0414 deficit round robin scheduler

7/31/2019 0414 Deficit Round Robin Scheduler

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Deficit Round Robin

Scheduler


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Outline

Introduction

Ordinary Problems

Deficit Round Robin Latency of DRR

Improvement of latencies


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Introduction

Implementation complexity is low

Bandwidth guarantees are still robust

Appealing whenever the flows to bescheduled have no stringent delayrequirements.


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Ordinary Problems

Not fair, if node A,B,C,D does notdiscriminate flows, and so give serveto source flows by half

F1 gets 1/8

F2 gets 1/8

F3 gets 1/4


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Deficit Round Robin

1. Add quantum size to Deficit counterof corresponding flow

2.

Process the first job of flow andminus job count from counter

3. Serve other flows if current counter

is not enough for next job4. Set counter to zero if job is all done

in a flow

5. Maintain a link list of active flows to


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Deficit Round Robin


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Deficit Round Robin


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Deficit Round Robin

Round Robin Listhead

Flow 1

Flow 2

Flow 3

Flow 4


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Deficit Round Robin


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Demers-Keshav-Shenkerbound

Let packet p of flow i arrive with s bits,and other flows have no tasks

Uses bit-by-bit round robin Assume there is no more than n active

flows in any time

Suppose B is bandwidth of output line(bit/s)

The latency bound is about n*s/B

n:number of flows, s=packet size,


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Latency of DRR

Reducing quantum size does not helpimprove the worst-case latency

Solution:

use separate

fair queuingalgorithm


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Define two classes of flows:

latency critical

best effort Latency critical must contract send

no more than x bytes in some period

T If a latency critical flow f meet the

contract, put the flow in the head of

round-robin list


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Define a state bit for flows, and atimer with countdown time T

Set when a packet is arrived Cleared if the timer expires

Reset timer to T if a packet arrives

If a packet arrive and the bit isalready set, put the flow to the endof the round-robin list (move to best-

effort class)


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Delayed by

n is number of latency critical flows

MAX is max bit length of package inlist

B is speed of output line

s is size of current package

Only delayed by other latency critical


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0414 deficit round robin scheduler

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