Why does this code give this output for the thread's priority?

Question

I have the following code onCreate()

Log.d(TAG, "Setting priority background");  
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);  
Log.d(TAG, Thread.currentThread().getId() + ": " + Thread.currentThread().getPriority());
Log.d(TAG, Process.getThreadPriority(Process.myTid()) + " - myTid() " + Process.myTid() + " Thread.getId() = " + Thread.currentThread().getId());
// start a thread here
Thread thread = new Thread(() -> {
            Log.d(TAG, " In new thread");
            Log.d(TAG, Thread.currentThread().getId() + ": " +   Thread.currentThread().getPriority());
            Log.d(TAG, Process.getThreadPriority(Process.myTid()) + " - myTid() " + Process.myTid() + "Thread.getId() = " + Thread.currentThread().getId());

}

The output is:

Setting priority background  
1: 5  
10 - myTid() 8798 Thread.getId() = 1  

In new thread  
7534: 10  
-8 - myTid() 8819 Thread.getId() = 7534

Can someone please explain:
1) Even though I set setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); the log in the next line shows priority 5. Why?
2) Where is the -8 coming from in the output for the priority of the second thread?
3) Where is the 10 for the new thread coming from too?

Update:
If I remove the line Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
the output remains the same

JakeB · Answer 1 · 2019-08-13T23:29:07.180

2

Reading here, THREAD_PRIORITY_BACKGROUND delegates a lower than normal priority to the task on that thread.

Taking information from here, using Thread.setPriority() contains a value from MIN_PRIORITY(1) to MAX_PRIORITY(10) whereas Process.setThreadPriority() supports value from -20 to 19.

The image below shows the Android Scheduler and the Linux Scheduler and their priority levels. Calling Process and Thread to get thread priority will return two diffrent values as they are two different schedulers, like in your logs.

Update:

This post provided some insight into this:

Process.myTid() is the linux thread ID
Thread.getId() is a simple sequential long number.

"Thread.getId() is simply a "java layer" static long being auto-increment for each thread."

ThreadID is not equal to ProcessID.

However, casting the long to int and providing the ThreadID to the Process returns an expcted value Process.getThreadPriority((int) Thread.currentThread().getId()).

edited Aug 13 '19 at 23:29

answered Aug 13 '19 at 22:15

JakeB

2,043
3
12
19

That mapping does not explain where the value `-8` came from though I believe – Jim Aug 13 '19 at 22:25
Also as per that mapping `5 <=> 0` but 10 is printed – Jim Aug 13 '19 at 22:27
Updated with some more info. – JakeB Aug 13 '19 at 23:21
Thanks for the update but I am not using `Process.getThreadPriority((int) Thread.currentThread().getId())`. How is this called? – Jim Aug 14 '19 at 07:09
I mean I am not doing `Process.getThreadPriority((int) Thread.currentThread().getId())` anywhere. Why is that affecting something? – Jim Aug 14 '19 at 17:04
`However, casting the long to int and providing the ThreadID to the Process ` where is this happening? – Jim Aug 14 '19 at 17:18
No, i was saying thats what i did. I dont know anymore without searching more into it. Seems like Process.myTid is switching threads at some point? It seems like its a pool of threads rather than a single thread. – JakeB Aug 14 '19 at 18:18
Ok, I just noticed that in the Thread class the priority is only updated if someone calls the setPriority method!!! So the Thread class does not "see" the change of the priority happening from Process.setThreadPriority!!!! Those don't sync! So this explains why the priorities returned by the 2 APIs are different. – Jim Aug 14 '19 at 20:09

Why does this code give this output for the thread's priority?

1 Answers1