The Design of an EDF- Scheduled Resource-Sharing Open Environment Nathan Fisher Wayne State...

The Design of an EDF-Scheduled Resource-Sharing Open EnvironmentNathan Fisher

Wayne State UniversityMarko Bertogna

Scuola Superiore Santa’Anna of Pisa

Sanjoy BaruahThe University of North Carolina at

Chapel Hill

Outline Background: Open Environments.

• Motivation.• Challenge.

Prior Work. Our Results:

• System Model.• Resource-Sharing Framework:• Formal Properties.

Summary & Future Work.

Motivation: Traditional Real-Time System Design

Application A:

1

2

3

Application B:

’1

’2

Real-Time Tasks

Background – Prior Work– Our Results– Summary


Application A:

1

2

3

Application B:

’1

’2

CPU Scheduler

Schedulability Test

Task

Specifications



Application A:

1

2

3

Application B:

’1

’2

CPU Scheduler

Each task submits jobs

directly to CPU scheduler.

Assumption: All tasks of all real-time applications are known at system-validation time.


Motivation: Traditional Real-Time System DesignDrawbacks:

1. All tasks in the system need to be validated together and known to system designer, a priori.

• Monolithic system design.

2. Each application on shared platform must use same scheduling algorithm.

3. Temporally-bad behavior of one task may affect other tasks.

Violation of System Design Principles:

• Encapsulation & Abstraction.

• Modularity & Hierarchical Design.

• Fault-containment.

Solution?Solution?


Virtual Processor BVirtual Processor B

LocalScheduler

Virtual Processor AVirtual Processor A

LocalScheduler

Motivation: Real-Time Open Environments

CPU Scheduler

Application Interface:• VP Speed.• Jitter tolerance.• …Application A:

1

2

3

Application B:

’1

’2

IA

IB

Global



CPU Scheduler

Application A:

1

2

3


Application B:

’1

’2


LocalScheduler

LocalScheduler

IA

IB

Application-LevelSchedulability Test

Global



CPU Scheduler

Application A:

1

2

3


Application B:

’1

’2


LocalScheduler

LocalScheduler

IA

IB Application-LevelSchedulability Test

Each application independently developed

& validated.

Global



CPU Scheduler

Application A:

1

2

3


Application B:

’1

’2


LocalScheduler

LocalScheduler

IA

IB

Composability Test

Global



CPU Scheduler

Application A:

1

2

3


Application B:

’1

’2


LocalScheduler

LocalScheduler

IA

IB

Global


Motivation: Real-Time Open EnvironmentsAdvantages:

1. Application’s temporal constraints may be validated independently and need not be known a priori.

• Component-based design.

• Service-oriented design.

2. Each application on shared platform may use different scheduling algorithm.

3. Virtual processors isolate temporally-bad behavior of an application.

Adherence to System Design Principles:

• Encapsulation & Abstraction.

• Modularity & Hierarchical Design.

• Fault-containment.


Challenge: Real-Time Open Environments

CPU Scheduler

Application A:

1

2

3

Application Server AApplication Server A

Application B:

’1

’2

Application Server BApplication Server B

LocalScheduler

LocalScheduler

IA

IB

Global RR11 RR22 RRmm……

Challenge: Tasks may access shared global resources.

Implication: Applications not completely independent.


Prior Work: Real-Time Open Environments “First Generation” Open Environments:

• Examples:• Resource Kernels [Rajkumar et al., MMCM 1998].

• Resource Partitions [Mok, Feng, & Chen, RTAS 2001].• Periodic Resource Model [Shin & Lee, RTSS 2003].

• …

• Periodic tasks.

• Not all consider shared resources.

[Deng & Liu, RTSS 1997]


Prior Work: Real-Time Open Environments “Second Generation” Open Environments:

• Recent Work:• Davis & Burns [RTSS 2006].

• Behnam et al. [RTSS-WiP 2006].• …

• Sporadic tasks.

• Non-preemptive sharing of global resources.

[Deng & Liu, RTSS 1997]

Drawback: May cause blocking among & within applications.Our Work: Allow for preemptive

sharing (when needed).


Our Results: System Model

Applications: A1, A2, …, Aq.

Global Resources: R1, R2, …, Rm.

