Android AlarmManager setExact() is not exact

Question

I need to plan sheduled task every 10 minutes.

As in Lollipop and higher version setRepeating() is inexact, I use setExact() and (on alarm firing) I set new exact alarm in 10 minutes.

private void setAlarm(long triggerTime, PendingIntent pendingIntent) {
        int ALARM_TYPE = AlarmManager.ELAPSED_REALTIME_WAKEUP;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
            alarmManager.setExact(ALARM_TYPE, triggerTime, pendingIntent);
        } else {
            alarmManager.set(ALARM_TYPE, triggerTime, pendingIntent);
        }
    }

triggerTime is calculated SystemClock.elapsedRealtime() + 600_000;

When alarm fires, firstly I plan new one, only after that I run my sheduled task.

setAlarm();
mySheduledTask;

I do have WAKE_LOCK permission in my manifest.

When I test this on Android 4 - it works perfect (deviation might be 12-15 milliseconds).

But when I run app on Xiaomi Redmi Note 3 Pro (5.1.1) - deviation can be up to 15 seconds!

For example, I see in my log file: first run was at 1467119934477 (of RTC time), second - at 1467120541683. Difference is 607_206 milliseconds, not 600_000, as it was planned!

What am I missing? What is a way to simulate behaviour of system alarm (it's the most close usecase that can describe my tack)?

PS. I use IntentService for PendingIntent = PendingIntent.getService(context, 0, myIntent, 0);

Answer 1

The OS chooses how the alarms will work, with consideration of the time you've specified. Because of that, when the phone gets into a 'semi-sleep' mode, it won't necessary use the resource at the time you wish it to. Basically, it waits for 'windows' that the OS opens for it, and only then the alarm you want to run will run, that's why you're experiencing time gaps.

This was introduced on Marshmallow OS and will continue on Nougat OS as well, as part of Google trying to improve the device's battery.

Here's the thing, you have 2 options:

1. Accept the time delays (but maybe consider using JobScheduler which is more recommended and will save you battery).
2. Use setExactAndAllowWhileIdle which might cause you battery issues (use this carefully, too many alarms will be bad for your battery). This method isn't repeating, so you have to declare the next job to be run at the service which the pendingIntent opens.

If you choose option 2, here's the start:

AlarmManager am = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
int ALARM_TYPE = AlarmManager.RTC_WAKEUP;
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M)
    am.setExactAndAllowWhileIdle(ALARM_TYPE, calendar.getTimeInMillis(), pendingIntent);
else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT)
    am.setExact(ALARM_TYPE, calendar.getTimeInMillis(), pendingIntent);
else
    am.set(ALARM_TYPE, calendar.getTimeInMillis(), pendingIntent);