1 a reservation scheduler for real-time operating systems david matschulat, césar marcon, fabiano...
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1
A Reservation Scheduler for Real-Time
Operating Systems
David Matschulat, César Marcon, Fabiano Hessel
PUCRS - Brazil
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
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.
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.
Job and Task Models
Task is a set of Jobs
Each Job has a Release time, Execution time, Relative deadline and Absolute deadline
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.
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.
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.
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
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
System Architecture - SoC
SoC Xilinx Spartan-3 FPGA MIPS processor + EPOS Serial Display SRAM (1 MB) Internal Memory (8 KB)
InternalMemory
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
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
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.
18
Thank You
Questions?