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help / color / mirror / Atom feedFrom: Melanie Plageman <melanieplageman@gmail.com>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Make local buffers pin limit more conservative
Date: Wed, 05 Aug 2026 15:44:52 +0000
Message-ID: <E1wrdnY-00000000JoH-2FyR@gemulon.postgresql.org> (raw)
Make local buffers pin limit more conservative
GetLocalPinLimit() and GetAdditionalLocalPinLimit(), currently in use
only by the read stream, previously allowed a backend to pin all
num_temp_buffers local buffers. This meant that the read stream could
use every available local buffer for read-ahead, leaving none for other
concurrent pin-holders.
In 18, this was reported as a sequential scan of a temporary table
failing with "no empty local buffer available": with
effective_io_concurrency >= 64 and default io_combine_limit and
temp_buffers, the scan's read stream budget,
(effective_io_concurrency + 1) * io_combine_limit, meets or exceeds
num_temp_buffers, so look-ahead could pin the entire local buffer pool.
Any additional pin request made while returning tuples -- such as
fetching TOASTed values from the table's (also temporary) TOAST table --
then found no unpinned buffer to evict.
Cap the local pin limit to num_temp_buffers / 4, providing some
headroom. This doesn't guarantee that all needed pins will be available
-- for example, a backend can still open more cursors than there are
buffers -- but it makes it less likely that read-ahead will exhaust the
pool.
This is a backpatch of commit da6874635db, which was applied to master
only during the 19 development cycle, where the issue surfaced as a
regression test failure after on-access pruning began setting the
visibility map. Reports of the failure above on 18 prompted
backpatching it now.
No backpatch to 17, even though its read stream also allows a single
stream to pin the whole pool in principle. There, sequential scans pass
READ_STREAM_SEQUENTIAL, which disables fadvise-based look-ahead, so the
look-ahead distance cannot grow past io_combine_limit (at most 32
buffers, versus a minimum temp_buffers of 100) and the reported failure
is unreachable. The remaining theoretical paths require adversarial
settings and have never been reported. Moreover, 17 predates these
functions; the fix would have to be reimplemented in
LimitAdditionalLocalPins(), making the change not a simple cherry-pick.
Reported-by: Induja Sreekanthan <indujas@google.com>
Reported-by: Eduard Stepanov <crtxcz@gmail.com>
Reported-by: Feike Steenbergen <feikesteenbergen@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/97529f5a-ec10-46b1-ab50-4653126c6889%40gmail.com
Discussion: https://postgr.es/m/flat/CAFMO8-rYPSJbXsDdWDzDdpNi-fQ%2B6bKvgbXwE%2BR%3DsGko4epq0Q%40mail.gmail.com
Backpatch-through: 18
Branch
------
REL_18_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/ed8050370b7cf117dfb0de218695b1e003a51f36
Modified Files
--------------
src/backend/storage/buffer/localbuf.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)
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To: pgsql-committers@postgresql.org
Cc: melanieplageman@gmail.com, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Make local buffers pin limit more conservative
In-Reply-To: <E1wrdnY-00000000JoH-2FyR@gemulon.postgresql.org>
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