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System. Currenttimemillis() and system Nanotime() which is faster? Most people get the wrong answer!

2022-06-29 13:33:00 【Java technology stack】

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author ： A fabricated belief
source ：https://segmentfault.com/a/1190000041276485

Java There are two ways to get a timestamp ：System.currentTimeMillis() and System.nanoTime(), Their usage scenarios are different , At present, some articles on the Internet have some one-sided descriptions about the performance of these two methods , This article hopes to give a simple final answer .

System.currentTimeMillis() There are performance issues ？

The answer is No . The performance difference between the two methods depends on the operating system .

Windows:

stay Windows Next ,System.currentTimeMillis() Than System.nanoTime() Much faster , This is because Windows The system only provides a cache variable for the former , The latter is to get the count from the bottom of the hardware in real time .

So if your production environment is Windows, Please avoid using System.nanoTime().

Linux:

stay Linux Next , There is little difference in the execution time between the two , Whether it's single threaded or multi-threaded . in addition , Multithreaded series of interview questions and answers have been sorted out , WeChat search Java Technology stack , Send it in the background ： interview , You can read it online .

Different virtual machine implementations can lead to performance differences

Today's cloud hosts mainly include Xen and KVM Two ways of implementation , Some articles on the Internet found that they have performance differences in taking system time .

When your virtual machine uses Xen when , Taking time will be KVM More than ten times . However, the above also provides solutions to such problems .

You need to write a special class to improve System.currentTimeMillis() Performance ？

Unwanted . That's painting a snake to add feet .

My test code

My test code is as follows , Without any dependency , It can be used directly javac Compile and run . Readers who are interested can try ：

import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;

public class TimePerformance {

    public static final int LOOP_COUNT = 9999999;
    public static final int THREAD_COUNT = 30;

    public static void main(String[] args) {

        Runnable millisTest = () -> {

            long start = System.currentTimeMillis();
            for (int i = 0; i < LOOP_COUNT; i++) {
                System.currentTimeMillis();
            }
            long end = System.currentTimeMillis();

            System.out.printf("%s : %f ns per call\n",
                    Thread.currentThread().getName(), ((double)end - start) * 1000000 / LOOP_COUNT);
        };

        Runnable nanoTest = () -> {

            long start = System.currentTimeMillis();
            for (int i = 0; i < LOOP_COUNT; i++) {
                System.nanoTime();
            }
            long end = System.currentTimeMillis();

            System.out.printf("%s : %f ns per call\n",
                    Thread.currentThread().getName(), ((double)end - start) * 1000000 / LOOP_COUNT);
        };

        Consumer<Runnable> testing = test -> {
            System.out.println("Single thread test:");
            test.run();

            System.out.println(THREAD_COUNT + " threads test:");
            List<Thread> threads = new ArrayList<>();
            for (int i = 0; i < THREAD_COUNT; i++) {
                Thread t = new Thread(test);
                t.start();
                threads.add(t);
            }
            // Wait for all threads to finish
            threads.forEach(thread -> {
                try {
                    thread.join();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        };

        System.out.println(" Test System.nanoTime()");
        testing.accept(nanoTest);
        System.out.println(" Test System.currentTimeMillis()");
        testing.accept(millisTest);
    }
}

Because I'm using Windows, So the execution loses Out of the middle System.nanoTime() Obviously very slow .

I won't release the specific output content , Because it has no reference value , Most production environments use Linux.