I'm not a total expert on this...

Threads are really just a way of running multiple stacks/program
counters in the same address space. It's fair to assume they are
time-sliced. Services and Activities are run in a single UI thread
(AFAIK) and the precessing model is typical UI thread which calls
event methods on them sequentially (they don't really run like
classical processes). AsyncTasks run from a thread pool. The first few
can run concurrently, but after that the next one can't start until
another one is finished. Network connections are also pooled (AFAIK),
so that after you open a few, additional ones queue up.

To get workable background processing/networking, you'll probably need
to use alarms/saved state, since an app's process can be summarily
dismissed any time by the OS when other apps need resources. Although
some background processing that takes just seconds you might as well
do with an AsyncTask in an activity instead of a service.

Don't expect SDK apps to get any processing done when the device is
sleeping or turned off (I'm sure you know that already).

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