c75ebc657f
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 in48354581a4. 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 in5e89985928). 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
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/.