From: Justin Pryzby <pryzby@telsasoft.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Alexander Korotkov <a.korotkov@postgrespro.ru>
Cc: David Steele <david@pgmasters.net>
Cc: Ildus Kurbangaliev <i.kurbangaliev@gmail.com>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: [HACKERS] Custom compression methods (mac+lz4.h)
Date: Sun, 21 Mar 2021 18:43:24 -0500
Message-ID: <20210321234324.GC4203@telsasoft.com> (raw)
In-Reply-To: <504396.1616368310@sss.pgh.pa.us>
References: <301730.1616276227@sss.pgh.pa.us>
<20210320231957.GX11765@telsasoft.com>
<309337.1616287493@sss.pgh.pa.us>
<311370.1616290403@sss.pgh.pa.us>
<CAFiTN-tN-fi=9VEatyiZyhW28yhhQCmn-egYzbdxBTLkOnVQvg@mail.gmail.com>
<CAFiTN-t0w+Rc2U3S+y=7KWcLuOYNB5MfWeGdNa7+pg0UovVdcQ@mail.gmail.com>
<414943.1616358751@sss.pgh.pa.us>
<20210321204148.GB4203@telsasoft.com>
<456616.1616360715@sss.pgh.pa.us>
<504396.1616368310@sss.pgh.pa.us>
On Sun, Mar 21, 2021 at 07:11:50PM -0400, Tom Lane wrote:
> I wrote:
> > Justin Pryzby <pryzby@telsasoft.com> writes:
> >> On Sun, Mar 21, 2021 at 04:32:31PM -0400, Tom Lane wrote:
> >>> This seems somewhat repeatable (three identical failures in three
> >>> attempts). Not sure why I did not see it yesterday; but anyway,
> >>> there is something wrong with partial detoasting for LZ4.
>
> >> With what version of LZ4 ?
>
> > RHEL8's, which is
> > lz4-1.8.3-2.el8.x86_64
>
> I hate to be the bearer of bad news, but this suggests that
> LZ4_decompress_safe_partial is seriously broken in 1.9.2
> as well:
>
> https://github.com/lz4/lz4/issues/783
Ouch
> Maybe we cannot rely on that function for a few more years yet.
>
> Also, I don't really understand why this code:
>
> /* slice decompression not supported prior to 1.8.3 */
> if (LZ4_versionNumber() < 10803)
> return lz4_decompress_datum(value);
>
> It seems likely to me that we'd get a flat out build failure
> from library versions lacking LZ4_decompress_safe_partial,
> and thus that this run-time test is dead code and we should
> better be using a configure probe if we intend to allow old
> liblz4 versions. Though that might be moot.
The function existed before 1.8.3, but didn't handle slicing.
https://github.com/lz4/lz4/releases/tag/v1.8.3
|Finally, an existing function, LZ4_decompress_safe_partial(), has been enhanced to make it possible to decompress only the beginning of an LZ4 block, up to a specified number of bytes. Partial decoding can be useful to save CPU time and memory, when the objective is to extract a limited portion from a larger block.
Possibly we could allow v >= 1.9.3 || (ver >= 1.8.3 && ver < 1.9.2).
Or maybe not: the second half apparently worked "by accident", and we shouldn't
need to have intimate knowledge of someone else's patchlevel releases,
--
Justin
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To: pgsql-hackers@postgresql.org
Cc: pryzby@telsasoft.com, tgl@sss.pgh.pa.us, dilipbalaut@gmail.com, robertmhaas@gmail.com, andres@anarazel.de, tomas.vondra@2ndquadrant.com, a.korotkov@postgrespro.ru, david@pgmasters.net, i.kurbangaliev@gmail.com, 9erthalion6@gmail.com, x4mmm@yandex-team.ru, pgsql-hackers@lists.postgresql.org
Subject: Re: [HACKERS] Custom compression methods (mac+lz4.h)
In-Reply-To: <20210321234324.GC4203@telsasoft.com>
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