Thread Scheduling §

Nice Values §

Nice Values in Android are a measure of a thread’s priority. Threads with a higher nice value are lower priority - they are being nice to other threads.

The two most important priorities are default and background. The UI thread is given default priority.

In theory this would be enough but in practise this system is somewhat lacking - 20 background threads with one thread driving the UI would end up impacting the performance of the UI considerably, resulting in lag.

Cgroups §

To fix this issue, Android adopts foreground vs background scheduling from Linux using cgroups (control groups).

Threads with background priority are moved into a background cgroup, where they are limited to a small percentage of available CPU horsepower if threads in other groups are busy. This lets background threads make some progress without starving the UI, for instance.

Android also moves all threads belonging to applications that aren’t currently in the foreground to the background - ergo, the currently focused application will always have priority.