多线程并发一定比单线程快吗？

拼搏现实的明天。 2021-11-09 01:31 624阅读 0赞

很多时候我们都以为要想处理速度更快,那就多开几个线程跑!

确实多线程在一定情况下比单线程更快。

下面的代码演示串行和并发执行并累加操作的时间，请分析：下面的代码并发执行一定比串行执行快吗？当count的数量增加 1万 -> 10万 -> 100万 -> 1000万 -> 1亿

public class ConcurrencyTest {
        private static final long count = 10000l;
    
        public static void main(String[] args) throws InterruptedException {
            for(int i=0; i<10; i++) {
                concurrency();
                serial();
                System.out.println("------------------------------------------");
            }
    
        }
    
        private static void concurrency() throws InterruptedException {
            long start = System.currentTimeMillis();
            Thread thread = new Thread(new Runnable() {
                @Override
                public void run() {
                    int a = 0;
                    for (long i = 0; i < count; i++) {
                        a += 5;
                    }
                }
            });
            thread.start();
            int b = 0;
            for (long i = 0; i < count; i++) {
                b--;
            }
            long time = System.currentTimeMillis() - start;
            thread.join();
            System.out.println("concurrency :" + time + "ms,b=" + b);
        }
    
        private static void serial() {
            long start = System.currentTimeMillis();
            int a = 0;
            for (long i = 0; i < count; i++) {
                a += 5;
            }
            int b = 0;
            for (long i = 0; i < count; i++) {
                b--;
            }
            long time = System.currentTimeMillis() - start;
            System.out.println("serial:" + time + "ms,b=" + b + ",a=" + a);
        }
    }

**测试结果**  
这是count=1万的结果，基本上串行要快些

并发 :1ms,b=-10000
    串行:0ms,b=-10000,a=50000
    ------------------------------------------
    并发 :0ms,b=-10000
    串行:1ms,b=-10000,a=50000
    ------------------------------------------
    并发 :0ms,b=-10000
    串行:1ms,b=-10000,a=50000
    ------------------------------------------
    并发 :0ms,b=-10000
    串行:0ms,b=-10000,a=50000
    ------------------------------------------
    并发 :1ms,b=-10000
    串行:0ms,b=-10000,a=50000
    ------------------------------------------
    并发 :1ms,b=-10000
    串行:0ms,b=-10000,a=50000
    ------------------------------------------
    并发 :0ms,b=-10000
    串行:1ms,b=-10000,a=50000
    ------------------------------------------
    并发 :0ms,b=-10000
    串行:0ms,b=-10000,a=50000
    ------------------------------------------
    并发 :1ms,b=-10000
    串行:0ms,b=-10000,a=50000
    ------------------------------------------

这是count=10万l的结果，基本上串行要快点点

并发 :4ms,b=-100000
    串行:3ms,b=-100000,a=500000
    ------------------------------------------
    并发 :0ms,b=-100000
    串行:1ms,b=-100000,a=500000
    ------------------------------------------
    并发 :1ms,b=-100000
    串行:0ms,b=-100000,a=500000
    ------------------------------------------
    并发 :0ms,b=-100000
    串行:1ms,b=-100000,a=500000
    ------------------------------------------
    并发 :1ms,b=-100000
    串行:0ms,b=-100000,a=500000
    ------------------------------------------
    并发 :0ms,b=-100000
    串行:0ms,b=-100000,a=500000
    ------------------------------------------
    并发 :1ms,b=-100000
    串行:0ms,b=-100000,a=500000
    ------------------------------------------
    并发 :0ms,b=-100000
    串行:0ms,b=-100000,a=500000
    ------------------------------------------
    并发 :0ms,b=-100000
    串行:1ms,b=-100000,a=500000
    ------------------------------------------
    并发 :0ms,b=-100000
    串行:0ms,b=-100000,a=500000
    ------------------------------------------

这是count=100万l的结果，基本上并发就要比串行要快了

并发 :5ms,b=-1000000
    串行:5ms,b=-1000000,a=5000000
    ------------------------------------------
    并发 :3ms,b=-1000000
    串行:7ms,b=-1000000,a=5000000
    ------------------------------------------
    并发 :1ms,b=-1000000
    串行:7ms,b=-1000000,a=5000000
    ------------------------------------------
    并发 :1ms,b=-1000000
    串行:2ms,b=-1000000,a=5000000
    ------------------------------------------
    并发 :1ms,b=-1000000
    串行:2ms,b=-1000000,a=5000000
    ------------------------------------------
    并发 :2ms,b=-1000000
    串行:2ms,b=-1000000,a=5000000
    ------------------------------------------
    并发 :1ms,b=-1000000
    串行:3ms,b=-1000000,a=5000000
    ------------------------------------------
    并发 :1ms,b=-1000000
    串行:2ms,b=-1000000,a=5000000
    ------------------------------------------
    并发 :1ms,b=-1000000
    串行:3ms,b=-1000000,a=5000000
    ------------------------------------------
    并发 :1ms,b=-1000000
    串行:2ms,b=-1000000,a=5000000
    ------------------------------------------

余下的读者可以自己去调试哦。

**结论**  
　1.在数量比较小的时候，很明显串行要比并发更快。

2.因为数量较小的时候，并发会花费很多时间在线程调度上。

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文章出处：[https://www.cnblogs.com/yangmingsen/p/11290707.html][https_www.cnblogs.com_yangmingsen_p_11290707.html]  
作者： 杨铭森

[Link 1]: https://jq.qq.com/?_wv=1027&k=55dGrHe
[aHR0cHM6Ly91cGxvYWQtaW1hZ2VzLmppYW5zaHUuaW8vdXBsb2FkX2ltYWdlcy85NzI3Mjc1LTk2MDBkM2ZjNDA3YmMyMjYucG5nP2ltYWdlTW9ncjIvYXV0by1vcmllbnQvc3RyaXAlN0NpbWFnZVZpZXcyLzIvdy8xMDAwL2Zvcm1hdC93ZWJw]: /images/20211108/ec5209c71a824571b9a100eecb93931e.png
[https_www.cnblogs.com_yangmingsen_p_11290707.html]: https://www.cnblogs.com/yangmingsen/p/11290707.html