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Move the responsibility for advancing the NOTIFY queue tail pointer from the listener(s) to the notification sender, and only have the sender do it once every few queue pages, rather than after every batch of notifications as at present. This reduces the number of times we execute asyncQueueAdvanceTail, and reduces contention when there are multiple listeners (since that function requires exclusive lock). This change relies on the observation that we don't really need the tail pointer to be exactly up-to-date. It's certainly not necessary to attempt to release disk space more often than once per SLRU segment. The only other usage of the tail pointer is that an incoming listener, if it's the only listener in its database, will need to scan the queue forward from the tail; but that's surely a less performance-critical path than routine sending and receiving of notifies. We compromise by advancing the tail pointer after every 4 pages of output, so that it shouldn't get more than a few pages behind. Also, when sending signals to other backends after adding notify message(s) to the queue, recognize that only backends in our own database are going to care about those messages, so only such backends really need to be awakened promptly. Backends in other databases should get kicked if they're well behind on reading the queue, else they'll hold back the global tail pointer; but wakening them for every single message is pointless. This change can substantially reduce signal traffic if listeners are spread among many databases. It won't help for the common case of only a single active database, but the extra check costs very little. Martijn van Oosterhout, with some adjustments by me Discussion: https://postgr.es/m/CADWG95vtRBFDdrx1JdT1_9nhOFw48KaeTev6F_LtDQAFVpSPhA@mail.gmail.com Discussion: https://postgr.es/m/CADWG95uFj8rLM52Er80JnhRsTbb_AqPP1ANHS8XQRGbqLrU+jA@mail.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. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.