提振.线程在1.58中醒来太晚了

Boost.Thread wakes up too late in 1.58

本文关键字：醒来 线程 提振      更新时间：2023-10-16

我有一个需要在某些窗口内工作的应用程序(在这种情况下，窗口之间的间隔为30秒)。当时间不在一个窗口内时，计算到下一个窗口中间的时间，并且线程休眠这段时间(以毫秒为单位，使用boost::this_thread::sleep_for)。

使用Boost 1.55，我能够在我的容忍范围内(+/-100ms)以极高的可靠性击中窗口。在迁移到Boost 1.58后，我永远无法击中这些窗口。用std::this_thread::sleep_for代替boost::this_thread::sleep_for解决了这个问题;但是，我需要boost::thread的可中断特性和boost::this_thread::sleep_for提供的中断点。

下面是一些示例代码来说明这个问题: 
#include <boost/thread.hpp>
#include <boost/chrono.hpp>
#include <chrono>
#include <iostream>
#include <thread>
void boostThreadFunction ()
{
   std::cout << "Starting Boost thread" << std::endl;
   for (int i = 0; i < 10; ++i)
   {
      auto sleep_time = boost::chrono::milliseconds {29000 + 100 * i};
      auto mark = std::chrono::steady_clock::now ();
      boost::this_thread::sleep_for (sleep_time);
      auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(
         std::chrono::steady_clock::now () - mark);
      std::cout << "Boost thread:" << std::endl;
      std::cout << "tSupposed to sleep for:t" << sleep_time.count () 
                << " ms" << std::endl;
      std::cout << "tActually slept for:t" << duration.count () 
                << " ms" << std::endl << std::endl;
   }
}
void stdThreadFunction ()
{
   std::cout << "Starting Std thread" << std::endl;
   for (int i = 0; i < 10; ++i)
   {
      auto sleep_time = std::chrono::milliseconds {29000 + 100 * i};
      auto mark = std::chrono::steady_clock::now ();
      std::this_thread::sleep_for (sleep_time);
      auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(
         std::chrono::steady_clock::now () - mark);
      std::cout << "Std thread:" << std::endl;
      std::cout << "tSupposed to sleep for:t" << sleep_time.count () 
                << " ms" << std::endl;
      std::cout << "tActually slept for:t" << duration.count () 
                << " ms" << std::endl << std::endl;
   }
}
int main ()
{
   boost::thread boost_thread (&boostThreadFunction);
   std::this_thread::sleep_for (std::chrono::seconds (10));
   std::thread std_thread (&stdThreadFunction);
   boost_thread.join ();
   std_thread.join ();
   return 0;
}

以下是引用Boost 1.58作为包含目录并在我的工作站(Windows 7 64位)上运行时的输出:

Starting Boost thread
Starting Std thread
Boost thread:
        Supposed to sleep for:  29000 ms
        Actually slept for:     29690 ms
Std thread:
        Supposed to sleep for:  29000 ms
        Actually slept for:     29009 ms
Boost thread:
        Supposed to sleep for:  29100 ms
        Actually slept for:     29999 ms
Std thread:
        Supposed to sleep for:  29100 ms
        Actually slept for:     29111 ms
Boost thread:
        Supposed to sleep for:  29200 ms
        Actually slept for:     29990 ms
Std thread:
        Supposed to sleep for:  29200 ms
        Actually slept for:     29172 ms
Boost thread:
        Supposed to sleep for:  29300 ms
        Actually slept for:     30005 ms
Std thread:
        Supposed to sleep for:  29300 ms
        Actually slept for:     29339 ms
Boost thread:
        Supposed to sleep for:  29400 ms
        Actually slept for:     30003 ms
Std thread:
        Supposed to sleep for:  29400 ms
        Actually slept for:     29405 ms
Boost thread:
        Supposed to sleep for:  29500 ms
        Actually slept for:     29999 ms
Std thread:
        Supposed to sleep for:  29500 ms
        Actually slept for:     29472 ms
Boost thread:
        Supposed to sleep for:  29600 ms
        Actually slept for:     29999 ms
Std thread:
        Supposed to sleep for:  29600 ms
        Actually slept for:     29645 ms
Boost thread:
        Supposed to sleep for:  29700 ms
        Actually slept for:     29998 ms
Std thread:
        Supposed to sleep for:  29700 ms
        Actually slept for:     29706 ms
Boost thread:
        Supposed to sleep for:  29800 ms
        Actually slept for:     29998 ms
Std thread:
        Supposed to sleep for:  29800 ms
        Actually slept for:     29807 ms
Boost thread:
        Supposed to sleep for:  29900 ms
        Actually slept for:     30014 ms
Std thread:
        Supposed to sleep for:  29900 ms
        Actually slept for:     29915 ms

