jmm乱序问题

￡神魔★判官ぃ 2021-07-25 01:52 428阅读 0赞

# 一 点睛 #

cpu的读等待，同时指令执行。

cpu乱序执行的根源：[https://www.cnblogs.com/liushaodong/p/4777308.html][https_www.cnblogs.com_liushaodong_p_4777308.html]

读指令的同时，可以同时执行不影响其他指令。

而写的同时可以进行合并写。

这样CPU执行就是乱序的。

必须使用 Memory Barrier 来做好指令排序。

volatile 底层就是这么实现的（windows是lock指令）

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70][]

CPU为了提高指令执行效率，会在一条指令执行过程中（比如去内存读数据（慢100倍）），去同时执行另外一条指令，前提是，两条指令没有依赖关系。

写操作也可以合并进行。

# 二 生活中的例子 #

![20210619200645365.png][]

# 三 实战 #

## 1 代码 ##

package jmm;
    
    public final class WriteCombiningExt {
        private static final int    ITERATIONS = Integer.MAX_VALUE;
        private static final int    ITEMS      = 1 << 24;
        private static final int    MASK       = ITEMS - 1;
    
        private static final byte[] array1     = new byte[ITEMS];
        private static final byte[] array2     = new byte[ITEMS];
        private static final byte[] array3     = new byte[ITEMS];
        private static final byte[] array4     = new byte[ITEMS];
        private static final byte[] array5     = new byte[ITEMS];
        private static final byte[] array6     = new byte[ITEMS];
        private static final byte[] array7     = new byte[ITEMS];
        private static final byte[] array8     = new byte[ITEMS];
        private static final byte[] array9     = new byte[ITEMS];
        private static final byte[] array10     = new byte[ITEMS];
        private static final byte[] array11     = new byte[ITEMS];
        private static final byte[] array12     = new byte[ITEMS];
        private static final byte[] array13     = new byte[ITEMS];
        private static final byte[] array14     = new byte[ITEMS];
    
        public static void main(final String[] args) {
            for (int i = 1; i <= 3; i++) {
                System.out.println(i + " SingleLoop duration (ns) = " + runCaseOne());
                System.out.println(i + " SplitLoop  duration (ns) = " + runCaseTwo());
            }
        }
        
            // 循环中一次写
        public static long runCaseOne() {
            long start = System.nanoTime();
            int i = ITERATIONS;
            while (--i != 0) {
                int slot = i & MASK;
                byte b = (byte) i;
                array1[slot] = b;
                array2[slot] = b;
                array3[slot] = b;
                array4[slot] = b;
                array5[slot] = b;
                array6[slot] = b;
                array7[slot] = b;
                array8[slot] = b;
                array9[slot] = b;
                array10[slot] = b;
                array11[slot] = b;
                array12[slot] = b;
                array13[slot] = b;
                array14[slot] = b;
            }
            return System.nanoTime() - start;
        }
    
        // 分两次循环写
        public static long runCaseTwo() {
            long start = System.nanoTime();
            int i = ITERATIONS;
            while (--i != 0) {
                int slot = i & MASK;
                byte b = (byte) i;
                array1[slot] = b;
                array2[slot] = b;
                array3[slot] = b;
                array4[slot] = b;
                array5[slot] = b;
                array6[slot] = b;
                array7[slot] = b;
            }
            i = ITERATIONS;
            while (--i != 0) {
                int slot = i & MASK;
                byte b = (byte) i;
                array8[slot] = b;
                array9[slot] = b;
                array10[slot] = b;
                array11[slot] = b;
                array12[slot] = b;
                array13[slot] = b;
                array14[slot] = b;
            }
            return System.nanoTime() - start;
        }
    }

## 2 测试环境 ##

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 1][]

## 3 测试结果 ##

> SingleLoop duration (ns) = 19421047700
> 
> SplitLoop  duration (ns) = 11710882200

## 4 说明 ##

分两次循环写的性能优于循环中一次写，这处决于机器的 WC Buffer的个数。

不同机器的WC Buffer个数是不一样的，一般都是4个，但从测试来看，笔者的机器是8个。

[https_www.cnblogs.com_liushaodong_p_4777308.html]: https://www.cnblogs.com/liushaodong/p/4777308.html
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70]: /images/20210724/250c1a8c074b4a9bbd4db2a7245d4e2e.png
[20210619200645365.png]: /images/20210724/67ac71f9f21749059975e1c53be67c55.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 1]: /images/20210724/656e615881e14ed9b6cdcc70ce74055e.png