Primary and Secondary Storage Mechanisms

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From the course by Vanderbilt University

Android App Components - Services, Local IPC, and Content Providers

126 ratings

Vanderbilt University

Android App Components - Services, Local IPC, and Content Providers

126 ratings

Course 3 of 5 in the Specialization Android App Development

This 4 week MOOC builds upon the core Android app components and concurrency frameworks covered in Course 2 by focusing on started and bound services, local inter-process communication (IPC), and content providers. Case study apps will be examined from multiple perspectives to learn how to program these app components using Android's material design paradigm. Students will work incrementally on a hands-on project involving a material design-based RSS reader app. Each week you will add additional capabilities to the project, based on material covered in the lecture videos. You'll spend roughly 4 hours per week watching video lectures, taking quizzes, and programming assignments with Java and Android.

From the lesson

MOOC Overview

This module provides an introduction to the contents of the MOOC and summarizes effective learning strategies. Learning objectives are presented in the Module Introduction video lecture.

Primary and Secondary Storage Mechanisms6:19

Kernel IPC and Processing Mechanisms9:53

Android Kernel Extensions8:38

Meet the Instructors

Dr. Douglas C. Schmidt
Professor of Computer Science and Associate Chair of the Computer Science and Engineering Program
Electrical Engineering and Computer Science

[MUSIC]

Welcome to part one in our lesson on the Android Linux Kernel,

which describes its support for primary and secondary storage mechanisms.

After completing this part of lesson, you'll recognize these two types of

storage mechanisms supported by Android Linux.

Let's start by giving an overview.

Android's software instructions and data reside in two different types of storage.

One type is called primary storage, which is fast random access memory or RAM.

The contents of RAM are volatile,

which means that it's wiped out whenever a device is rebooted or loses power.

Another type of storage supported by Android Linux is called secondary storage,

which is slower flash memory.

Flash memory is persistent storage that could be erased and

reprogrammed electronically.

Primary and secondary storage form part of a so-called memory hierarchy.

In general, being higher in this hierarchy has several implications.

For example, it typically means that memory bandwidth is faster.

In other words, the rate in which the data can be read from or stored into is higher.

Likewise, CPU access latency is lower,

which is defined as the time interval between when a CPU initiates a call for

fetching or storing data, and when the call actually completes.

Yet another implication of being higher in the hierarchy is that the cost is greater.

Typically, the faster the memory access the costlier it is.

Processor cores operate on instructions and

their associated data that reside in random access memory.

For example, the Android Linux Kernel executes in so called kernel space RAM.

Kernel space is a protected region of RAM that's used to run

so-called privileged operations that not all applications can access.

Thus, Kernel space can be accessed by user processes only via system calls.

All Android applications execute in user space RAM.

User space is a more restrictive protection domain than kernel space.

What this means, of course, is that apps that are running in user space,

normally can't access the RAM of other apps unless it's explicitly allowed.

We'll talk later about anonymous shared memory in part 3 of this lesson to

give you some examples of cases where apps can access memory that are shared

with other apps.

The cost and performance of both primary and

secondary storage has improved substantially in recent years.

However, primary storage, or RAM,

on an Android mobile device is still typically constrained.

For example, if you take a look at a mobile device today,

the high end mobile devices, around this time frame, which is

towards the end of 2016, typically give you 1 to 2 gigabytes of memory.

In contrast, a desktop or laptop, typically has 8 to 32 gigabytes.

There's also issues of price point.

Lower cost mobile devices typically have much less RAM than higher cost devices.

So you're paying for the speed improvement.

Android Linux's virtual memory manager features address various constraints of

the limited memory that's available.

For example, virtual memory helps conserve RAM by not moving app instructions and

data from secondary to primary storage until they're actually accessed.

Keep in mind again that secondary storage is slower, in fact much slower,

than primary storage.

So the instructions and data need to be moved from the secondary storage to

the primary storage before they can be executed.

Another benefit of Android's virtual memory manager is it allows acceleration

of I/O operations by a so-called memory mapping of files & hardware devices.

And finally, virtual memory management protects an app's private data

in RAM from accidental or attempted malicious access by other apps.

Secondary storage in Android Linux is typically used to save data persistently.

There's a number of different mechanisms that Android provides

to support persistence.

One mechanism provided by Android are shared preferences,

which are used to store private primitive data in key value pairs

using a Java hash map whose contents are stored persistently in a file.

There's also something called external storage,

which is used to store public data on a shared external storage mechanism.

There's also internal storage, which is internal to an app,

and this is used to store private data on the device memory.

And there's also something called SQLight Databases, which we'll talk about later

in this MOOC, and they're used to store structured data in a private database.

Android Linux supports secondary storage by it's virtual file system or

VSF framework.

Each file system VFS is implemented via a kernel module

it registers the operations that it supports with VFS.

These operations include things like open, close, read, write, select, and so on.

Many Android Linux file systems supports flash meteor files that can be erased and

programmed electronically.

This is the end of the overview on the Android Linux Kernel

giving a brief description of its primary and secondary storage mechanisms.

[MUSIC]

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