819dc118c0
While testing a new potential use for ReadRecentBuffer(), Andres reported that it scales badly when called concurrently for the same buffer by many backends. Instead of a naive (but wrong) coding with PinBuffer(), it used the spinlock, so that it could be careful to pin only if the buffer was valid and holding the expected block, to avoid breaking invariants in eg GetVictimBuffer(). Unfortunately that made it less scalable than PinBuffer(), which uses compare-exchange instead. We can fix that by giving PinBuffer() a new skip_if_not_valid mode that doesn't pin invalid buffers. It might occasionally skip when it shouldn't due to the unlocked read of the header flags, but that's unlikely and perfectly acceptable for an opportunistic optimisation routine, and it can only succeed when it really should due to the compare-exchange loop. Note that this fixes ReadRecentBuffer()'s failure to bump the usage count. While this could be seen as a bug, there currently aren't cases affected by this in core, so it doesn't seem worth backpatching that portion. Author: Thomas Munro <thomas.munro@gmail.com> Reported-by: Andres Freund <andres@anarazel.de> Reviewed-by: Andres Freund <andres@anarazel.de> Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com> Discussion: https://postgr.es/m/20230627020546.t6z4tntmj7wmjrfh%40awork3.anarazel.de Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff
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.
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