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

From: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
To: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
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: 2018-07-12 18:27:14
Message-ID: alpine.DEB.2.21.1807122013040.27883@lancre
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>>> I don't understand the 0.5 second rule. For the tests, we only need to
>>> ensure that at least one progress report is printed, right?
>>
>> [...]
>
> I still don't understand.

Let's look at the code:

if (progress && thread->tid == 0)
{
...
if (last_report == thread_start || now - last_report >= 500000)
doProgressReport(thread, &last, &last_report, now, thread_start);

> 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.

>> 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.

>> 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.

--
Fabien.

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