Re: [HACKERS] Re: [COMMITTERS] pgsql: Remove pgbench "progress" test pending solution of its timing is (fwd)

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL Developers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] Re: [COMMITTERS] pgsql: Remove pgbench "progress" test pending solution of its timing is (fwd)
Date: 2018-07-16 15:35:03
Message-ID: 9af2f1bf-d3e5-f0eb-505f-7967ee9a88ec@iki.fi
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On 12/07/18 21:27, Fabien COELHO wrote:
>> For the testing, we just need to make sure that at least one progress report
>> is always printed, if -P is used. Right?
>
> Yep. That is the first condition above the last_report is set to
> thread_start meaning that there has been no report.
>
>> So where does the 0.5 second rule come in? Can't we just do "if (no
>> progress reports were printed) { print progress report; }" at the end?
>
> The second 0.5s condition is to print a closing report if some time
> significant time passed since the last one, but we do not want to print
> a report at the same second.
>
> pgbench -T 5 -P 2
>
> Would then print report at 2, 4 and 5. 0.5 ensures that we are not going
> to do "2 4.0[00] 4.0[01]" on -t whatever -P 2, which would look silly.
>
> If you do not like it then the second condition can be removed, fine with
> me.

As the code stands, you would get reports at "2 4.0[00]", right? Let's
keep it that way. I think the only change we need to make in the logic
is to check at the end, if *any* progress reports at all have been
printed, and print one if not. And do that only when the -P option is
smaller than the -T option, I suppose.

>>> It also adds a small feature which is that there is always a final
>>> progress when the run is completed, which can be useful when computing
>>> progress statistics, otherwise some transactions could not be reported in
>>> any progress at all.
>>
>> Any transactions in the last 0.5 seconds might still not get reported in any
>> progress reports.
>
> Yep. I wanted a reasonable threshold, given that both -T and -P are in
> seconds anyway, so it probably could only happen with -t.

Oh. I'm a bit surprised we don't support decimals, i.e. -P 0.5.
Actually, it seems to be acceptd, but it's truncated down to the nearest
integer. That's not very nice :-(. But it's a separate issue.

>>> Indeed… but then throttling would not be tested:-) The point of the test
>>> is to exercise all time-related options, including throttling with a
>>> reasonable small value.
>>
>> Ok. I don't think that's really worthwhile. If we add some code that only
>> runs in testing, then we're not really testing the real thing. I wouldn't
>> trust the test to tell much. Let's just leave out that magic environment
>> variable thing, and try to get the rest of the patch finished.
>
> If you remove the environment, then some checks need to be removed,
> because the 2 second run may be randomly shorten when there is nothing to
> do. If not, the test will fail underterminiscally, which is not
> acceptable. Hence the hack. I agree that it is not beautiful.
>
> The more reasonable alternative could be to always last 2 seconds under
> -T 2, even if the execution can be shorten because there is nothing to do
> at all, i.e. remove the environment-based condition but keep the sleep.

That sounds reasonable. It's a bit silly to wait when there's nothing to
do, but it's also weird if the test exits before the specified time is
up. Seems less surprising to always sleep.

- Heikki

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