Multi-Threading Mechanics?

Question

I've been experimenting with multithreading and I'm really confused about a test I did. I had been doing research and everything was talking about how multithreading can allow 2 processes to run at the same time.

I made this program so 3 different threads would use a for loop to count 1-10, 11-20, and 21-30 so that I could see if they actually ran at the same time how I expected them to.

After running the program the output is something like 1 2 3 4 5 6 7 8 9 10 21 22 23 24 25 26 27 28 29 30 11 12 13 14 15 16 17 18 19 20 Basically any variation order of the 3 sets of numbers. So they can all be in order or have 21-30 before 11-20 sometimes. That doesn't seem like it is running at the same time, just running one after another.

In the for loop in the println(i); if i change it to println(i + "a"); for 1-10 and b, c for 11-20, 21-30 The output is actually in a random order like I had expected. Like this: 1a 11b 21c 2a 22c 12b 13b 23c 3a

Does the program know its doing nothing but counting up and just throw all the numbers on the screen without actually doing it? Or does adding the string at the end make it slow enough for the other threads to sneak in between the operations? I know nothing about this ha.

public class Run{

    static Runnable updatePosition;
    static Runnable render;
    static Runnable checkCollisions;

    public static void main(String[] args) {
        // System.out.println(Runtime.getRuntime().availableProcessors());

        updatePosition = new Runnable() {

            @Override
            public void run() {

                for (int i = 1; i <= 10; i++) {
                    System.out.println(i);
                }
            }

        };

        render = new Runnable() {

            @Override
            public void run() {
                for (int i = 11; i <= 20; i++) {
                    System.out.println(i);
                }

            }

        };

        checkCollisions = new Runnable() {

            @Override
            public void run() {
                for (int i = 21; i <= 30; i++) {
                    System.out.println(i);
                }

            }

        };

        Thread updatePositionThread = new Thread(updatePosition);
        Thread renderThread = new Thread(render);
        Thread checkCollisionsThread = new Thread(checkCollisions);
        updatePositionThread.start();
        renderThread.start();
        checkCollisionsThread.start();

    }

}

Also, how do threads get assigned to CPU cores? In depth please, reasonably. What I mean by asking this is: If I were to use a single thread program and use an update method and a draw method and they together took too long and made my program lag, would putting them on separate threads make this not help, or does it not actually run side by side? Assuming I can deal with all of the concurrency.

Answer 1

Actually, JIT optimizations, processor architecture and several other things play a part in how these kind of situations turn out.

The actual output should not be an ordered execution of threads like 1-10, 21-30, 11-20.

Changing your code a little to :

for (int i = 1; i <= 10000; i++)
for (int i = 10001; i <= 20000; i++)
for (int i = 20001; i <= 30000; i++)

I get (as expected, complete execution of one thread is not happening, as one might assume in your case). It is all about how much time a thread gets.

1
2
3
...
250
251
252
253
20001
20002
20003
...
20127
20128
10001
10002
10003
..

Changing i to "a" + i leads to dynamic construction of new Strings using StringBuilder , this will indeed take some time (and hence CPU cycles). On the other hand, primitive ints don't have this delay .. So, you get such an output.