Chapter 28. Application timestamping

POSIX is a standard for implementing and representing time sources. You can assign a POSIX clock to an application without affecting other applications in the system. This is in contrast to hardware clocks which are selected by the kernel and implemented across the system.

The function used to read a given POSIX clock is clock_gettime(), which is defined at <time.h>. The kernel counterpart to clock_gettime() is a system call. When a user process calls clock_gettime():

However, the context switch from the user application to the kernel has a CPU cost. Even though this cost is very low, if the operation is repeated thousands of times, the accumulated cost can have an impact on the overall performance of the application. To avoid context switching to the kernel, thus making it faster to read the clock, support for the CLOCK_MONOTONIC_COARSE and CLOCK_REALTIME_COARSE POSIX clocks was added, in the form of a virtual dynamic shared object (VDSO) library function.

Time readings performed by clock_gettime(), using one of the _COARSE clock variants, do not require kernel intervention and are executed entirely in user space. This yields a significant performance gain. Time readings for _COARSE clocks have a millisecond (ms) resolution, meaning that time intervals smaller than 1 ms are not recorded. The _COARSE variants of the POSIX clocks are suitable for any application that can accommodate millisecond clock resolution.

The example code output shows using the clock_gettime function with the CLOCK_MONOTONIC_COARSE POSIX clock.

#include <time.h>

main()
{
	int rc;
	long i;
	struct timespec ts;

	for(i=0; i<10000000; i++) {
		rc = clock_gettime(CLOCK_MONOTONIC_COARSE, &ts);
	}
}

You can improve upon the example above by adding checks to verify the return code of clock_gettime(), to verify the value of the rc variable, or to ensure the content of the ts structure is to be trusted.