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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 845
814
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
Instead of trying to guess which processes should be killed, the Android
out-of-memory killer relies very strictly on information provided to it by user
space. The traditional Linux out-of-memory killer has a per-process
oom
adj
pa-
rameter that can be used to guide it toward the best process to kill by modifying the
process’ overall badness score. Android’s out-of-memory killer uses this same pa-
rameter, but as a strict ordering: processes with a higher
oom
adj
will always be
killed before those with lower ones. We will discuss later how the Android system
decides to assign these scores.
10.8.6 Dalvik
Dalvik implements the Java language environment on Android that is responsi-
ble for running applications as well as most of its system code.
Almost everything
in the
system
service
process—from the package manager, through the window
manager, to the activity manager—is implemented with Java language code ex-
ecuted by Dalvik.
Android is not, however, a Java-language platform in the traditional sense.
Java code in an Android application is provided in Dalvik’s bytecode format, based
around a register machine rather than Java’s traditional stack-based bytecode.
Dalvik’s bytecode format allows for faster interpretation, while still supporting
JIT
(
Just-in-Time
) compilation. Dalvik bytecode is also more space efficient, both on
disk and in RAM, through the use of string pooling and other techniques.
When writing Android applications, source code is written in Java and then
compiled into standard Java bytecode using traditional Java tools. Android then
introduces a new step: converting that Java bytecode into Dalvik’s more compact
bytecode representation.
It is the Dalvik bytecode version of an application that is
packaged up as the final application binary and ultimately installed on the device.
Android’s system architecture leans heavily on Linux for system primitives, in-
cluding memory management, security, and communication across security bound-
aries. It does not use the Java language for core operating system concepts—there
is little attempt to abstract away these important aspects of the underlying Linux
operating system.
Of particular note is Android’s use of processes.
Android’s design does not
rely on the Java language for isolation between applications and the system, but
rather takes the traditional operating system approach of process isolation.
This
means that each application is running in its own Linux process with its own
Dalvik environment, as are the
system
server
and other core parts of the platform
that are written in Java.
Using processes for this isolation allows Android to leverage all of Linux’s
features for managing processes, from memory isolation to cleaning up all of the
resources associated with a process when it goes away. In addition to processes,
instead of using Java’s SecurityManager architecture, Android relies exclusively on
Linux’s security features.
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