[Someone]

I would be wary of using gettimeofday to measure such short periods. As https://pubs.opengroup.org/onlinepubs/009604599/functions/ge... says, the resolution of the system clock is unspecified, and 20,000 × ≈10 ≈ 200k instructions easily run in under a millisecond on modern hardware.

The benchmark probably get rids of that by doing it 40,000 times in quick succession, but why not measure the time of all 40,000 iterations in one go, and decrease the risk (and the overhead of calling gettimeofday 39,999 times)?

[kllrnohj]

The resolution of the clock is unspecified by the POSIX standard but that does not mean it's unspecified by the platform this is actually running on. If this was trying to be portable code you'd have a big issue there but it's not. It is still limited by gettimeofday maxing out at microsecond precision, though, which is quite poor. And seems to be using a realtime clock, which introduces errors from network time sync & such. That's unlikely to crop up here, but it's still a risk. clock_gettime_nsec_np(CLOCK_MONOTONIC) is what should be used here.

[volta83]

Agree, you probably want to just read some cpu time stamp counter before and after, and make sure you ar pinning your threwads, locking the clocks, etc. so that you can get a reliable time from there, or... just use cycles as your unit of measure.

[titzer]

On Linux and MacOS, gettimeofday is accurate to microseconds. Not only is the returned struct is expressed in microseconds, but I have personally observed that it is accurate to microseconds.