CPU 寄存器和缓存一致性

发布于 2024-08-15 20:02:58 字数 634 浏览 7 评论 0原文

当涉及到 MESI 等缓存一致性协议时，CPU 寄存器和 CPU 缓存之间有什么关系？如果某个值存储在CPU的缓存中，并且还存储在寄存器中，那么如果缓存行被标记为“脏”，会发生什么情况？据我了解，即使缓存已更新（由于 MESI），也不能保证寄存器会更新其值。

Hench 这段代码：（

 static void Main()  
  {  
  bool complete = false;   
  var t = new Thread (() =>  
  {  
    bool toggle = false;  
    while (!complete) toggle = !toggle;  
  });  
  t.Start();  
  Thread.Sleep (1000);  
  complete = true;  
  t.Join();        // Blocks indefinitely  
}

假设编译器没有优化循环外“完整”的加载）
据我了解，第二个线程看不到“完成”的更新，因为它的值保存在寄存器内（但是CPU 2的缓存已更新）。

放置内存屏障是否会强制“刷新”所有寄存器？寄存器和缓存有什么关系？寄存器和内存屏障又如何呢？

原文

What's the relation between CPU registers and CPU cache when it comes to cache coherence protocols such as MESI? If a certain value is stored in the CPU's cache, and is also stored in a register, then what will happen if the cache line will be marked as "dirty"? To my understanding there is no guarantee that the register will update it's value even though the cache was updated (due to MESI).

Hench this code:

 static void Main()  
  {  
  bool complete = false;   
  var t = new Thread (() =>  
  {  
    bool toggle = false;  
    while (!complete) toggle = !toggle;  
  });  
  t.Start();  
  Thread.Sleep (1000);  
  complete = true;  
  t.Join();        // Blocks indefinitely  
}

(let's assume the compiler didn't optimized the load for 'complete' outside the loop)
to my understanding, the update to "complete" isn't visible to the second thread since it's value is held inside a register (CPU 2's cache was update however).

Does placing a memory barrier forces to "flush" all of the registers? What's the relation of registers to the cache? and what about registers and memory barriers?

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凡尘雨 2024-08-22 20:02:58

没有关系。使用“易失性”关键字。

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断肠人 2024-08-22 20:02:58

现代 C++ | C++11

你应该使用 std::atomic

#include <thread>
#include <atomic>
#include <chrono>

std::atomic_bool g_exit{ false }, g_exited{ false };

using namespace std::chrono_literals;

void fn()
{
    while (!g_exit)
    {
        // do something (lets say it took 5s)
        std::this_thread::sleep_for(5s);
    }

    g_exited = true;
}

int main()
{
    std::thread wt(fn);
    wt.detach();

    // do something (lets say it took 2s)
    std::this_thread::sleep_for(2s);

    // Exit

    g_exit = true;

    for (int i = 0; i < 5; i++) { 
        std::this_thread::sleep_for(1s);
        if (g_exited) {
            break;
        }
    }
}

Modern C++ | C++11

You should use std::atomic

#include <thread>
#include <atomic>
#include <chrono>

std::atomic_bool g_exit{ false }, g_exited{ false };

using namespace std::chrono_literals;

void fn()
{
    while (!g_exit)
    {
        // do something (lets say it took 5s)
        std::this_thread::sleep_for(5s);
    }

    g_exited = true;
}

int main()
{
    std::thread wt(fn);
    wt.detach();

    // do something (lets say it took 2s)
    std::this_thread::sleep_for(2s);

    // Exit

    g_exit = true;

    for (int i = 0; i < 5; i++) { 
        std::this_thread::sleep_for(1s);
        if (g_exited) {
            break;
        }
    }
}

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