多线程并发一定比单线程快吗？

Posted 2022-06-05 yangmingsen

　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

 1 public class ConcurrencyTest 
 2     private static final long count = 10000l;
 3 
 4     public static void main(String[] args) throws InterruptedException 
 5         for(int i=0; i<10; i++) 
 6             concurrency();
 7             serial();
 8             System.out.println("------------------------------------------");
 9         
10 
11     
12 
13     private static void concurrency() throws InterruptedException 
14         long start = System.currentTimeMillis();
15         Thread thread = new Thread(new Runnable() 
16             @Override
17             public void run() 
18                 int a = 0;
19                 for (long i = 0; i < count; i++) 
20                     a += 5;
21                 
22             
23         );
24         thread.start();
25         int b = 0;
26         for (long i = 0; i < count; i++) 
27             b--;
28         
29         long time = System.currentTimeMillis() - start;
30         thread.join();
31         System.out.println("concurrency :" + time + "ms,b=" + b);
32     
33 
34     private static void serial() 
35         long start = System.currentTimeMillis();
36         int a = 0;
37         for (long i = 0; i < count; i++) 
38             a += 5;
39         
40         int b = 0;
41         for (long i = 0; i < count; i++) 
42             b--;
43         
44         long time = System.currentTimeMillis() - start;
45         System.out.println("serial:" + time + "ms,b=" + b + ",a=" + a);
46     
47 

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。