How to run tasks with priority?

[George Lin]

Hello everyone,


I want to run tasks with priority, i.e. among several simultaneous running tasks, the task with the higher priority will have more chances to occupy CPU time. I have the following 2 issues dealing with the implementation of such feature.

- To define each task as a thread or as a method? How to change the priority of each task dynamically when they are running?

- The priority feature of Java thread does not meet my requirement, since I can not define priority precisely. For example, I want to define that a task with priority A will occupy CPU time 3 times than a task with priority B.

I am wondering whether I can find similar open source projects or tutorials?


Thanks in advance,
George

[Stuart Gray]

I doubt you can do this since these precise aspects of the JVMs operation are not defined by the specification.

[George Lin]

Thanks Stuart,

Originally posted by Stuart Gray:
I doubt you can do this since these precise aspects of the JVMs operation are not defined by the specification.

Sorry that I do not express my ideas very clearly. In my mind, I do not want to control the processing time very precisely, and I just want to make the processing time conforming with priority approximately. Do you have any ideas?


regards,
George

[Stuart Gray]

No, I think it is still dependant on the implementation of the JVM. There is no to guarantee a specific priority or order of execution AFAIK.

[George Lin]

Thanks Stuart,

Originally posted by Stuart Gray:
No, I think it is still dependant on the implementation of the JVM. There is no to guarantee a specific priority or order of execution AFAIK.

Your reply is very helpful to guide me to make the decision, i.e. whether I will use Java to implement this feature. I want to know whether your opinions are Java is not suitable for this feature.


regards,
George

[Henry Wong]

Originally posted by George Lin:
Thanks Stuart,

Your reply is very helpful to guide me to make the decision, i.e. whether I will use Java to implement this feature. I want to know whether your opinions are Java is not suitable for this feature.

regards,
George

The reason that the Java specification does not strictly enforce how the priority will affect the timeslice of the thread is because most JVMs are written to use the underlying threading model.

However, the threading model on Windows will try to give more time to a higher priority thread, while making sure that the low priority thread do not starve. So if you are using Java on Windows, it will behave somewhat close to what you want. (Be warned, there are many other factors that affect what happens to a thread)

Henry

[Stuart Gray]

As Henry confirms, I think 'try' is the closest you are going to get when it comes to thread management.

Question: Will the thread management vary even across different versions of Windows (I would imagine so, at least because 9x only supports single processors, while the NT kernel supports multiple processors).

[John Smith]

-How to change the priority of each task dynamically when they are running?

You may want to experiment with the setPriority() method to see what it does. Here is an example:

class TaskA extends Thread {
    public void run() {
        for (int i = 0; i < 100; i++) {
            System.out.println("running taskA");
        }
 
    }
}
 
class TaskB extends Thread {
    public void run() {
        for (int i = 0; i < 100; i++) {
            System.out.println("running taskB");
        }
 
    }
}
 
public class ThreadPriorityTest {
    public static void main(String args[]) {
        TaskA taskA = new TaskA();
        taskA.setPriority(Thread.MAX_PRIORITY);
        TaskB taskB = new TaskB();
        taskB.setPriority(Thread.MIN_PRIORITY);
 
        taskA.start();
        taskB.start();
    }
}

- The priority feature of Java thread does not meet my requirement, since I can not define priority precisely. For example, I want to define that a task with priority A will occupy CPU time 3 times than a task with priority B.


Does the CPU time need to be sliced precisely? If you could live with just minimum and maximum priorities, the threading model may work for you. Otherwise, perhaps you need to consider some other solution. What's the problem you are trying to address?

[George Lin]

Thanks John,

Originally posted by John Smith:
-How to change the priority of each task dynamically when they are running?

You may want to experiment with the setPriority() method to see what it does. Here is an example:

class TaskA extends Thread {
    public void run() {
        for (int i = 0; i < 100; i++) {
            System.out.println("running taskA");
        }
 
    }
}
 
class TaskB extends Thread {
    public void run() {
        for (int i = 0; i < 100; i++) {
            System.out.println("running taskB");
        }
 
    }
}
 
public class ThreadPriorityTest {
    public static void main(String args[]) {
        TaskA taskA = new TaskA();
        taskA.setPriority(Thread.MAX_PRIORITY);
        TaskB taskB = new TaskB();
        taskB.setPriority(Thread.MIN_PRIORITY);
 
        taskA.start();
        taskB.start();
    }
}

- The priority feature of Java thread does not meet my requirement, since I can not define priority precisely. For example, I want to define that a task with priority A will occupy CPU time 3 times than a task with priority B.


