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。
原文地址：https://www.cnblogs.com/yangmingsen/p/11290707.html
时间： 2024-08-30 11:16:57