BTW, returning to the original topic of this thread:
The new exact-delays table from pg_test_timing is really quite
informative. For example, on my M4 Macbook:
Observed timing durations up to 99.9900%:
ns % of total running % count
0 62.2124 62.2124 118127987
41 12.5826 74.7951 23891661
42 25.1653 99.9604 47783489
83 0.0194 99.9797 36744
84 0.0096 99.9893 18193
125 0.0020 99.9913 3784
...
31042 0.0000 100.0000 1
The fact that the clock tick is about 40ns is extremely
obvious in this presentation.
Even more interesting is what I got from an ancient PPC Macbook
(mamba's host, running NetBSD):
Testing timing overhead for 3 seconds.
Per loop time including overhead: 731.26 ns
...
Observed timing durations up to 99.9900%:
ns % of total running % count
705 39.9162 39.9162 1637570
706 17.6040 57.5203 722208
759 18.6797 76.2000 766337
760 23.7851 99.9851 975787
813 0.0002 99.9853 9
814 0.0004 99.9857 17
868 0.0001 99.9858 4
922 0.0001 99.9859 3
...
564950 0.0000 100.0000 1
I had previously reported that that machine had microsecond
timing precision, but this shows that the real problem is that
it takes most of a microsecond to go 'round the timing loop.
It seems clear that the system clock ticks about every 50ns,
even on this decades-old hardware.
regards, tom lane
