Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Traversal time for weakly synchronized CAN bus

Hugo Daigmorte, Marc Boyer

ONERA – The French aerospace lab

24th International Conference on Real-Time Networks and SystemsRTNS’2016

19th October 2016C

onsi

st

ent *

Complete

* Well d

ocu

mented * Easy to

reuse

* * E

valuate

d *

RTN

S *

Artifact

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 1 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Context and goalGlobal clockLocal clockBounded phases

Table of Contents

1 Context and goalContext and goalGlobal clockLocal clockBounded phases

2 Computing an upper bound on network delay

3 Experimental results

4 Conclusion

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 2 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Context and goalGlobal clockLocal clockBounded phases

Context

Context

Real-time networked system

Bus network: CAN

Periodic flows with Offsets

reduces contentions ⇒ reduces delaysrequires synchronization

CAN Node n°N

CAN Node n°2

CAN Node n°1

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 3 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Context and goalGlobal clockLocal clockBounded phases

Model and Goal

Model

Flows Fi : Pi ,Si ,Oi

N nodes, each node j has a clock: cj (t)

Sending frame k: cj (t) = Oi + kPi

Goal

Accurate bound on network traversal time

aka Worst Case Traversal Time (WCTT)

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 4 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Context and goalGlobal clockLocal clockBounded phases

Global clock

∀j , j ′ : cj (t) = cj ′(t)

Advantage: efficient schedule ⇒ no contention

Drawback: perfect synchronization (HW/SW cost)

A,1 B,1 A,2

C,3C,1 C,2

A,1 B,1 A,2C,1 C,2 C,3BUS

N1

N2

BUS

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 5 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Context and goalGlobal clockLocal clockBounded phases

Local clock

Advantage:

efficient schedule ⇒ no contention intra-nodes

efficient schedule ⇒ workload spread over time

N1

N2

BUSBUS

A,1 B,1 A,2

C,3C,1 C,2

A,1 B,1 A,2C,1 C,2 C,3

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 6 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Context and goalGlobal clockLocal clockBounded phases

Bounded phases

∀j , j ′ : cj (t)− cj ′(t) ≤ Φj ,j ′

Objectives:

affordable synchronization

reduces delays wrt no sync/local clock

N1

N2

BUSBUS

A,1 B,1 A,2

C,3C,1 C,2

A,1 B,1 A,2C,1 C,2 C,3

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 7 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

Table of Contents

1 Context and goal

2 Computing an upper bound on network delayNetwork CalculusMethodology

3 Experimental results

4 Conclusion

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 8 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

Reality modeling

Network Calculus is a theory designed to compute memory anddelay bounds in networks.

A αA+A′

Flow : Cumulative curve AA(t) : amount of data sent up to time tProperties: null at 0 (and before), non decreasing

Server: simple arrival/departure relation:Property: departure produced after arrival:

AS−→ D =⇒ A ≥ D

Worst delay:d(A,S) = sup

t∈R+{inf{d ∈ R+|A(t) ≤ D(t + d)}}

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 9 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

Arrival curve and services

Real behaviors are unknown at design time ⇒ use of contracts

Traffic contract: arrival curve

A flow A has arrival curve α iff:

∀t, d ∈ R+ : A(t + d)− A(t) ≤ α(d)

Server contract: service curve

For t, s in the same busy/backlogged period, a server S offers astrict minimal service of curve β iff:

D(t)− D(s) ≥ β(t − s)

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 10 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

A common period

Problem transformation

Initial problem: Flow : Fi =< Ti ,Oi , Si >, F = {F1, ..,Fn}1 sourceperiodic messagesSize, Period, OffsetPriority

Transformed problem: A = {A1, ..,Am}Period T = ppcm(Ti )Offset Oj

Size Sj

Donner un exemple (avec figure)

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 11 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

A common period

Problem transformation

Initial problem: Flow : Fi =< Ti ,Oi , Si >, F = {F1, ..,Fn}1 sourceperiodic messagesSize, Period, OffsetPriority

Transformed problem: A = {A1, ..,Am}Period T = ppcm(Ti )Offset Oj

Size Sj

Donner un exemple (avec figure)

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 11 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

A common period

Problem transformation

Initial problem: Flow : Fi =< Ti ,Oi , Si >, F = {F1, ..,Fn}1 sourceperiodic messagesSize, Period, OffsetPriority

Transformed problem: A = {A1, ..,Am}Period T = ppcm(Ti )Offset Oj

Size Sj

Donner un exemple (avec figure)

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 11 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

Arrival curve

Arrival curve α1..k (Theorem 5)

Capture the synchronization

Efficient algorithm

Required common period

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 12 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

AO

P α

A′

O ′P α

A + A′2α αA+A′

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 13 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

State of the art: hDev(Ai ,Di ) ≤ hDev(αi , β −∑j<iαj − L)

1 Method 1: hDev(Ai ,Di ) ≤ hDev(αi , β − α1..i−1 − L)

Taking into account synchronization between flows A1..Ai−1

αi

β − α1..i−1 − LhDev(αi , βi − α1..i−1)

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 14 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

