Background Introduction

Android OS Architecture

Feasibility and Necessity

Android RTS ArchitectureLimitation

Improvement

Lei Cui

Evaluating Android OS for Embedded Real-

Time System

Background IntroductionAndroid platform has been instantly improved either in terms of features or supported hardware, as well as the extension-based new types of devices, which has attracted much attention from the various industries fields gradually. In other words, the utilization of Android OS based on Linux kernel has covered the whole daily life related to the E-control.

However, there is a feature that has not been explored yet - the RT capabilities. This paper provides an idea of adding some RT features into Android platform to make it a RT architecture in order to be used in open real-time environment.

Android OS Architecture

Android Architecture

Application Uppermost layer; a core set of applications; out of the box

ApplicationFramework

APIs; Services;

Libraries libc: the standard C system library for embedded Linux-based devices

Android Runtime

Android VM (Dalvik): target for memory/CPU-constrained devices; Infinite register-based machine

Linux Kernel

Drivers; Resource Management; Power Management; Hardware Control Management, etc; HAL: interaction with HW via Drivers

Details of Android Architecture

Android OS Architecture(DVM)1. Why does Android architecture utilize DVM but JVM?

Less instruction dispatch and memory access——30%Less CPU resource and computation timeMore bytes in the instruction stream——35%

2. What is the principle of Dalvik VM?

Dalvik-VM

Linux Kernel

Resource Management Capability

Feasibility and Necessity

Feasibility and Necessity(Dalvik-VM)

Fork()

VM father process

Child processes

Pre-initialization&

Pre-loadingOf Android APIs and Classes

(initial Andriod Runtime & setup VM)

All the scheduling

activities are delegated by the VM to the

kernel

Feasibility and Necessity(Linux Kernel)

Feasibility and Necessity(Resource Management Capability)

Android RTS Architecture

Android Full RT Platform

Prospredict priority inversion; better resource management; more-seamless integration; predictability and determinism;ConsDrivers supported natively implement during integration phase; Android Extended

Difference(RTSJ)

Both works ensure resource reservation & RT tasks scheduling in order to guarantee resource bandwidth to apps, within an interval of time

Pros: beneficial for incorporating a more deterministic RT behavior in the VM level;

Con: keep up with release cycle of Android

Pros: bound-based memory management; RT scheduler in RT-JVM; better synchronisation mechanisms; avoid priority inversion;Cons: integration issues

Android RTS Architecture

Android Partly RT Android with a RT Hypervisor

Guest OS

Parallel

High PriorityHigh Priority

No need for VM

Apps that need VM will not benefit from this model

Apps are limited by RT Hypervisor;If RT apps hangs, the sys may hang;

Low Integration

RT Linux Limitation

1. Linux system scheduling unit is 10ms, which is coarse-granularity and cannot provide the precise timing;

2. Until now, when a task or process is called into kernel state from userland it is unpreempted that cannot guarantee the predictability & undeterministics;

3. The utilization of virtual memory technology;

4. …….

5. Lack of effective and efficient RT scheduling algorithms & policies;

Improvement

1. Improve the accuracy of the CPU clock via Clock Counter Register to solve the Linux coarse-granularity issues; eg. RT-Linux & Kurt-Linux;

2. By insert preempted points into kernel functions to solve unpreempted issues that cannot guarantee the predictability & undeterministics; for instance, RED-Linux;

3. …….

4. Define OS scheduling RT framework & algorithms to solve the issues lack of effective and efficient RT scheduling algorithms & policies; RED-Linux, Qlinux, H-SFQ, Cello, SILK, etc.

Background Introduction

Architecture

Implementation & Evaluation

Lei Cui

Real-Time Android: Deterministic Ease of Use

Background IntroductionThe preferences of end-usersDevelopers with consistent usability A convenient development environment

Android Platform(upper)

Rich & Mature feature set Linux Kernel(bottom)

Android Architecture

Android stack with RT capabilitiesin a minimally invasive way via adding a crucial Linux building block

Exclusive concern in the industrial embedded domain

Remedies for user & programmerof embedded RTS

Architecture

Linux Kernel(Kernel State)

Android Framework (User State)

Communication & Data exchange

Architecture---Linux Kernel

RT support by the OS kernel is obviously a mandatory requirement for RT architectures

RTAIRTLinux,Xenomai

Preempt_rtLitmus-RT

…Preempt_rt is not a RT kernel owing to the complexity of the Linux kernel

——impossible to prove upper bounds on all operations under all possible circumstances——allow to determine upper latency bounds

Architecture--- User State

Android Full RT Platform Android Extended

The Android userland is heavily based on the Dalvik VM, based on register rather than stack architecture. Obsolete--why

Exceed constrained developer resources

Provide an insufficient return on investment

Linux C without translation by VM

Architecture--- Data exchange

Communication Channel

Reject unbounded delays

Shared-memory & Mutual exclusion avoid data corruptionEasily result in unbounded

latenciesShared-memory & Synchronization algorithm lock-free & wait-free

Implementation & Evaluation

A. Basis Component IntegrationBased on the preempt_rt release 2.6.33, back-

port the BSP from 36 to 33, and forward-port the Android patch set from 32 to 33.

B. Shared-memory

1. A system service creates a shared-memory via mmap() sys call;2. The service registers at Binder;3. All programs acquire reference via Binder call; 4. Native clients receive pointer directly;JNI library provides access methods to Android apps.

Implementation & EvaluationC. Demo

D. Latency Measurements E. Efforts