Application Interface for Ak:

IA = (k, k, Hk()) k: virtual processor speed.

k: jitter tolerance.

• Hk(Rℓ): Ak’s resource-hold time of Rℓ.

Each application Ak comprised of sporadic tasks system (Ak).


kMaximum continuous

interval that Ak may lock Rℓ.

Our Results: Resource-Sharing Framework Bounded-delay Resource Open

Environment (BROE) Server:• Application virtual processor.

• Maintains 3 server variables for Ak:

• Ek: budget.

• Pk: replenishment period.

• Dk: server deadline.

• BROE Servers are scheduled by EDF plus Stack Resource Protocol (SRP) [Baker, 1991] w.r.t. server deadline and period.


Our Results: Resource-Sharing Framework

IA = (k, k, Hk()) BROE Server Rules:1. Initialize to “normal” replenishment values:

• Period:• Maximum Budget: • Deadline:

kFor all intervals of size t > k, execution over interval

should be at least (t- k ) k

k

Pk =k

2(1-k)Ek =

kk

2(1-k)Dk = + tcur

k

2(1-k)Task system (Ak) scheduled within server

allocation by Ak’s local scheduler.Earlier-deadline applications are executing.


Our Results: Resource-Sharing FrameworkBROE Server Rules:1. Initialize to “normal” replenishment values.

2. If server is executing, budget is decremented at rate 1.

Budgettime0

Ek =-1, if Ak executing,

0, otherwise.d

dt

IA = (k, k, Hk()) k


Our Results: Resource-Sharing FrameworkBROE Server Rules:1. Initialize to “normal” replenishment values.

2. If server is executing, budget is decremented at rate 1.

3. If task of (Ak) requests resource Rℓ when Ek < Hk(Rℓ ), then defer execution and update replenishment time & next deadline:

Task requests RTask requests Rℓℓ, ,

but Ebut Ekk < H < Hkk(R(Rℓℓ))

Access to RAccess to Rℓℓ is is

granted heregranted herekExecution over interval > (t- k)k

IA = (k, k, Hk()) k


Our Results: Formal Properties Composability Test:

Bk: largest Hi(Rℓ) value that can block Ak.

Theorem: Applications A1, A2, …, Aq composable on a unit-speed processor if for all k {1,…, q}:

j + 1Bk

PkPj Pk


Our Results: Formal Properties Composability Test:

Local-Scheduler “Optimality”:

Theorem: If Ak independently validated on processor of speed k and each job completes k prior to its deadline, then it will meet all deadlines on BROE server with interface IA (k, k, Hk()) when using EDF + SRP.

Theorem: Applications A1, A2, …, Aq composable on a unit-speed processor if for all k {1,…, q}:

j + 1Bk

PkPj Pk


k

Our Results: Resource-Hold Times How do you determine Hk(Rℓ)?

1. If (Ak) feasible when non-preemptively executing Rℓ on VP, execute non-preemptively on BROE server.

Hk(Rℓ) equals duration of longest critical section of (Ak) on Rℓ.


Our Results: Resource-Hold Times How do you determine Hk(Rℓ)?

1. If (Ak) feasible when non-preemptively executing Rℓ on VP, execute non-preemptively on BROE server.

2. If (Ak) infeasible on VP using EDF+SRP, then by “optimality” theorem, (Ak) cannot be scheduled on server of speed k.

3. If neither above hold: devise local scheduling technique for minimizing application resource hold time.

Previous paper [RTAS 2007] describes Hk(Rℓ) minimization for EDF+SRP.


Our Results: Blocking Reduction Intra-application preemption + SRP:

reduces blocking of “higher-priority” tasks. Deferring resource execution: prevents

blocking after budget exhausted. Minimization of resource-hold times:

reduces an application’s impact on other applications.


Summary & Future Work

Open Environments:• System design benefits.• Composability of independently-validated applications.

Challenge: Shared Resources. Our Contributions:

• “Clean” interface.• Simple composability test.• Resource-hold times (allowing preemption, if needed).

Future Work:• Interface selection.• General task models & different schedulers.• Multiprocessor platforms.

Thank You!

Questions?Questions?

[email protected]@cs.wayne.edu

The Design of an EDF- Scheduled Resource-Sharing Open Environment Nathan Fisher Wayne State...

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