From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> |
Cc: | PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: PSA: we lack TAP test coverage on NetBSD and OpenBSD |
Date: | 2019-01-22 17:12:29 |
Message-ID: | 11101.1548177149@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> writes:
>> I'm not following this argument. The test case is basically useless
>> for its intended purpose with that parameter, because it's highly
>> likely that the failure mode it's supposedly checking for will be
>> masked by the "random" function's tendency to spit out the same
>> value all the time.
> The first value is taken about 75% of the time for N=1000 and s=2.5, which
> means that a non deterministic implementation would succeed about 0.75² ~
> 56% of the time on that one.
Right, that's about what we've been seeing on OpenBSD.
> Also, the drawing procedure is less efficient when the parameter is close
> to 1 because it is more likely to loop,
That might be something to fix, but I agree it's a reason not to go
overboard trying to flatten the test case's distribution right now.
> If you want something more drastic, using 1.5 instead of 2.5 would reduce
> the probability of accidentaly passing the test by chance to about 20%, so
> it would fail 80% of the time.
I think your math is off; 1.5 works quite well here. I saw one failure
to produce distinct values in 20 attempts. It's not demonstrably slower
than 2.5 either. (1.1 is measurably slower; probably not by enough for
anyone to care, but 1.5 is good enough for me.)
regards, tom lane
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