Question

我在我的函数中使用chrono::high_resolution_clock::now()来计算循环花费多长时间来获得x次重复的估计完成时间。但是，当使用future从main调用时，它会给出错误的时间。我使用过Visual C ++和intel c编译器。怎么解决这个问题？

Answer 1

这是一个已知的bug in VC和其他人，标题使用的系统时钟分辨率不是很高。如果你想要真正准确的计时，可以使用boost或rdtsc。

使用boost你会像使用std :: chrono一样使用它，链接中的例子：

 boost::chrono::high_resolution_clock::time_point t3 = boost::chrono::high_resolution_clock::now();
 boost::chrono::high_resolution_clock::time_point t4 = boost::chrono::high_resolution_clock::now();
 while (t3==t4 ) {
    t4 = boost::chrono::high_resolution_clock::now();
 }
 boost::chrono::nanoseconds ns = t4 - t3;

rdtsc的可能实现

// On some processors in certain circumstances the value can be off
// on some old processors rdtsc doesn't exist.
// only valid for x86/x64.
// only valid when no thread switch happens between calls.
// will be wrong if cpu uses turbo mode or changes speed.
// ToDO check where. make fallback timer.
// will blow up if you run over the epoch, shouldn't happen as the epoch is ~136 years
// beware of cpu instruction reordering if timing very short functions.
static uint64_t tick() noexcept {
    unsigned lo, hi;
    asm volatile("rdtsc" : "=a" (lo), "=d" (hi));
    return int64_t(hi) << 32 | lo;
}

如何在线程中使用chrono :: high_resolution_clock :: now（）

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