Re: pg_test_timing: fix unit typo and widen diff type

> On Apr 2, 2026, at 12:17, Lukas Fittl <lukas(at)fittl(dot)com> wrote:
>
> Hi Chao,
>
> On Wed, Apr 1, 2026 at 8:10 PM Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:
>>
>> Hi,
>>
>> This morning, as part of my usual routine, I synced the master branch and read through the recent commits. While reading 82c0cb4e672, I noticed a mistake in an error message. The relevant code is like:
>> ```
>> diff = INSTR_TIME_GET_NANOSEC(diff_time);
>>
>> fprintf(stderr, _("Time warp: %d ms\n"), diff);
>> ```
>>
>> Here, “diff" is in nanoseconds, but the error message prints ms as the unit, which is incorrect.
>
> Good catch!
>
> It looks like the use of nanoseconds for "diff" got introduced last
> year in 0b096e379e6f9bd49 (as you note later in the email, today's
> commit didn't actually change that part), CCing Tom and Hannu as
> authors of that earlier change.
>
> That said, its a bit odd that we were using INSTR_TIME_GET_MICROSEC
> there before that earlier commit, but called it "ms" (i.e.
> milliseconds) in the error message.
>
>>
>> To fix that, I think there are two possible options:
>>
>> 1. Use INSTR_TIME_GET_MILLISEC to get “diff"
>> 2. Change “ms" to “ns" in the error message
>>
>> After reading through the whole file, I think option 2 is the right fix. While doing that, I also noticed another issue.
>>
>> “diff" is currently defined as int32. Although one might think that is enough for a time delta, I believe it should be int64 for two reasons:
>>
>> * INSTR_TIME_GET_NANOSEC() explicitly returns int64:
>> ```
>> #define INSTR_TIME_GET_NANOSEC(t) \
>> ((int64) (t).ticks)
>> ```
>>
>> * The current code has a sanity check for backward clock drift:
>> ```
>> /* Did time go backwards? */
>> if (unlikely(diff < 0))
>> {
>> fprintf(stderr, _("Detected clock going backwards in time.\n"));
>> fprintf(stderr, _("Time warp: %d ms\n"), diff);
>> exit(1);
>> }
>> ```
>> Clock jumping forward is also possible, and a forward jump of about 2.14 seconds would overflow int32 when expressed in nanoseconds, making the value appear negative. In that case, the code could report a “backwards” clock jump when the actual jump was forwards, which would be misleading.
>
> I agree it doesn't seem sound to use an int32 for storing the result
> of INSTR_TIME_GET_NANOSEC. It looks like we may also need to adjust
> the call to pg_leftmost_one_pos32 though if we actually accept that
> large a "diff" value, as in your patch.

You are right. Changed to pg_leftmost_one_pos64 in v2.

>
> Maybe we should error out if the diff is larger than an int32, noting
> a positive time drift?

I agree we should warn/fail upon clock forwards drift. But I doubt int32 is too big (~2.14 seconds), I consider 1 second could be a too big threshold. Let’s wait for more voices on this.

>
> Independently of that, its worth noting we could easily emit the diff
> in a larger unit (micro or milliseconds) for easier interpretation, by
> just calling INSTR_TIME_GET_MICROSEC / INSTR_TIME_GET_MILLISEC on the
> "diff_time" again.
>

Given the error should rarely happen, I personally feel that might not be super helpful. Also, if the drift is just beyond the threshold, bumping to microsecond or millisecond might print just 0.

PFA v2 - updated 0002 for pg_leftmost_one_pos64.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/