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Since the dawn of time (aka Postgres95) multiple pins of the same buffer by one backend have been optimized not to modify the shared refcount more than once. This optimization has always used a NBuffer sized array in each backend keeping track of a backend's pins. That array (PrivateRefCount) was one of the biggest per-backend memory allocations, depending on the shared_buffers setting. Besides the waste of memory it also has proven to be a performance bottleneck when assertions are enabled as we make sure that there's no remaining pins left at the end of transactions. Also, on servers with lots of memory and a correspondingly high shared_buffers setting the amount of random memory accesses can also lead to poor cpu cache efficiency. Because of these reasons a backend's buffers pins are now kept track of in a small statically sized array that overflows into a hash table when necessary. Benchmarks have shown neutral to positive performance results with considerably lower memory usage. Patch by me, review by Robert Haas. Discussion: 20140321182231.GA17111@alap3.anarazel.de |
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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: 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/.