38b602b028
There are two ways for shared libraries to allocate their own LWLock tranches. One way is to call RequestNamedLWLockTranche() in a shmem_request_hook, which requires the library to be loaded via shared_preload_libraries. The other way is to call LWLockNewTrancheId(), which is not subject to the same restrictions. However, LWLockNewTrancheId() does require each backend to store the tranche's name in backend-local memory via LWLockRegisterTranche(). This API is a little cumbersome and leads to things like unhelpful pg_stat_activity.wait_event values in backends that haven't loaded the library. This commit moves these LWLock tranche names to shared memory, thus eliminating the need for each backend to call LWLockRegisterTranche(). Instead, the tranche name must be provided to LWLockNewTrancheId(), which immediately makes the name available to all backends. Since the tranche name array is append-only, lookups can ordinarily avoid locking as long as their local copy of the LWLock counter is greater than the requested tranche ID. One downside of this approach is that we now have a hard limit on both the length of tranche names (NAMEDATALEN-1 bytes) and the number of dynamically-allocated tranches (256). Besides a limit of NAMEDATALEN-1 bytes for tranche names registered via RequestNamedLWLockTranche(), no such limits previously existed. We could avoid these new limits by using dynamic shared memory, but the complexity involved didn't seem worth it. We briefly considered making the tranche limit user-configurable but ultimately decided against that, too. Since there is still a lot of time left in the v19 development cycle, it's possible we will revisit this choice. Author: Sami Imseih <samimseih@gmail.com> Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Reviewed-by: Rahila Syed <rahilasyed90@gmail.com> Reviewed-by: Andres Freund <andres@anarazel.de> Discussion: https://postgr.es/m/CAA5RZ0vvED3naph8My8Szv6DL4AxOVK3eTPS0qXsaKi%3DbVdW2A%40mail.gmail.com |
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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/.