C++如何在Linux中使计时器准确

Question

考虑这段代码:

#include <iostream>
#include <vector>
#include <functional>
#include <map>
#include <atomic>
#include <memory>
#include <chrono>
#include <thread>
#include <boost/asio.hpp>
#include <boost/thread.hpp>
#include <boost/asio/high_resolution_timer.hpp>

static const uint32_t FREQUENCY = 5000; // Hz
static const uint32_t MKSEC_IN_SEC = 1000000;

std::chrono::microseconds timeout(MKSEC_IN_SEC / FREQUENCY);
boost::asio::io_service ioservice;
boost::asio::high_resolution_timer timer(ioservice);

static std::chrono::system_clock::time_point lastCallTime = std::chrono::high_resolution_clock::now();
static uint64_t deviationSum = 0;
static uint64_t deviationMin = 100000000;
static uint64_t deviationMax = 0;
static uint32_t counter = 0;

void timerCallback(const boost::system::error_code &err) {
  auto actualTimeout = std::chrono::high_resolution_clock::now() - lastCallTime;
  std::chrono::microseconds actualTimeoutMkSec = std::chrono::duration_cast<std::chrono::microseconds>(actualTimeout);
  long timeoutDeviation = actualTimeoutMkSec.count() - timeout.count();
  deviationSum += abs(timeoutDeviation);
  if(abs(timeoutDeviation) > deviationMax) {
    deviationMax = abs(timeoutDeviation);
  } else if(abs(timeoutDeviation) < deviationMin) {
    deviationMin = abs(timeoutDeviation);
  }

  ++counter;
  //std::cout << "Actual timeout: " << actualTimeoutMkSec.count() << "\t\tDeviation: " << timeoutDeviation << "\t\tCounter: " << counter << std::endl;

  timer.expires_from_now(timeout);
  timer.async_wait(timerCallback);
  lastCallTime = std::chrono::high_resolution_clock::now();
}

using namespace std::chrono_literals;

int main() {
  std::cout << "Frequency: " << FREQUENCY << " Hz" << std::endl;
  std::cout << "Callback should be called each: " << timeout.count() << " mkSec" << std::endl;
  std::cout << std::endl;

  ioservice.reset();
  timer.expires_from_now(timeout);
  timer.async_wait(timerCallback);
  lastCallTime = std::chrono::high_resolution_clock::now();
  auto thread = new std::thread([&] { ioservice.run(); });
  std::this_thread::sleep_for(1s);

  std::cout << std::endl << "Messages posted: " << counter << std::endl;
  std::cout << "Frequency deviation: " << FREQUENCY - counter << std::endl;
  std::cout << "Min timeout deviation: " << deviationMin << std::endl;
  std::cout << "Max timeout deviation: " << deviationMax << std::endl;
  std::cout << "Avg timeout deviation: " << deviationSum / counter << std::endl;

  return 0;
}

它运行定时器以指定频率定期调用 timerCallback(..)。在此示例中，回调必须每秒调用 5000 次。可以玩一下频率，看看实际(测量的)调用频率与期望频率不同。事实上，频率越高，偏差越大。我用不同的频率做了一些测量，这里是总结: https://docs.google.com/spreadsheets/d/1SQtg2slNv-9VPdgS0RD4yKRnyDK1ijKrjVz7BBMSg24/edit?usp=sharing

当所需频率为 10000Hz 时，系统会错过 10%(~ 1000)的调用。 当所需频率为 100000Hz 时，系统会错过 40%(~ 40000)的调用。

问题:在Linux\C++环境下是否可以达到更好的精度？怎么做？我需要它以 500000Hz 的频率工作而没有明显偏差

附言我的第一个想法是它是 timerCallback(..) 方法本身的主体导致延迟。我测量了它。执行时间稳定少于 1 微秒。所以不影响进程。

Answer 1

我自己在这个问题上没有经验，但我猜想(如引用资料所述)操作系统的调度程序以某种方式干扰了您的回调。 因此，您可以尝试使用实时调度程序并尝试将任务的优先级更改为更高的优先级。

希望这能给你找到答案的方向。

调度器: http://gumstix.8.x6.nabble.com/High-resolution-periodic-task-on-overo-td4968642.html

优先级: https://linux.die.net/man/3/setpriority