From: Thomas Munro <tmunro@postgresql.org>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Fix unfairness in all-cached parallel seq scan.
Date: Sat, 31 Aug 2024 05:38:07 +0000
Message-ID: <E1skGoJ-002EFP-4n@gemulon.postgresql.org> (raw)
Fix unfairness in all-cached parallel seq scan.
Commit b5a9b18c introduced block streaming infrastructure with a special
fast path for all-cached scans, and commit b7b0f3f2 connected the
infrastructure up to sequential scans. One of the fast path
micro-optimizations had an unintended consequence: it interfered with
parallel sequential scan's block range allocator (from commit 56788d21),
which has its own ramp-up and ramp-down algorithm when handing out
groups of pages to workers. A scan of an all-cached table could give
extra blocks to one worker, when others had finished. In some plans
(probably already very bad plans, such as the one reported by
Alexander), the unfairness could be magnified.
An internal buffer of 16 block numbers is removed, keeping just a single
block buffer for technical reasons.
Back-patch to 17.
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/63a63690-dd92-c809-0b47-af05459e95d1%40gmail.com
Branch
------
REL_17_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/3ed3683618cb6a9b10dc5297751fa8b7fe7e36e1
Modified Files
--------------
src/backend/storage/aio/read_stream.c | 81 ++++++++++-------------------------
1 file changed, 23 insertions(+), 58 deletions(-)
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To: pgsql-committers@postgresql.org
Cc: tmunro@postgresql.org, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Fix unfairness in all-cached parallel seq scan.
In-Reply-To: <E1skGoJ-002EFP-4n@gemulon.postgresql.org>
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