On Mon, Sep 22, 2025 at 11:30:00PM +0300, Alexander Lakhin wrote:
> I reproduced the issue locally and found that
> /* total elapsed time in this function call */
> INSTR_TIME_SET_CURRENT(total);
> INSTR_TIME_SUBTRACT(total, fcu->start);
> sometimes gives total.ticks = 0.
>
> I tried the test program from [2] and got on my VM:
> went backwards 0 out of 10000000 times
> (three times)
>
> But I've created my own test program (see attached), which shows:
> for i in {1..1000}; do printf "ITERATION $i "; ./tt 100 || break; done
> ITERATION 1 t1: 55873639081080, t2: 55873639084090, t2 - t1: 3010 (r: 4950)
> ITERATION 2 t1: 55873641019440, t2: 55873641025700, t2 - t1: 6260 (r: 4950)
> ITERATION 3 t1: 55873642794200, t2: 55873642797130, t2 - t1: 2930 (r: 4950)
> ...
> ITERATION 23 t1: 55873675001590, t2: 55873675001590, t2 - t1: 0 (r: 4950)
>
> I don't know how to test the patch committed, but if you can, that would
> be nice.
We've had this exact same issue of a clock going backwards with one of
the netbsd animals on an older version not supported anymore by
upstream and that has been kicked out of the buildfarm, as far as I
recall. This has created some disturbance in the regression tests
causing EXPLAIN plan outputs we did not expect, in terms of extra
negative signs and the animal showing red periodically. So yes, this
random factor would be annoying in the buildfarm.
--
Michael
