Summary of the use of TIMERFD

TIMERFD is a timer interface that Linux provides for user programs. This interface, which is based on a file descriptor, is timed out by a readable event of the file descriptor, so it can be used for select/poll scenarios. TIMERFD is an excuse to add to the Linux kernel version 2.6.25. TIMERFD, EVENTFD, SIGNALFD with epoll use, you can construct a 0 polling program, but the program does not handle events, the program is blocked. In this case, the program is more power-saving on some mobile devices.
The Clock_gettime function can obtain the system clock, accurate to nanosecond. Libraries need to be specified at compile time:-LRT. You can get two types of events: Clock_realtime: Relative time, from 1970.1.1 to the current time. Changing the system time changes the obtained value. That is, it is in the system time coordinates. Clock_monotonic: In contrast to Clock_realtime, it is based on absolute time, the time it takes to reboot the system to the present time, and the change in system time alignment has no effect.
Timerfd_create: Generates a Timer object that returns the file descriptor associated with it. Receive two incoming parameters, one is Clockid, fill in clock_realtime or clock_monotonic, the parameter meaning ditto. The second can pass the control flag: Tfd_nonblock (non-blocking), tfd_cloexec (same o_cloexec)
Note: The progress of TIMERFD is higher than usleep.
Timerfd_settime: Ability to start and stop timers; You can set a second parameter: flags,0 is a relative timer, Tfd_timer_abstime is an absolute timer. The third parameter sets the timeout time, and if 0 indicates the stop timer. Timer Set Timeout method: 1, set timeout time is required to call Clock_gettime to get the current time, if it is an absolute timer, then need to get clock_realtime, in addition to the time to timeout. If it's a relative timer, get clock_monotonic time.                2, data structure: struct TIMESPEC {time_t tv_sec; * Seconds * *
Long tv_nsec; * nanoseconds * *
};

struct Itimerspec {
struct Timespec it_interval; /* Interval for periodic timer */
struct Timespec it_value; /* Initial Expiration * *
}; It_value is the first timeout, the time required to fill out from Clock_gettime, plus the time to timeout.      It_interval is the subsequent periodic timeout and how much time is required.            Pay attention to a place that is easy to make mistakes: tv_nsec plus go after must determine whether to exceed 1000000000 (if more than a second plus one), or set a failure.      A it_interval of not 0 indicates a periodic timer. Both It_value and It_interval are 0 to stop the timer.
Note: The first parameter of Timerfd_create and Clock_gettime is the second parameter of Clock_realtime or clock_monotonic,timerfd_settime is 0 (relative timer) or TFD _timer_abstime, the relationship between the three: 1, if the timerfd_settime is set to Tfd_timer_abstime (determines the time), then the time after must be obtained by Clock_gettime, and set clock_ when getting Realtime or clock_monotonic depends on the value of the Timerfd_create setting.     2, if the Timerfd_settime set to 0 (relative timer), then the time must be followed by relative time, that is: New_value.it_value.tv_nsec = 500000000;     New_value.it_value.tv_sec = 3;     new_value.it_interval.tv_sec = 0; New_value.it_interval.tv_nsec = 10000000;
The read function reads Timerfd, reads as uint_64, and indicates the number of timeouts.
TIMERFD Simple performance test: Apply for 1000 timers, super time positioning 1s, timeout once per second, found CPU occupancy rate in 3.0G CPU on about 1%, 10,000 timers and then 7%, and will not occur at the same time timeout of two cases, if there is printf to the foreground , the timer timeout is generally repeated (3-5) before callback.