Thread

Re: What is a typical precision of gettimeofday()?

Hannu Krosing <hannuk@google.com> — 2025-07-08T19:37:07Z

On Tue, Jul 8, 2025 at 8:07 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> BTW, returning to the original topic of this thread:
>
> The new exact-delays table from pg_test_timing is really quite
> informative.

Maybe we should collect some of it in the PostgreSQL Wiki for easy reference ?

I had some interesting results with some RISC-V SBC which were similar
to ARM but
seemed to indicate that the chosen "CPU CLOCK"  used by the
rtdsc-equivalent instruction was counting in exact multiples of a
nanosecond

And "the rtdsc-equivalent instruction" on both ARM and RISC-V is
reading a special register which exposes the faked "clock counter".


> 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

Do we have a fencepost error in the limit code so that it stops before
printing out the 99.9900% limit row ?

The docs in your latest patch indicated that it prints out the row >=  99.9900%

---
Hannu