CPU Execution Time Shared by Multiple Threads

This section provides a tutorial example on how to measuring CPU execution time allocated to threads with different setPriorities.

Question: If there are 5 application programs running on a single CPU system, and there 5 threads running in each of the 5 running application programs, how all the 25 threads get executed simultaneously on that one CPU?

The answer is that those 25 threads do not get executed simultaneous at any given instance of time. They get executed one at a time in turns to share the processing time of the single CPU. The following simple example illustrates how the processing time can be shared by multiple threads.

Assuming that:

If we let the system runs for 0.1 second, each application program will be executed 20 times for a total time of 20 milliseconds, and each thread will be executed 400 times for a total time of 4 milliseconds. So, for a fraction of a second (1/10 second), all threads in all applications programs will be executed for about 4 milliseconds. This will make you feel like all threads get executed simultaneously.

Now let's use the following program to see how each thread is getting its share of the CPU time:

/* RacingThread.java
 - Copyright (c) 2014, HerongYang.com, All Rights Reserved.
import java.util.*;
class RacingThread extends Thread {
   private static final long s_maxi = 1000000; // maximum # of steps
   private static final int t_maxi = 5; // maximum # of threads
   private static long[] t_done = new long[t_maxi];
   private static int t_last = -1; // index of the last thread
   private int t_indx; 
   private static int n_prime = 0;
   public static void main(String[] a) {
      System.out.println("Priority: (min, norm, max) = ("+
         Thread.MIN_PRIORITY+", "+
         Thread.NORM_PRIORITY+", "+
      long start_time = new Date().getTime();
      for (int i=0; i<t_maxi; i++) {
         RacingThread t = new RacingThread();
         if (i==0) t.setPriority(Thread.MIN_PRIORITY);
         else if (i==1) t.setPriority(Thread.NORM_PRIORITY);
         else t.setPriority(Thread.MAX_PRIORITY);
      System.out.print("Threads: ");
      for (int i=0; i<t_maxi; i++) {
         System.out.print(i+" ");
      while(true) {
         try {
         } catch (InterruptedException e) {
         System.out.print("\n  Steps: ");
         for (int i=0; i<t_maxi; i++) {
            System.out.print(t_done[i]+" ");
         System.out.print((new Date()).getTime()-start_time);
   public RacingThread() {
      t_indx = t_last;
      t_done[t_indx] = 0;
   public void run() {
      for (long s=0; s<s_maxi; s++) {
      	 int n = 0;
      	 for (int i=3; i<100; i++) {// keep it busy for some time
      	    boolean is_prime = true;
      	    for (int j=2; j<i; j++) {
      	       is_prime = (i%j>0);
      	       if (!is_prime) break;
      	    if (is_prime) n++;
      	 n_prime = n;
         t_done[t_indx] = s;

Notes on the program design:

See the next section for execution output of this program.

Last update: 2014.

Table of Contents

 About This Book

 Installing JDK 1.8 on Windows

 Execution Process, Entry Point, Input and Output

 Primitive Data Types and Literals

 Bits, Bytes, Bitwise and Shift Operations

 Managing Bit Strings in Byte Arrays

 Reference Data Types and Variables

 StringBuffer - The String Buffer Class

 System Properties and Runtime Object Methods

 Generic Classes and Parameterized Types

 Generic Methods and Type Inference

 Lambda Expressions and Method References

Execution Threads and Multi-Threading Java Programs

 What Are Processes and Threads?

 The "Thread" Class - Creating Thread Objects With Thread Sub Classes

 The "Runnable" Interface - Creating Thread Objects with Runnable Objects

CPU Execution Time Shared by Multiple Threads

 CPU Execution Time Shared by Multiple Threads - Test Output

 Application Data Shared by Multiple Threads

 Application Data Shared by Multiple Threads - Test Results

 interrupt() - Method to Terminate Thread

 ThreadGroup Class and "system" ThreadGroup Tree

 Synchronization Technique and Synchronized Code Blocks

 Deadlock Condition Example Programs

 Garbage Collection and the gc() Method

 Outdated Tutorials


 PDF Printing Version