Java Thread priority has no effect
Java Thread priority has no effect
Thread priorities are highly OS specific and on many operating systems often have minimal effect. Priorities help to order the threads that are in the run queue only and will not change how often the threads are run in any major way unless you are doing a ton of CPU in each of the threads.
Your program looks to use a lot of CPU but unless you have fewer cores than there are threads, you may not see any change in output order by setting your thread priorities. If there is a free CPU then even a lower priority thread will be scheduled to run.
Also, threads are never starved. Even a lower priority thread will given time to run quite often in such a situation as this. You should see higher priority threads be given time sliced to run more often but it does not mean lower priority threads will wait for them to finish before running themselves.
Even if priorities do help to give one thread more CPU than the others, threaded programs are subject to race conditions which help inject a large amount of randomness to their execution. What you should see however, is the max priority thread is more likely to spit out its 0 message more often than the rest. If you add the priority to the println(), that should become obvious over a number of runs.
It is also important to note that System.out.println(...) is synchronized method that is writing IO which is going to dramatically affect how the threads interact and the different threads are blocking each other. In addition, Thread.yield(); can be a no-op depending on how the OS does its thread scheduling.
but a random result (every time I run it changes):
Right. The output from a threaded program is rarely if ever "perfect" because by definition the threads are running asynchronously. We want the output to be random because we want the threads to be running in parallel independently from each other. That is their power. If you expecting some precise output then you should not be using threads.
Let's keep it simple and go straight to the source ...
Every thread has a priority. When there is competition for processing resources, threads with higher priority are generally executed in preference to threads with lower priority. Such preference is not, however, a guarantee that the highest priority thread will always be running, and thread priorities cannot be used to reliably implement mutual exclusion.
- from the Java Language Specification (2nd Edition) p.445.
Also ...
Although thread priorities exist in Java and many references state that the JVM will always select one of the highest priority threads for scheduling [52, 56, 89], this is currently not guaranteed by the Java language or virtual machine specifications [53, 90]. Priorities are only hints to the scheduler [127, page 227].
from Testing Concurrent Java Components (PhD Thesis, 2005) p. 62.
Reference 127, page 227 (from the excerpt above) is from Component Software: Beyond Object-Oriented Programming (by C. Szyperski), Addison Wesley, 1998.
In short, do not rely on thread priorities.
Thread priority does not guarantee execution order. It comes into play when resources are limited. If the System is running into constraints due to memory or CPU, then the higher priority threads will run first. Assuming that you have sufficient system resources (which I would assume so for a simple program and the system resources you posted), then you will not have any system constraints. Here is a blog post (not my post) I found that provides more information about it: Why Thread Priority Rarely Matters.
Thread priority is implementation dependent. In particular, in Windows:
Thread priority isn't very meaningful when all threads are competing for CPU. (Source)
The book "Java Concurrency in Practice" also says to
Avoid the temptation to use thread priorities, since they increase platform dependence and can cause liveness problems. Most concurrent applications can use the default priority for all threads.