c – boost :: thread_sleep vs boost :: chrono :: thread_clock不一致

#include <iostream>
#include <boost/thread.hpp>
#include <boost/chrono/thread_clock.hpp>

void foo() {
    boost::this_thread::sleep(boost::posix_time::microseconds(500));
}


int main() {
    boost::chrono::thread_clock::time_point start = boost::chrono::thread_clock::Now();    
    foo();   
    boost::chrono::thread_clock::time_point stop = boost::chrono::thread_clock::Now();    
    std::cout << "duration = " 
              << boost::chrono::duration_cast<boost::chrono::microseconds>(stop-start).count() 
              << " microsec\n";
}

我得到以下输出：

duration = 121 microsec
duration = 121 microsec
duration = 110 microsec
duration = 114 microsec

此外,当我用不同的值(例如200)替换500时,输出始终大约为100.使用-O3进行编译.为什么时机不一致？

PS：我读到睡眠已被弃用,但是当我用它替换它时

boost::this_thread::sleep_for(boost::chrono::microseconds(200));

输出在~20微秒的范围内.