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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 877
846
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
8.
Activity manager sends to the new process any pending operations, in
this case ‘‘start activity X.’’
9.
New process receives the command to start an activity, instantiates the
appropriate Java class, and executes it.
System_server process
Application process
Activity instance
Application code
Android framework
PackageManagerService
startActivity()
ActivityManagerService
"Who am I?"
Load this app s code
Instantiate this class
Zygote process
Create a new process
Resolve Intent
2
1
3
5
6
8
9
7
4
Figure 10-67.
Steps in launching a new application process.
Note that when we started this activity, the application’s process may already
have been running.
In that case, the activity manager will simply skip to the end,
sending a new command to the process telling it to instantiate and run the ap-
propriate component.
This can result in an additional activity instance running in
the application, if appropriate, as we saw previously in Fig. 10-56.
Process Lifecycle
The activity manager is also responsible for determining when processes are
no longer needed.
It keeps track of all activities, receivers, services, and content
providers running in a process; from this it can determine how important (or not)
the process is.
Recall that Android’s out-of-memory killer in the kernel uses a process’s
oom
adj
as a strict ordering to determine which processes it should kill first. The
activity manager is responsible for setting each process’s
oom
adj
appropriately
Page 878
SEC. 10.8
ANDROID
847
based on the state of that process, by classifying them into major categories of use.
Figure 10-68 shows the main categories, with the most important category first.
The last column shows a typical
oom
adj
value that is assigned to processes of this
type.
Category
Description
oom
adj
SYSTEM
The system and daemon processes
−
16
PERSISTENT
Always-running application processes
−
12
FOREGROUND
Currently interacting with user
0
VISIBLE
Visible to user
1
PERCEPTIBLE
Something the user is aware of
2
SERVICE
Running background services
3
HOME
The home/launcher process
4
CACHED
Processes not in use
5
Figure 10-68.
Process importance categories.
Now, when RAM is getting low, the system has configured the processes so
that the out-of-memory killer will first kill
cached
processes to try to reclaim
enough needed RAM, followed by
home
,
service
, and on up.
Within a specific
oom
adj
level, it will kill processes with a larger RAM footprint before smaller
ones.
We’ve now seen how Android decides when to start processes and how it cate-
gorizes those processes in importance.
Now we need to decide when to have proc-
esses exit, right?
Or do we really
need
to do anything more here?
The answer is,
we do not. On Android,
application processes never cleanly exit.
The system just
leaves unneeded processes around, relying on the kernel to reap them as needed.
Cached processes in many ways take the place of the swap space that Android
lacks. As RAM is needed elsewhere, cached processes can be thrown out of active
RAM. If an application later needs to run again, a new process can be created,
restoring any previous state needed to return it to how the user last left it.
Behind
the scenes, the operating system is launching, killing, and relaunching processes as
needed so the important foreground operations remain running and cached proc-
esses are kept around as long as their RAM would not be better used elsewhere.
Process Dependencies
We at this point have a good overview of how individual Android processes are
managed. There is a further complication to this, however: dependencies between
processes.
As an example, consider our previous camera application holding the pictures
that have been taken. These pictures are not part of the operating system; they are
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