Does the CPU time need to be sliced precisely? If you could live with just minimum and maximum priorities, the threading model may work for you. Otherwise, perhaps you need to consider some other solution. What's the problem you are trying to address?

Your reply is very great! I found that if I use priority feature of Java thread, then I can only have (MAX_PRIORITY - MIN_PRIORITY) different priorities, which may limit my application. I am wondering whether there are any alternate solutions which may have more different priorities.

The problem I am meeting with is to develop a simple QoS prototype, in which different tasks can run according to user defined priorities.


regards,
George

[John Smith]

I found that if I use priority feature of Java thread, then I can only have (MAX_PRIORITY - MIN_PRIORITY) different priorities, which may limit my application. I am wondering whether there are any alternate solutions which may have more different priorities.


If you look at the implementation of the java.lang.Thread class, you will see that the priority is a simple int and the range is defined between 1 and 10:

    /**
     * The minimum priority that a thread can have. 
     */
    public final static int MIN_PRIORITY = 1;
 
    /**
     * The maximum priority that a thread can have. 
     */
    public final static int MAX_PRIORITY = 10;
 

So, yes, you can call setPriority and pass any int from 1 to 10. However, don't get too excited. As other posters noted, there are no guarantees that the underlying OS will comply with your code. For one thing, consider this: Solaris supports 2^31 priority levels, and Windows NT supports only 7. When you specify one of the Java's 10 priorities, they will be mapped into one of the OS's priorities, and this mapping is undefined. Your best bet is to limit your choices to Thread.MAX_PRIORITY, Thread.MIN_PRIORITY, and Thread.NORM_PRIORITY, but even then the scheduling will vary from one OS to another.

The best way to think about Java thread priorities is as a mere hint to the underlying OS scheduler. As such, if your task is to prioritize the tasks in a reliable, platform-independent way, most people would probably advise you against using Java priorities.

Let me throw you something alternative to you: how about giving up on threads and priorities, and making use of a priority queue which contains the tasks to be executed in a sequence? That way, you can rearrange the tasks dynamically, at any time, in a very precise fashion, your code will be portable, and the results will be predictable.

You may also consider two celebrated books on Java thread programming:
Java Thread Programming by Paul Hyde and Concurrent Programming in Java by Doug Lea
[ May 31, 2005: Message edited by: John Smith ]

[George Lin]

Thanks Henry,

Originally posted by Henry Wong:


The reason that the Java specification does not strictly enforce how the priority will affect the timeslice of the thread is because most JVMs are written to use the underlying threading model.

However, the threading model on Windows will try to give more time to a higher priority thread, while making sure that the low priority thread do not starve. So if you are using Java on Windows, it will behave somewhat close to what you want. (Be warned, there are many other factors that affect what happens to a thread)

Henry

Your reply is very helpful! Do you mean using setPriority of Java Thread? If so, could you please recommend some good tutorials dealing with how to use priority feature of Java thread efficiently? I do not have much experience on this feature.

Another issue is that, it seems Windows and Linux use different thread models in your reply, could you please explain it in more details or provide some online resources?


regards,
George

[George Lin]

Thanks Stuart,

Originally posted by Stuart Gray:
As Henry confirms, I think 'try' is the closest you are going to get when it comes to thread management.

Question: Will the thread management vary even across different versions of Windows (I would imagine so, at least because 9x only supports single processors, while the NT kernel supports multiple processors).

Your reply is very helpful. I am wondering where can I find related resources (open source projects, tutorials or papers) dealing with different thread models on different platforms, just as you have mentioned.


regards,
George

[George Lin]

Thanks John,

Originally posted by John Smith:

Let me throw you something alternative to you: how about giving up on threads and priorities, and making use of a priority queue which contains the tasks to be executed in a sequence? That way, you can rearrange the tasks dynamically, at any time, in a very precise fashion, your code will be portable, and the results will be predictable.

Your idea of using priority based queue is great!! I think you are an expert of this topic. I do not have very much exporience about this topic before, could you provide me a simple sample? Are there any online resources (open source projects, tutorials or papers) dealing with this topic?


regards,
George