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October 3, 2005 CIS 700 1
Compositional Real-Time Scheduling Framework
Insik Shin
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October 3, 2005 CIS 700 2
Outline• Compositional scheduling framework
– Scheduling component model
– Periodic resource model • Schedulability analysis• Utilization bound• Component timing abstraction
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October 3, 2005 CIS 700 3
Traditional Scheduling Framework• Single real-time task in a single application
CPU
OS Scheduler
Task Task Task
Application Application Application
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Hierarchical Scheduling Framework (HFS)
October 3, 2005 CIS 700 4
• Multiple real-time tasks with a scheduler in a single application, forming a hierarchy of scheduling
CPU
OS Scheduler
ApplicationScheduler
Task Task
ApplicationScheduler
Task Task
ApplicationScheduler
Task Task
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October 3, 2005 CIS 700 5
Compositional Scheduling Framework
CPU
OS Scheduler
Java Virtual Machine
J1(50,3) J2(75,5)
VM Scheduler
Multimedia
T2(33,10)
Digital Controller
T1(25,5)
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October 3, 2005 CIS 700 6
VM Scheduler’s Viewpoint
CPU
OS Scheduler
Multimedia
T2(33,10)
Digital Controller
T1(25,5)
Java Virtual Machine
J1(50,3) J2(75,5)
VM Scheduler
CPU Share
Real-Time Guarantee on CPU Supply
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October 3, 2005 CIS 700 7
Problems & Approach I
• Resource supply modeling– Characterize temporal property of resource allocations
• we propose a periodic resource model
– Analyze schedulability with a new resource model
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October 3, 2005 CIS 700 8
OS Scheduler’s Viewpoint
CPU
OS Scheduler
Java Virtual Machine
J1(50,3) J2(75,5)
VM Scheduler
Multimedia
T2(33,10)
Digital Controller
T1(25,5) Real-Time Task
Real-Time Demand
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October 3, 2005 CIS 700 9
Problems & Approach II
• Real-time demand composition– Combine multiple real-time requirements into a single
real-time requirement
Real-Time Constraint
Real-Time Constraint
Real-Time Constraint
EDF / RM
T1 (p1, e1) T2 (p2, e2) T (p, e)
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Compositional Real-time Scheduling Framework
October 3, 2005 CIS 700 10
• Goal – to support compositionality
for timeliness aspect– to achieve system-level
schedulability analysis usingthe results of component-levelschedulability analysis
• Scheduling component modeling
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October 3, 2005 CIS 700 11
Scheduling Component Modeling• Scheduling
– assigns resources to workloads by scheduling algorithms
• Scheduling Component Model : C(W,R,A)– W : workload model– R : resource model– A : scheduling algorithm
Resource
Scheduler
WorkloadPeriodic Task WorkloadPeriodic Task
EDF / RM
???
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October 3, 2005 CIS 700 12
Resource Modeling• Dedicated resource : always available at full capacity
• Shared resource : not a dedicated resource– Time-sharing : available at some times
– Non-time-sharing : available at fractional capacity
0 time
0 time
0 time
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October 3, 2005 CIS 700 13
Resource Modeling• Time-sharing resources
– Bounded-delay resource model [Mok et al., ’01]characterizes a time-sharing resource w.r.t. a non-time-sharing resource
– Periodic resource model Γ(Π,Θ) [Shin & Lee, RTSS ’03]characterizes periodic resource allocations
0 1 2 3 4 5 6 7 8 9 time
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October 3, 2005 CIS 700 14
Schedulability Analysis• A workload set is schedulable under a scheduling algorithm
with available resources if its real-time requirements are satisfiable
• Schedulability analysis determines whether
resource demand,which a workload set
requires under a scheduling algorithm
resource supply,which available
resources provide≤
scheduler
