4b4b680c3d
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
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.