Android memory management granularity - Activity or Process?

Question

I am seeing inconsistent documentation and discussion regarding what happens when Android is low on memory and the steps the OS takes to re-claim memory. More specifically, does Android kill at the granularity of activity/fragment, or entire process?

For example, if Activity B is launched in front of Activity A (and both activities are part of the same app/process), can Activity A be killed by the OS while Activity B is in the foreground and the user is interacting with Activity B (assume: screen remains on, current app remains in the foreground, no orientation change occurs)?

This SO answer from 2011 (by Dianne Hackborn on the Android team at Google) suggests that Android kills at the granularity of a process, NOT an activity.

On the Android Developer pages on Recreating an Activity, it says:

The system may also destroy your activity if it's currently stopped and hasn't been used in a long time or the foreground activity requires more resources so the system must shut down background processes to recover memory.

Notice the ambiguity: "the system must shut down background PROCESSES".

On the Android Developer pages for onSaveInstanceState, it says:

For example, if activity B is launched in front of activity A, and at some point activity A is killed to reclaim resources, activity A will have a chance to save the current state of its user interface via this method

After reading through these and many other doc pages and online discussion, it is not clear what the correct answer is.

I also have the same question regarding fragments: Can a backgrounded fragment be killed due to low memory, without its entire process being killed?

Answer 1

Memory management happens at two distinct levels: through garbage collection (recycling unreferenced objects) and at the process level, as explained in this Android blog post. There is no concept of killing just a single activity (remember: Android is based on Linux and Linux has no concept of activities or components, just processes).

This SO answer from 2011 (by Dianne Hackborn on the Android team at Google) suggests that Android kills at the granularity of a process, NOT an activity.

This is still correct.

On the Android Developer pages on Recreating an Activity, it says...

Yes, the 'background processes' it mentions is exactly the process category mentioned in the above blog and the documentation. This refers to activities that previously existed, but are no longer part of the current foreground/visible processes.

On the Android Developer pages for onSaveInstanceState, it says:

Yes, they are discussing the case where you launch an Activity from another process (as is likely when you are using implicit intents). During this time, your process is not the foreground process and therefore is certainly possible to be killed if the combination of foreground activity+foreground services is too much.

I also have the same question regarding fragments: Can a backgrounded fragment be killed due to low memory, without its entire process being killed?

No, fragments cannot be killed due to low memory.