resourceworkload workload
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October 3, 2005 CIS 700 15
Resource Demand Bound• Resource demand bound during an interval of length t
– dbf(W,A,t) computes the maximum possibleresource demand that W requires under algorithm A during a time interval of length t
• Periodic task model T(p,e) [Liu & Layland, ’73]– i.e., T(3,2)
0 1 2 3 4 5 6 7 8 9 10
tdem
and
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October 3, 2005 CIS 700 16
Demand Bound Function - EDF• For a periodic workload set W = {Ti(pi,ei)},
– dbf (W,A,t) for EDF algorithm [Baruah et al.,‘90]
– Example: W = {T1(3,2), T2(4,1)}
iWT i
ept
i
⋅⎥⎦
⎥⎢⎣
⎢= ∑
∈
t)EDF,(W, dbf
0 1 2 3 4 5 6 7 8 9 10
tdem
and
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October 3, 2005 CIS 700 17
Resource Supply Bound• Resource supply during an interval of length t
– sbfR(t) : the minimum possible resource supply by resource R over all intervals of length t
• For a single periodic resource model, i.e., Γ(3,2)– we can identify the worst-case resource allocation
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October 3, 2005 CIS 700 18
Resource Supply Bound• supply =
• supply =
• sbfR(i) = 1
3
0 time
0 time
1
R(3,2)
i
R(3,2)
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October 3, 2005 CIS 700 19
Resource Supply Bound• Resource supply during an interval of length t
– sbfR(t) : the minimum possible resource supply by resource R over all intervals of length t
• For a single periodic resource model, i.e., Γ(3,2)– we can identify the worst-case resource allocation
0 1 2 3 4 5 6 7 8 9 10
tsup
ply
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October 3, 2005 CIS 700 20
Supply Bound Function• Resource supply during an interval of length t
– sbfΓ(t) : the minimum possible resource supply by resource R over all intervals of length t
• For a single periodic resource model Γ(Π,Θ)
[ ]otherwise
)1(,2)1( if)1(
))(1()t(sbf
Θ−Π+Θ−Π+∈
⎩⎨⎧
Θ−Θ−Π+−
=Γkkt
kkt
0 1 2 3 4 5 6 7 8 9 10
tsup
ply
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October 3, 2005 CIS 700 21
Schedulability Conditions (EDF)• A workload set W is schedulable over a resource
model R under EDF if and only if for all interval i of length t
dbfw(i) ≤ t [BHR90]
dbfw(i) ≤ sbfR(i)
– sbfR(i) : the minimum resource supply by resource Rduring an interval i
– dbfw(i) : the resource demand of workload W during an interval i
Resource demandin an interval
Resource supply during the interval
(from a dedicated resource)
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October 3, 2005 CIS 700 22
Schedulability Condition - EDF
024
68
101214
161820
1 4 7 10 13 16 19 22 25 28
supplydemand
• A periodic workload set W is schedulable under EDF over a periodic resource model Γ(Π,Θ)if and only if
)t(sbf t)EDF,dbf(W, 0t Γ≤>∀
time
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October 3, 2005 CIS 700 23
Schedulability Condition - RM• A periodic workload set W is schedulable under
EDF over a periodic resource model Γ(Π,Θ)if and only if
• For a periodic workload set W = {Ti(pi,ei)}, – dbf (W,A,t,i) for RM algorithm [Lehoczky et al., ‘89]
)t(sbf i) t,RM,dbf(W,W T 0t i Γ≤∈∀>∀
kTHPT k
i epte
ik
⋅⎥⎥
⎤⎢⎢
⎡+= ∑
∈ )( i) t,RM,(W, dbf
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October 3, 2005 CIS 700 24
Utilization Bounds
• Utilization bound (UB) of a resource model R– given a scheduling algorithm A and a resource
model R, UBR,A is a number s. t. a workload set W is schedulable if
A,RW UB U ≤
∑∈WT i
i
i pe
• For a periodic workload T(p,e), utilization UT = e/p• For a periodic workload set W, utilization UW is
scheduler
workload
resource
workload
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October 3, 2005 CIS 700 25
Utilization Bounds
• Example: – Consider a periodic resource Γ(Π,Θ), where Π =
10 and Θ = 4, and suppose UB Γ,EDF = 0.4. – Then, a set of periodic task W is schedulable if
– W = {T1(20,3), T2(50,5)} s.t. UW = 0.25, is schedulable
0.4 UW ≤
EDF
workload
Γ(Π=10,Θ=4)
workload
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October 3, 2005 CIS 700 26
