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When a write transaction commits, it must clear its XID advertised via the ProcArray, which requires that we hold ProcArrayLock in exclusive mode in order to prevent concurrent processes running GetSnapshotData from seeing inconsistent results. When many processes try to commit at once, ProcArrayLock must change hands repeatedly, with each concurrent process trying to commit waking up to acquire the lock in turn. To make things more efficient, when more than one backend is trying to commit a write transaction at the same time, have just one of them acquire ProcArrayLock in exclusive mode and clear the XIDs of all processes in the group. Benchmarking reveals that this is much more efficient at very high client counts. Amit Kapila, heavily revised by me, with some review also from Pavan Deolasee.
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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.