1 a reservation scheduler for real-time operating systems david matschulat, césar marcon, fabiano...

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A Reservation Scheduler for Real-Time

Operating Systems

David Matschulat, César Marcon, Fabiano Hessel

PUCRS - Brazil

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Introduction – Quality of Service

Increase demand for embedded multimedia platforms (health devices, cellphones,…)

End-to-end QoS has become harder new requirements imposed by multimedia systems and new

codification techniques have emerged

QoS requirements has become essential Quality of Service (QoS)

Offer Guarantees• Bandwidth• Delay• Jitter• Deadline miss rate

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Introduction – QoS for Operating Systems

Mapping What?

Admission Test When?

Resource Reservation Protocol How?

In a end-to-end manner. Some applications demand end-to-end predictability. Each part of a Operating System can (or should) take part in

the end-to-end quality provision.

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Motivation

Flexibility and predictability are desirable for applications and can be accomplished through end-to-end QoS provision.

The Process Scheduler of an OS is a key player for QoS provision.

Current solutions do not offer hard real-time coexistence support with other task classes.

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Job and Task Models

Task is a set of Jobs

Each Job has a Release time, Execution time, Relative deadline and Absolute deadline

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R-EDF – Reservation Based EDF β = Best-effort Reservation Θ(J) = Job Utilization Θ(T) = Task Utilization: the mean utilization of all jobs. Ψ(T) = Peak Utilization: the utilization of its largest job. PCRT = Peak Capacity: sum of the peaks (Ψ) of all jobs of a task.

The system is overloaded when PCRT > 1 – β.

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R-EDF – Reservation Based EDF A task reserves Θ(T), its mean utilization. The overrun state protects the system. A job enters the overrun state when its reservation is reached. Admission control is limited by ΣΘ(T) <= 1.

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R-EDF – Reservation Based EDF

CPU Reservation: R-EDF has room for improvement: Better performance for soft real-time tasks. Support for hard real-time tasks.

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ER-EDF – Enhanced R-EDF

Based on R-EDF Includes:

Hard real-time support Improved performance for soft real-time tasks

Components: Admission Control Algorithm Scheduling Algorithm

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ER-EDF – Admission Control

β= 0.1 TA: θ=0.5 (1/2), Ψ=0.66 (2/3)

TB: θ=0.33 (1/3), Ψ=0.66 (2/3)

Task ER-EDF Authorized? Overloaded

-

CRT = 0

PCRT = 0

CTS = 1- No

A

CRT = 0.5

PCRT = 0.66

CTS = 0.5Yes No

A + B

CRT = 0.83

PCRT = 1.32

CTS = 0.17Yes Yes

A + BH

CRT = 1.16

PCRT = 1.32

CTS = -0.16No Yes

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ER-EDF – Scheduling

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System Architecture - SoC

SoC Xilinx Spartan-3 FPGA MIPS processor + EPOS Serial Display SRAM (1 MB) Internal Memory (8 KB)

InternalMemory

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Experiment - First

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Experiment - First

Deadline Miss Rate Comparison

0,00%

20,00%

40,00%

60,00%

80,00%

100,00%

120,00%

EDF R-EDF R-EDF hard ER-EDF ER-EDF hard

Task 1 Task 2 Task 3 Task 4

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Experiment - Second

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Experiment - Second

Deadline Miss Rate Comparison

0,0%

20,0%

40,0%

60,0%

80,0%

100,0%

EDF R-EDF ER-EDF

Task 1 Task 2

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Conclusions

Scheduling plays a important role in QoS provision. A new scheduler, ER-EDF was introduced.

Support for hard real-time tasks. Better performance for soft real-time tasks when compared to

its predecessor. Fallback to classic EDF when the system is underloaded.

The algorithm allows the developer to parameterize the OS to fulfill applications requirements, allowing flexibility and predictability.

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Thank You

Questions?