C++11 wait_for timeout

#include <future>
#include <iostream>
#include <string>

int DoTimeCostWork(int nId)
{
//      std::this_thread::sleep_for(std::chrono::milliseconds(10));
        return nId;
}

int main()
{
        for ( int i = 0 ; i < 100000 ; i ++ )
        {
                std::future<int> fut = std::async(std::launch::async,DoTimeCostWork, i);

                std::chrono::steady_clock::duration tSpan(300000);
                if (fut.wait_for(tSpan) == std::future_status::timeout)
                {
                        std::cout << "timeout" << std::endl;
                }
                else
                        std::cout << "ret := " << fut.get() << std::endl;
        }
        return 0;
}

Why does the timeout occur from time to time? Through strace, the following error messages are found。

11:13:45.333546 clock_gettime(CLOCK_REALTIME, {1568862825, 333557480}) = 0 <0.000009> 11:13:45.333575 futex(0xa88064, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 1, {1568862825, 333857480}, ffffffff) = -1 ETIMEDOUT (Connection timed out) <0.000760> 11:13:45.334370 clock_gettime(CLOCK_REALTIME, {1568862825, 334387226}) = 0 <0.000010>

How to solve timeout？