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Andres Freund c75ebc657f bufmgr: Allow some buffer state modifications while holding header lock 
Until now BufferDesc.state was not allowed to be modified while the buffer
header spinlock was held. This meant that operations like unpinning buffers
needed to use a CAS loop, waiting for the buffer header spinlock to be
released before updating.

The benefit of that restriction is that it allowed us to unlock the buffer
header spinlock with just a write barrier and an unlocked write (instead of a
full atomic operation). That was important to avoid regressions in
48354581a4. However, since then the hottest buffer header spinlock uses have
been replaced with atomic operations (in particular, the most common use of
PinBuffer_Locked(), in GetVictimBuffer() (formerly in BufferAlloc()), has been
removed in 5e89985928).

This change will allow, in a subsequent commit, to release buffer pins with a
single atomic-sub operation. This previously was not possible while such
operations were not allowed while the buffer header spinlock was held, as an
atomic-sub would not have allowed a race-free check for the buffer header lock
being held.

Using atomic-sub to unpin buffers is a nice scalability win, however it is not
the primary motivation for this change (although it would be sufficient). The
primary motivation is that we would like to merge the buffer content lock into
BufferDesc.state, which will result in more frequent changes of the state
variable, which in some situations can cause a performance regression, due to
an increased CAS failure rate when unpinning buffers.  The regression entirely
vanishes when using atomic-sub.

Naively implementing this would require putting CAS loops in every place
modifying the buffer state while holding the buffer header lock. To avoid
that, introduce UnlockBufHdrExt(), which can set/add flags as well as the
refcount, together with releasing the lock.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff
2025-11-06 16:42:10 -05:00
.github Add CODE_OF_CONDUCT.md, CONTRIBUTING.md, and SECURITY.md. 2024-07-02 13:03:58 -05:00
config Remove traces of support for Sun Studio compiler 2025-09-12 07:39:05 +02:00
contrib bufmgr: Allow some buffer state modifications while holding header lock 2025-11-06 16:42:10 -05:00
doc Doc: use uppercase keywords in SQLs 2025-11-06 16:03:02 +13:00
src bufmgr: Allow some buffer state modifications while holding header lock 2025-11-06 16:42:10 -05:00
.cirrus.star ci: Simplify ci-os-only handling 2025-08-14 12:09:34 -04:00
.cirrus.tasks.yml ci: Improve OpenBSD core dump backtrace handling. 2025-11-06 21:14:05 +13:00
.cirrus.yml ci: Per-repo configuration for manually trigger tasks 2025-08-14 11:54:03 -04:00
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aclocal.m4 autoconf: Move export_dynamic determination to configure 2022-12-06 18:55:28 -08:00
configure Fix spurious output in configure 2025-11-06 08:00:57 +01:00
configure.ac Fix spurious output in configure 2025-11-06 08:00:57 +01:00
COPYRIGHT Align organization wording in copyright statement 2025-05-16 11:20:07 -04:00
GNUmakefile.in Allow selecting the git revision to be packaged by "make dist". 2024-05-03 11:08:50 -04:00
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Makefile Remove AIX support 2024-02-28 15:17:23 +04:00
meson.build MSVC: Improve warning options set 2025-11-06 07:56:02 +01:00
meson_options.txt Add support for basic NUMA awareness 2025-04-07 23:08:17 +02:00
README.md Revise the style of a paragraph in README.md. 2024-03-21 10:16:41 -05:00

README.md

PostgreSQL Database Management System

This directory contains the source code distribution of the PostgreSQL database management system.

PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings.

Copyright and license information can be found in the file COPYRIGHT.

General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.

The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.