2 Method 2: hDev(Ai ,Di ) ≤ (β − α1..i − L)−1(0)

Bound busy period for high priority flows (1..i)Pessimistic if several messages of the same flow are in thesame busy period

( - β α1.. i - L)-1(0)

i,1

i,2

i,1 i,2

8,6cm

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 15 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Network CalculusMethodology

3 Method 3 (Theorem 3): hDev(Ai ,Di ) ≤ hDev(Ai ,Di)

Requires good knowledge of Ai

Ai

DiDi

hDev(Ai ,Di )

hDev(Ai ,Di)

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 16 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Phases bounded by 0msPhases bounded by ±1msPhases bounded by ±5msPhases bounded by ±10ms

Table of Contents

1 Context and goal

2 Computing an upper bound on network delay

3 Experimental resultsPhases bounded by 0msPhases bounded by ±1msPhases bounded by ±5msPhases bounded by ±10ms

4 Conclusion

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 17 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Phases bounded by 0msPhases bounded by ±1msPhases bounded by ±5msPhases bounded by ±10ms

Configuration under study

250 kbit/s

10 nodes

62 flows

Load: 35%

Period: {20,50,100,200,500,1000}Payload: 1-8 bytes

0 1 2 3 4 5 6 7 8 9Frame payload (bytes)

101

102

103

Perio

d(m

s)

1

1

1

1

2

3

2

1

3

1

2

1

2

2

1

1

1

1

2

3

1

3

2

1

1

1

4

2

3

2

1

3

1

1

3

1

Method Synchronization WCTT

Method 1 Phases BoundMethod 2 Phases BoundMethod 3 Phases BoundMethod 4 Local clocks ExactMethod 5 Global clock BoundMethod 6 No offsets Exact

Phases : cj (t)− c ′j (t) ≤ Φ, Local clocks : Φ =∞,

Global clocks : Φ = 0, No offset : Oi = 0/unknown

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 18 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Phases bounded by 0msPhases bounded by ±1msPhases bounded by ±5msPhases bounded by ±10ms

0 10 20 30 40 50 60Frame by decreasing priority

0

2

4

6

8

10

12

14R

espo

nse

times

inm

s

Local clocks: Method 4Global clock: Method 5No offsets: Method 6

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 19 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Phases bounded by 0msPhases bounded by ±1msPhases bounded by ±5msPhases bounded by ±10ms

0 10 20 30 40 50 60Frame by decreasing priority

0

2

4

6

8

10

12

14R

espo

nse

times

inm

s

Phases: Method 1Phases: Method 2Phases: Method 3Local clocks: Method 4Global clock: Method 5No offsets: Method 6

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 20 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Phases bounded by 0msPhases bounded by ±1msPhases bounded by ±5msPhases bounded by ±10ms

0 10 20 30 40 50 60Frame by decreasing priority

0

2

4

6

8

10

12

14R

espo

nse

times

inm

s

Phases: Method 1Phases: Method 2Phases: Method 3Local clocks: Method 4No offsets: Method 6

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 21 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Phases bounded by 0msPhases bounded by ±1msPhases bounded by ±5msPhases bounded by ±10ms

0 10 20 30 40 50 60Frame by decreasing priority

0

2

4

6

8

10

12

14R

espo

nse

times

inm

s

Phases: Method 1Phases: Method 2Phases: Method 3Local clocks: Method 4No offsets: Method 6

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 22 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Phases bounded by 0msPhases bounded by ±1msPhases bounded by ±5msPhases bounded by ±10ms

0 10 20 30 40 50 60Frame by decreasing priority

0

2

4

6

8

10

12

14R

espo

nse

times

inm

s

Phases: Method 1Phases: Method 2Phases: Method 3Local clocks: Method 4No offsets: Method 6

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 23 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Offsets pro/cons

Pro: reduced contention and delaysCons: global clock has HW/SW cost

Is there a benefit even with a weak inter-nodessynchronization ?

Great results

Phases 10% minimal period ⇒ Gains 75%Phases 50% minimal period ⇒ Gains 45%

Further work

Model clock drift and re-synchronizationEnhance the analysisCompare analytic results to simulation resultsCreate a dedicated offset algorithm

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 24 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Offsets pro/cons

Pro: reduced contention and delaysCons: global clock has HW/SW cost

Is there a benefit even with a weak inter-nodessynchronization ?

Great results

Phases 10% minimal period ⇒ Gains 75%Phases 50% minimal period ⇒ Gains 45%

Further work

Model clock drift and re-synchronizationEnhance the analysisCompare analytic results to simulation resultsCreate a dedicated offset algorithm

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 24 / 24

Context and goalComputing an upper bound on network delay

Experimental resultsConclusion

Offsets pro/cons

Pro: reduced contention and delaysCons: global clock has HW/SW cost

Is there a benefit even with a weak inter-nodessynchronization ?

Great results

Phases 10% minimal period ⇒ Gains 75%Phases 50% minimal period ⇒ Gains 45%

Further work

Model clock drift and re-synchronizationEnhance the analysisCompare analytic results to simulation resultsCreate a dedicated offset algorithm

H. Daigmorte, M. Boyer Traversal time for weakly synchronized CAN bus 24 / 24