Utilization Bound - EDF
• For a scheduling component C(W, Γ(Π,Θ),A), where A = EDF, its utilization bound is
• Pmin is the minimum task period (deadline) in W.• k represents the relationship between resource period Π
and the minimum task period Pmin, k ≈ Pmin /Π
)1(2)P(UB minEDF,
Γ
ΓΓ
−+×
=Uk
Uk
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October 3, 2005 CIS 700 27
EDF Utilization Bound - Intuition• Observation for a component C(W, Γ(Π,Θ),EDF)
– C is schedulable iff dbf (W,EDF,t) ≤ sbfΓ (t)
Pmint
sbfΓ (t)
dbf (W,EDF,t)
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October 3, 2005 CIS 700 28
EDF Utilization Bound - Intuition• Observation for a component C(W, Γ(Π,Θ),EDF)
– C is schedulable iff dbf (W,EDF,t) ≤ sbfΓ (t) – dbf (W,EDF,t) ≤ UW· t
UW· t
Pmint
sbfΓ (t)
dbf (W,EDF,t)
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October 3, 2005 CIS 700 29
EDF Utilization Bound - Intuition• Observation for a component C(W, Γ(Π,Θ),EDF)
– C is schedulable iff dbf (W,EDF,t) ≤ sbfΓ (t) – dbf (W,EDF,t) ≤ UW· t– UΓ(t-2(Π-Θ)) ≤ sbfΓ (t)
UW· t
UΓ(t-2(Π-Θ))
Pmint
sbfΓ (t)
dbf (W,EDF,t)
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October 3, 2005 CIS 700 30
EDF Utilization Bound - Intuition• Observation for a component C(W, Γ(Π,Θ),EDF)
– C is schedulable iff dbf (W,EDF,t) ≤ sbfΓ (t) – dbf (W,EDF,t) ≤ UW· t– UΓ(t-2(Π-Θ)) ≤ sbfΓ (t) – Therefore, C is schedulable if UW· t ≤ UΓ(t-2(Π-Θ))
UW· t
UΓ(t-2(Π-Θ))
Pmint
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October 3, 2005 CIS 700 31
EDF Utilization Bound - Intuition• For a component C(W, Γ(Π,Θ),EDF)
– for all t > Pmin, if UW· t ≤ UΓ(t-2(Π-Θ))then C is schedulable.
UW ≤ UΓ(t-2(Π-Θ)) / t
UW· t
UΓ(t-2(Π-Θ))
Pmint
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October 3, 2005 CIS 700 32
Utilization Bound - RM
• For a scheduling component C(W, Γ(Π,Θ),A), where A = RM, its utilization bound is – [Saewong, Rajkumar, Lehoczky, Klein, ’02]
– We generalize this earlier result, where k ≈ Pmin/ Π.
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛−⎟
⎠⎞
⎜⎝⎛
×−−
=Γ
ΓΓ 1
233)(UB
n1
RM,U
Unn
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛−⎟⎟
⎠
⎞⎜⎜⎝
⎛−+−+
×=Γ
ΓΓΓ 1
)1(2)1(22)P,(UB
n1
minRM,UkUknUn
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October 3, 2005 CIS 700 33
Component Abstraction• Component timing abstraction
– To specify the collective real-time demands of a component as a timing interface
Periodic (50,7)
EDF
Periodic (70,9) timing
interfacevirtual
real-time task
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October 3, 2005 CIS 700 34
Component Abstraction• Component timing abstraction
– To specify the collective real-time demands of a component as a timing interface
Periodic (50,7)
EDF
Periodic (70,9) timing
interface
periodic interfaceΓ(Π,Θ)
periodicresourceΓ(Π,Θ)
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October 3, 2005 CIS 700 35
Component Abstraction (Example)• In this example, a solution space of a periodic
resource Γ(Π,Θ) that makes C(W, Γ(Π,Θ),EDF) schedulable is
(a) Solution Space under EDF
0
0.2
0.4
0.6
0.8
1
1 10 19 28 37 46 55 64 73
resource period
reso
urce
cap
acity
Periodic (50,7)
EDF
Periodic (70,9)
Γ(Π,Θ)Γ(Π,Θ)
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October 3, 2005 CIS 700 36
Component Abstraction (Example)• An approach to pick one solution out of the solution space
– Given a range of Π, we can pick Γ(Π,Θ) such that UΓ is minimized. (for example, 28 ≤ Π ≤ 46)
(a) Solution Space under EDF
0
0.2
0.4
0.6
0.8
1
1 10 19 28 37 46 55 64 73
resource period
reso
urce
cap
acity
Periodic (50,7)
EDF
Periodic (70,9)
Γ(29,9.86)Γ(Π,Θ)
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October 3, 2005 CIS 700 37
Component Timing Abstraction• Component timing abstraction
– To abstract the collective real-time demands of a component as a timing interface
Periodic (50,7)
EDF
Periodic (70,9) periodic
interfaceΓ(29, 9.86)
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October 3, 2005 CIS 700 38
Compositional Real-Time Guarantees
T21(25,4)
T22(40,5)
R(?, ?)
