservation Law/Max-min fair
share)Various scheduling techniquesResearch directions in scheduling
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What is scheduling?
Packets from multiple flows compete for same outgoing link.
Which packets should be given preference?
How many packets should be transmitted from a flow?
Simple solution: First come best servedComplex solution: Provide QoS
guarantees.
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Scheduling Goals
Sharing bandwidthFairness to competing flowsMeeting bandwidth guarantees (max and
min)Meeting loss guarantees (multiple level)Meeting delay guarantees (multiple level)Reducing delay variations
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Conservation law
Sum of the mean queuing delays received by the set of multiplexed connections, weighted by their share of link’s load is independent of the scheduling discipline – Kleinrock
Constii
iii
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i
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Conservation Law Contd
flowsofnumber
scheduleratiflowoftimewaitmeani
iflowfrompacketsoftimeservicemeani
iflowofratearrivalmeani
iflowofnutilizatiomeani
N
q
x
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Max-min fair share
Allocates the smallest of all demands from all flows
Distribute remaining resources equally competing of the flows
Guarantees fairness
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Scheduling Disciplines
First come first serve (FCFS)Priority (PQ)Round Robin (RR)/Weighed round robinDeficit round robin (DRR)Weighted fair queuing (WFQ)Class based queuing (CBQ)
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First Come First Serve
Packets enqueued into a common bufferServer serves packet from front of queueNo fair sharing of bandwidthNo flow isolationNo priority or QoS guarantee
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FCFS example
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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Priority Queuing
Multiple queues with priority 0 to n-1Priority 0 served firstPriority i served only if 0 to i-1 emptyHighest priority – lowest delay/loss,
highest bandwidthPossible starvation of lower class
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Priority Queue example
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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Generalized processor sharing
Ideal work conserving schemeFlows kept in separate queueServe infinitesimal amount of data from
each queueServe all active queues in finite timeWeight can be associated with each queueAchieves max-min fair share
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GPS Continued
In GPS terminology, a connection is called backlogged when it has data present in queue.
Lets assume that there are K flows to be served by a server implementing GPS with weights w(1), .. w(k) Service rate of ith flow in interval [τ, t] is represented as R(i, τ,t). For any backlogged flow i in interval [τ,t] and for another flow j, the following equation holds: )(/)(),,(/),,( jwiwtjRtiR
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Round Robin
Flows kept in separate queueServe one packet from each active queueFair share but no bandwidth guaranteeWhat if packet size variable
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Weighted Round Robin
Allows variable length packetServes n packet from a queuen adjusted to specific fraction of link shareFairness problem at small time scaleNeeds to know packet size a priori Assume 3 ATM sources (small cell size) with
weights 0.75, 1.0 and 1.5. If these weights are normalised to integer values, each source will be served 3, 4 and 6 cells in each round.
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Deficit Round Robin
No need to know packet size a prioriInitially serves each queue quantum worth of
bitsIf packet less than or equal to quantum, serve
itElse increment deficit_counter by quantumIf no more outstanding packet, reset
deficit_counter (Why?)Set quantum to minimum MTU of all incoming
linksFairness problem at smaller time scale
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DRR Example 1
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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DRR Example 2
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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DRR Example 3
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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DRR Example 4
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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Weighted Fair Queuing
Packets tagged with a value identifying the time last bit of packet should be transmitted using GPS simulation
Packet with lowest tag value transmitted by scheduler
Uses complex finish time calculationHard to implement with variable packet
sizeQoS guarantees possible (gets bandwidth
in proportion of weight) )(/)( jwiRwThroughputMin
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WFQ Delay bounds
Delay can be bounded if flows can be policed (token bucket)
Flows regulated by token bucket are put in different queues
Each queue has assigned weight With token bucket policing, assume that intially
the token bucket is full and a brust of bi packets arrive for a flow of class i. Last packet to complete service will suffer a maximum delay of dmax given by equation ))(/)(/(max jwiRwbd i
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WFQ Delay with Token bucket
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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Finish Time Calculation
Following equation shows the finish time calcuation where R(t) is called round number. Pc
m is the time required to transmit mth packet from cth connection and w(c) is the weight of connection c.
)(/))(,max( 1)( cwPtRFF cm
cmm
c
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Round Number
This is the number a bit-by-bit round robing scheduler (in place of GPS’s non-implementable infinitesimal data) has completed at a given time. The round number is a variable that depends on number of active queues to be served (inversely proportional to the active queue number). The more queues to serve, the longer a round will take to complete (example and figure in section 3.2.7 of text)
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Virtual Clock
Also known as Fair QueuingWFQ finish time calculation is very
complexVirtual clock replaces round time with real
time as per the following equation (here, Am is the real-arrival time of packet m):
cmm
cmm
c PAFF ),max( 1)(
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VC Example
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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Class Based Queuing
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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CBQ Contd
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
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Scheduling Research Directions
Worst-case fair weighted Fair queuing (WF2Q)
Self clocked fair queuing (SCFQ)Start time fair queuing (SFQ)Core state fair queuing (CSFQ)Score others