我希望std::thread和boost::thread的睡眠时间相同;然而，当要求睡眠29.1 - 29.9秒时，boost::thread似乎想要睡眠约30秒。是我误用了boost::thread接口，还是这是1.55以来引入的错误?

我是提交上述更改到Boost.Thread的人。1.58中的这一变化是在与Boost社区和微软进行了一段时间的磋商后设计出来的，它可能会极大地改善移动设备的电池寿命。c++标准不保证任何定时等待实际等待，或等待正确的周期，或任何接近正确的周期。因此，任何假设定时等待工作或准确的代码都是有bug的。未来的Microsoft STL可能会对Boost进行类似的更改。线程，因此STL行为将与Boost.Thread相同。我可能会补充说，在任何非实时操作系统上，任何时间等待都是不可预测的，可能会比请求的时间晚很多。因此，社区认为此更改有助于暴露STL的错误使用。

该更改允许Windows可选地延迟一定数量的计时器。它实际上可能不会这样做，实际上只是试图延迟常规中断，作为Windows最新版本的无麻烦内核设计的一部分。即使您指定了周的公差，因为正确的截止日期总是发送给Windows，计时器到期后发生的下一个系统中断总是会触发计时器，所以计时器最多不会延迟超过几秒钟。

这个修改修复的一个bug是系统睡眠问题。之前的实现可能会因为系统休眠而感到困惑，因为定时等待永远不会唤醒(好吧，29天后它们会唤醒)。这个实现使用Boost正确地处理了系统休眠和代码的随机挂起。由系统休眠引起的线程问题已经成为过去。

最后，我个人认为定时等待需要在STL中保证硬度/柔软度。然而，这是一个相当大的变化。即使实现了，除了在硬实时操作系统上，定时等待的困难也只能是最好的努力。这就是为什么它们一开始就被排除在c++标准之外，因为在移动设备功耗被认为重要到足以修改api之前，c++ 11就已经完成了。

尼尔

从Windows上的Boost 1.58开始，sleep_for()利用SetWaitableTimerEx()(而不是SetWaitableTimer())传递一个容忍时间来利用合并计时器。

在lib/thread/src/win32/thread.cpp中，公差为睡眠时间的5%或32ms，以较大者为准:

// Preferentially use coalescing timers for better power consumption and timer accuracy
    if(!target_time.is_sentinel())
    {
        detail::timeout::remaining_time const time_left=target_time.remaining_milliseconds();
        timer_handle=CreateWaitableTimer(NULL,false,NULL);
        if(timer_handle!=0)
        {
            ULONG tolerable=32; // Empirical testing shows Windows ignores this when <= 26
            if(time_left.milliseconds/20>tolerable)  // 5%
                tolerable=time_left.milliseconds/20;
            LARGE_INTEGER due_time=get_due_time(target_time);
            bool const set_time_succeeded=detail_::SetWaitableTimerEx()(timer_handle,&due_time,0,0,0,&detail_::default_reason_context,tolerable)!=0;
            if(set_time_succeeded)
            {
                timeout_index=handle_count;
                handles[handle_count++]=timer_handle;
            }
        }
    }

由于29.1秒的5%是1.455秒，这就解释了为什么使用boost::sleep_for的睡眠时间是如此不准确。

如果我需要sleep_for:

的可中断性，我使用这段代码作为变通方法
        ::Sleep(20);
        boost::this_thread::interruption_point();
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