EDF
RM
R2(?, ?)R2(10, 4.4)
T11(25,4)
T12(40,5)EDF
R1(?, ?)R1(10, 3.1)
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October 3, 2005 CIS 700 39
Compositional Real-Time Guarantees
T21(25,4)
T22(40,5)
R(?, ?)
EDF
RM
R(5, 4.4)
R2(10, 4.4)
T2(10, 4.4)
T11(25,4)
T12(40,5)EDF
R1(10, 3.1)
T1(10, 3.1)
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October 3, 2005 CIS 700 40
Abstraction Overhead
• For a scheduling component C(W, Γ(Π,Θ), A), its abstraction overhead (OΓ) is 1-
UU
W
Γ
Periodic (50,7)
EDF
Periodic (70,9) periodic
interfaceΓ(29, 9.86)
UW=0.27 UΓ=0.34
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October 3, 2005 CIS 700 41
Abstraction Overhead Bound
• For a scheduling component C(W, Γ(Π,Θ), A), its abstraction overhead (OΓ) is
– A = EDF
– A = RM
W
W
UkU×+−×
≤Γ2
)1(2O EDF,
( )( )
1
12122log
1O RM, −
⎟⎟⎠
⎞⎜⎜⎝
⎛−+−+
≤Γ
W
W
UkUk
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October 3, 2005 CIS 700 42
Abstraction Overhead• Simulation Results
– with periodic workloads and periodic resource under EDF/RM
– the number of tasks n : 2, 4, 8, 16, 32, 64– the workload utilization U(W) : 0.2~0.7– the resource period : represented by k
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October 3, 2005 CIS 700 43
Abstraction Overhead
00.05
0.10.15
0.20.25
0.30.35
2 4 8 16 32 64
The Number of Tasks
Analytical Bound - EDF Simulation Result - EDF
• k= 2, U(W) = 0.4
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October 3, 2005 CIS 700 44
Summary• Compositional real-time scheduling framework
- with the periodic model [Shin & Lee, RTSS ‘03]
1. resource modeling– utilization bounds (EDF/RM)
2. schedulability analysis• exact schedulability conditions (EDF/RM)
3. component timing abstraction and composition• overhead evaluation
– upper-bounds and simulation results
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October 3, 2005 CIS 700 45
Future Work• Extending our framework for handling
– Soft real-time workload models– non-periodic workload models– task dependency
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October 3, 2005 CIS 700 46
References
• Insik Shin & Insup Lee,“Periodic Resource Model for Compositional Real-Time Gaurantees”, the Best Paper of RTSS 2003.
• Insik Shin & Insup Lee,“Compositional Real-time Scheduling Framework”, RTSS 2004.
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October 3, 2005 CIS 700 47
THE
THE END
END
ENDTHE
THANK YOU
Compositional Real-Time Scheduling FrameworkOutlineTraditional Scheduling FrameworkHierarchical Scheduling Framework (HFS)Compositional Scheduling FrameworkVM Scheduler’s ViewpointProblems & Approach IOS Scheduler’s ViewpointProblems & Approach IICompositional Real-time Scheduling FrameworkScheduling Component ModelingResource ModelingResource ModelingSchedulability AnalysisResource Demand BoundDemand Bound Function - EDFResource Supply BoundResource Supply BoundResource Supply BoundSupply Bound FunctionSchedulability Conditions (EDF)Schedulability Condition - EDFSchedulability Condition - RMUtilization BoundsUtilization BoundsUtilization Bound - EDFEDF Utilization Bound - IntuitionEDF Utilization Bound - IntuitionEDF Utilization Bound - IntuitionEDF Utilization Bound - IntuitionEDF Utilization Bound - IntuitionUtilization Bound - RMComponent AbstractionComponent AbstractionComponent Abstraction (Example)Component Abstraction (Example)Component Timing AbstractionCompositional Real-Time GuaranteesCompositional Real-Time GuaranteesAbstraction OverheadAbstraction Overhead BoundAbstraction OverheadAbstraction OverheadSummaryFuture WorkReferences