Understanding the different clocks of clock_gettime()

Question

Hi I wanted to use the clock_gettime() function for measuring the performance of my code.

I am unable to understand the difference between the different kinds of clocks used in the function from the man page descriptions. esp

CLOCK_REALTIME,

CLOCK_PROCESS_CPUTIME_ID

CLOCK_THREAD_CPUTIME_ID

Can someone explaing what each of these clocks do?

Answer 1

CLOCK_REALTIME reports the actual wall clock time.

CLOCK_MONOTONIC is for measuring relative real time. It advances at the same rate as the actual flow of time but it's not subject to discontinuities from manual or automatic (NTP) adjustments to the system clock.

CLOCK_PROCESS_CPUTIME_ID is for measuring the amount of CPU time consumed by the process.

CLOCK_THREAD_CPUTIME_ID is for measuring the amount of CPU time consumed by the thread. It's supported by modern kernels and glibc since 2.6.12, but on older linux kernels glibc emulates it badly by simply returning the amount of CPU time consumed by the process since the moment the thread was created.

http://man7.org/linux/man-pages/man2/clock_gettime.2.html