浅谈 Cache

来源：互联网发布：夜神模拟器修改mac 编辑：程序博客网时间：2024/06/04 00:05

先来看一段代码：

#include <Windows.h>#include <stdio.h>int x, y, r1, r2;HANDLE Event1, Event2, EventMain1, EventMain2;DWORD_stdcallThreadProc1 (     LPVOID lpThreadParameter    ){    while (TRUE) {        WaitForSingleObject(Event1, INFINITE);        while (rand() % 8 != 0);        x = 1;        r1 = y;        SetEvent(EventMain1);    }}DWORD_stdcallThreadProc2 (     LPVOID lpThreadParameter    ){    while (TRUE) {        WaitForSingleObject(Event2, INFINITE);        while (rand() % 8 != 0);        y = 1;        r2 = x;        SetEvent(EventMain2);    }}void main(){    HANDLE Thread1, Thread2;    Thread1 = CreateThread(NULL, 0, ThreadProc1, NULL, 0, NULL);    Thread2 = CreateThread(NULL, 0, ThreadProc2, NULL, 0, NULL);    SetThreadAffinityMask(Thread1, 1);    SetThreadAffinityMask(Thread2, 2);    Event1 = CreateEvent(NULL, FALSE, FALSE, NULL);    Event2 = CreateEvent(NULL, FALSE, FALSE, NULL);    EventMain1 = CreateEvent(NULL, FALSE, FALSE, NULL);    EventMain2 = CreateEvent(NULL, FALSE, FALSE, NULL);    int dectected = 0;    for (int i = 0; ;i++) {        x = 0;        y = 0;        SetEvent(Event1);        SetEvent(Event2);        WaitForSingleObject(EventMain1, INFINITE);        WaitForSingleObject(EventMain2, INFINITE);        if (r1 == 0 && r2 == 0) {            printf("detected %d, i %d\n", dectected++, i);        }    }}

描述一下这段代码的重点，起两个线程在两个cpu 核心上执行，分别执行 x = 1, r1= y; y = 1, r2 = x；无论哪个线程先执行，或一起执行，按照常理执行结果一般为 r1 = 1 或 r2 = 1 或者两个都为 1，也就是在 main 函数中的判断 r1 和 r2 同为 0 的判断是不可能成立的，打印也不会出现。

当你真实在一台至少双核机器上运行时，你应该不会相信你的眼睛。

detected 305, i 202654detected 306, i 204704detected 307, i 205029detected 308, i 206454detected 309, i 207135detected 310, i 208178detected 311, i 208610detected 312, i 208879detected 313, i 208896detected 314, i 208960detected 315, i 209003detected 316, i 209869detected 317, i 210573detected 318, i 211259detected 319, i 211275detected 320, i 211487detected 321, i 212646detected 322, i 212963detected 323, i 213317detected 324, i 213382detected 325, i 213473detected 326, i 213588detected 327, i 214940detected 328, i 215055detected 329, i 215271

（由于贴图不方便，直接把本机的打印复制下来了）

从打印中可以得出，在两个线程运行完后，r1== r2 == 0 的情况居然出现了，难道 CPU 在逗我们么。

CPU 没有逗我们，是我们不够了解它。（待续）

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