多线程并发一定比单线程快吗？

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。

最后

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