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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 846
SEC. 10.8
ANDROID
815
The use of Linux processes and security greatly simplifies the Dalvik environ-
ment, since it is no longer responsible for these critical aspects of system stability
and robustness. Not incidentally, it also allows applications to freely use native
code in their implementation, which is especially important for games which are
usually built with C++-based engines.
Mixing processes and the Java language like this does introduce some chal-
lenges. Bringing up a fresh Java-language environment can take a second, even on
modern mobile hardware. Recall one of the design goals of Android, to be able to
quickly launch applications, with a target of 200 msec.
Requiring that a fresh
Dalvik process be brought up for this new application would be well beyond that
budget. A 200-msec launch is hard to achieve on mobile hardware, even without
needing to initialize a new Java-language environment.
The solution to this problem is the
zygote
native daemon that we briefly men-
tioned previously.
Zygote
is responsible for bringing up and initializing Dalvik, to
the point where it is ready to start running system or application code written in
Java.
All new Dalvik-based processes (system or application) are forked from
zygote
, allowing them to start execution with the environment already ready to go.
It is not just Dalvik that
zygote
brings up.
Zygote
also preloads many parts of
the Android framework that are commonly used in the system and application, as
well as loading resources and other things that are often needed.
Note that creating a new process from
zygote
involves a Linux
fork
, but there is
no
exec
call. The new process is a replica of the original
zygote
process, with all
of its preinitialized state already set up and ready to go.
Figure 10-41 illustrates
how a new Java application process is related to the original
zygote
process. After
the
fork
, the new process has its own separate Dalvik environment, though it is
sharing all of the preloaded and initialed data with
zygote
through copy-on-write
pages. All that now remains to have the new running process ready to go is to give
it the correct identity (UID etc.), finish any initialization of Dalvik that requires
starting threads, and loading the application or system code to be run.
In addition to launch speed, there is another benefit that
zygote
brings. Because
only a
fork
is used to create processes from it, the large number of dirty RAM
pages needed to initialize Dalvik and preload classes and resources can be shared
between
zygote
and all of its child processes.
This sharing is especially important
for Android’s environment, where swap is not available; demand paging of clean
pages (such as executable code) from ‘‘disk’’ (flash memory) is available. However
any dirty pages must stay locked in RAM; they cannot be paged out to ‘‘disk.’’
10.8.7 Binder IPC
Android’s system design revolves significantly around process isolation, be-
tween applications as well as between different parts of the system itself.
This re-
quires a large amount of interprocess-communication to coordinate between the
different processes, which can take a large amount of work to implement and get
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