Horton

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

Hannu Krosing <hannuk@google.com>

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From: Hannu Krosing <hannuk@google.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Peter Eisentraut <peter@eisentraut.org>, "Andrey M. Borodin" <x4mmm@yandex-team.ru>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2024-07-02T17:31:21Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Force LC_NUMERIC to C while running TAP tests.

  2. Minor tweaks for pg_test_timing.

  3. Change pg_test_timing to measure in nanoseconds not microseconds.

Attachments

Hi Tom,

On various Intel CPUs I got either steps close to single nanosecond or
sometimes a little more on older ones

One specific CPU moved in in 2 tick increments while the ration to ns
was 2,1/1 or 2100 ticks per microsecond.

On Zen4 AMD the step seems to  be 10 ns, even though the tick-to-ns
ratio is 2.6 / 1 , so reading ticks directly gives 26, 54, ...

Also, reading directly in ticks on M1 gave "loop time including
overhead: 2.13 ns" (attached code works on Clang, not sure about GCC)


I'll also take a look at the docs and try to propose something

Do we also need tests for this one ?

----
Hannu



On Tue, Jul 2, 2024 at 7:20 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> BTW, getting back to the original point of the thread: I duplicated
> Hannu's result showing that on Apple M1 the clock tick seems to be
> about 40ns.  But look at what I got with the v2 patch on my main
> workstation (full output attached):
>
> $ ./pg_test_timing
> ...
> Per loop time including overhead: 16.60 ns
> ...
> Timing durations less than 128 ns:
>       ns   % of total  running %      count
>       15       3.2738     3.2738    5914914
>       16      49.0772    52.3510   88668783
>       17      36.4662    88.8172   65884173
>       18       9.5639    98.3810   17279249
>       19       1.5746    99.9556    2844873
>       20       0.0416    99.9972      75125
>       21       0.0004    99.9976        757
> ...
>
> It sure looks like this is exact-to-the-nanosecond results,
> since the modal values match the overall per-loop timing,
> and there are no zero measurements.
>
> This is a Dell tower from 2021, running RHEL8 on an Intel Xeon W-2245.
> Not exactly top-of-the-line stuff.
>
>                         regards, tom lane
>