使用 Atomic Builtins 旋转线程屏障

Question

我正在尝试使用原子实现旋转线程屏障，特别是 __sync_fetch_and_add。 https://gcc.gnu.org/onlinedocs/gcc-4.4.5/gcc/Atomic-Builtins.html

我基本上想要 pthread barrier 的替代品。我在可以运行大约一百个并行线程的系统上使用 Ubuntu。

int bar = 0;                      //global variable
 int P = MAX_THREADS;              //number of threads

 __sync_fetch_and_add(&bar,1);     //each thread comes and adds atomically
 while(bar<P){}                    //threads spin until bar increments to P
 bar=0;                            //a thread sets bar=0 to be used in the next spinning barrier

由于显而易见的原因，这不起作用（一个线程可能设置 bar=0，而另一个线程陷入无限 while 循环等）。我在这里看到了一个实现：Writing a (spinning) thread barrier using c++11 atomics，但是它看起来太复杂了，我认为它的性能可能比 pthread barrier 差。

由于 bar 的高速缓存行在线程之间进行 ping-pong，此实现预计还会在内存层次结构中产生更多流量。

关于如何使用这些原子指令制作简单的屏障有什么想法吗？另外，通信优化方案也会有所帮助。

Answer 1

与其在线程的计数器上自旋，不如在通过的障碍数上自旋更好，这将仅由最后一个线程递增，面临障碍。这样您还可以减少内存缓存压力，因为旋转变量现在仅由单个线程更新。

int P = MAX_THREADS;
int bar = 0; // Counter of threads, faced barrier.
volatile int passed = 0; // Number of barriers, passed by all threads.

void barrier_wait()
{
    int passed_old = passed; // Should be evaluated before incrementing *bar*!

    if(__sync_fetch_and_add(&bar,1) == (P - 1))
    {
        // The last thread, faced barrier.
        bar = 0;
        // *bar* should be reseted strictly before updating of barriers counter.
        __sync_synchronize(); 
        passed++; // Mark barrier as passed.
    }
    else
    {
        // Not the last thread. Wait others.
        while(passed == passed_old) {};
        // Need to synchronize cache with other threads, passed barrier.
        __sync_synchronize();
    }
}

请注意，您需要使用 volatile 修饰符来旋转变量。

C++ 代码可能比 C 代码快一些，因为它可以使用 acquire/release 内存屏障而不是 full 一个，这是 __sync 函数唯一可用的屏障：

int P = MAX_THREADS;
std::atomic<int> bar = 0; // Counter of threads, faced barrier.
std::atomic<int> passed = 0; // Number of barriers, passed by all threads.

void barrier_wait()
{
    int passed_old = passed.load(std::memory_order_relaxed);

    if(bar.fetch_add(1) == (P - 1))
    {
        // The last thread, faced barrier.
        bar = 0;
        // Synchronize and store in one operation.
        passed.store(passed_old + 1, std::memory_order_release);
    }
    else
    {
        // Not the last thread. Wait others.
        while(passed.load(std::memory_order_relaxed) == passed_old) {};
        // Need to synchronize cache with other threads, passed barrier.
        std::atomic_thread_fence(std::memory_order_acquire);
    }
}

使用 Atomic Builtins 旋转线程屏障

Spinning thread barrier using Atomic Builtins

c++

multithreading

atomic

barrier

compare-and-swap