[dan-robertson]

I don’t think this is right. I am pretty doubtful of the discussion of the granularity of the results of clock_gettime, but I failed to find any sources and I don’t have a suitable computer to hand to experiment.

But here are two more doubts:

1. On at least some systems, clock_gettime will be implemented by a vdso call that (on x86) uses rdtsc to give you the time, so you should expect its results to be pretty highly correlated with an rdtsc immediately afterwards, so you aren’t really adding useful entropy (whereas you are losing time-locality of your identifiers which may be desirable if they are to end up as primary keys somewhere)

2. On some systems (eg some AMD processors), rdtsc gave me pretty low precision results (eg rounding to the nearest 10 cycles) so you also won’t necessarily get the entropy you would expect from the description of the instruction. I failed to find a reference for this too apart from an offhand mention in a paper about timing attacks.