System. Currenttimemillis() and system Nanotime (), which is faster? Don't use it wrong!

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 .

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 in the execution output System.nanoTime() Obviously very slow .

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

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