Why does busy-wait of less than 15ms are inconsistent?

Question

I am doing simulation project where i have hundred of CPU-Bound jobs running for 10 to 50 milliseconds. The Job is a Runnable object with specified running-time for which the job will keep CPU busy. There are 10 threads waiting in a thread pool for job arrival. I set request rate to 40 requests per second and all jobs running-time is set to 10ms. But results are so much terrible . All jobs run for at least 15ms. There is no job that runs for 10 ms. i test the experiment with jobs at 15ms and i got correct result. Why does 10ms jobs run for atleast 15ms?(I am using WINDOWS8).

public class CpuBoundJob implements Runnable {
long runningTime
     public CpuBoundJob(long runningTime) {
        this.runningTime=runningTime;
          }
    @Override
    public void run() {

         long timeToWait = this.runningTime;
         long startTime = System.currentTimeMillis();
         while(startTime + timeToWait > System.currentTimeMillis());
}
}

Answer 1

On many systems (Windows specifically, IIRC), System.currentTimeMillis() is backed by a clock that is only accurate to with 15 milliseconds or so.

However, it's even worse: System.currentTimeMillis() measures time since the Unix epoch as measured by your system clock. So if you change the time on your computer (e.g. due to syncing your system clock to a time source, or due to a leap second, or any number of other things) then currentTimeMillis() can jump forward or backward arbitrarily large amounts of time.

If you want to measure elapsed time, NEVER use currentTimeMillis(). Use System.nanoTime() instead. As a bonus, on most systems, it's also substantially cheaper to invoke and substantially more accurate.