Feedback-ID: i0fe9450f:Fastmail
Date: Wed, 24 Sep 2025 08:31:27 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Michael Banck <mbanck@gmx.net>, Tom Lane <tgl@sss.pgh.pa.us>,
	pgsql-hackers@lists.postgresql.org,
	Thomas Munro <thomas.munro@gmail.com>
Subject: Re: GNU/Hurd portability patches
Message-ID: <aNMtzyqgin8I4r5H@paquier.xyz>
References: <6862e8d1.050a0220.194b8d.76fa@mx.google.com>
 <3245994.1751388110@sss.pgh.pa.us>
 <68643ea2.050a0220.128f53.3f77@mx.google.com>
 <aGR04gPotq3SOaVo@paquier.xyz>
 <2874644f-6431-41f4-abe2-99e5ab052606@gmail.com>
 <aNB18ZFIXXv-FR6B@paquier.xyz>
 <68d0f931.050a0220.3185e0.19ae@mx.google.com>
 <283c3d69-ef32-4391-9ea3-68e47c9dea31@gmail.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha512;
	protocol="application/pgp-signature"; boundary="io6hRejmZiwGjXrd"
Content-Disposition: inline
In-Reply-To: <283c3d69-ef32-4391-9ea3-68e47c9dea31@gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/aNMtzyqgin8I4r5H%40paquier.xyz>
Precedence: bulk


--io6hRejmZiwGjXrd
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Mon, Sep 22, 2025 at 11:30:00PM +0300, Alexander Lakhin wrote:
> I reproduced the issue locally and found that
> =A0=A0=A0=A0=A0=A0=A0 /* total elapsed time in this function call */
> =A0=A0=A0=A0=A0=A0=A0 INSTR_TIME_SET_CURRENT(total);
> =A0=A0=A0=A0=A0=A0=A0 INSTR_TIME_SUBTRACT(total, fcu->start);
> sometimes gives total.ticks =3D 0.
>=20
> I tried the test program from [2] and got on my VM:
> went backwards 0 out of 10000000 times
> (three times)
>=20
> But I've created my own test program (see attached), which shows:
> for i in {1..1000}; do printf "ITERATION $i "; ./tt 100 || break; done
> =A0ITERATION 1 t1: 55873639081080, t2: 55873639084090, t2 - t1: 3010 (r: =
4950)
> =A0ITERATION 2 t1: 55873641019440, t2: 55873641025700, t2 - t1: 6260 (r: =
4950)
> =A0ITERATION 3 t1: 55873642794200, t2: 55873642797130, t2 - t1: 2930 (r: =
4950)
> ...
> =A0ITERATION 23 t1: 55873675001590, t2: 55873675001590, t2 - t1: 0 (r: 49=
50)
>=20
> I don't know how to test the patch committed, but if you can, that would
> be nice.

We've had this exact same issue of a clock going backwards with one of
the netbsd animals on an older version not supported anymore by
upstream and that has been kicked out of the buildfarm, as far as I
recall.  This has created some disturbance in the regression tests
causing EXPLAIN plan outputs we did not expect, in terms of extra
negative signs and the animal showing red periodically.  So yes, this
random factor would be annoying in the buildfarm.
--
Michael

--io6hRejmZiwGjXrd
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----
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=YCLM
-----END PGP SIGNATURE-----

--io6hRejmZiwGjXrd--