I am not challenging Android's memory management technique (like killers do). I just want to give an app higher priority even when it's not in foreground or visible (with notification).
AutoKiller Memory Optimizer is able to do it on rooted device, but the change lasts for few seconds if I multi-task.
I want to do this permanently: perhaps running a background daemon to do change continuously should work. How to do this?
On XDA I found a post discussing how to PIN an app to not get killed by OOM. As your question indicates your device is rooted, this could be useful for you. The principle described there can be applied to your case as well by simply changing the oom_adj value specified (the -17 would permanently PIN it, highest prio -- so use a larger value to "weaken" that).
The script snipped posted there looks like this:
#!/system/bin/sh
sleep 60
PPID=$(pidof com.estrongs.android.safer)
echo "-17" > /proc/$PPID/oom_adj
An anonymous user noted you could also set this file to read-only permissions to prevent Android from updating it, adding chmod 444 /proc/$PPID/oom_adj to the above script (the proposed 444 means nothing but read permission, but doesn't affect the system's ability to remove the file when the process ends). I have not verified that, but this could in fact save you from looping/repeatedly running the script, making the set value permanent for as long as the process lives.
As said, replace the -17 by your chosen value. Also replace the app's package name (com.estrongs.android.safer) by the package name of your target app. You can also lower the sleep value, or even ommit the sleep at all.
Now you need a way to auto-execute this as soon as your app is running. For this part, you could e.g. use Tasker: Context would be "app running" (and your app selected), and the task "run shell", with above script as parameter. Then, as soon as you start your app (and it comes to foreground), Tasker would execute the script -- and the oom_adj value should be applied.
It can't be done from a application's code (background service or thread or whatever). Reason is well described in this post, http://debuggingisfun.blogspot.com/2014/03/android-foolproof-low-memory-killer.html
ROM Toolbox Pro by Jrummy has built-in OOM manager.
