## 17.4. Resource Consumption

### 17.4.1. Memory

`shared_buffers` (`integer`)

  Sets the number of shared memory buffers used by the database server. The default is typically 1000, but may be less if your kernel settings will not support it (as determined during initdb). Each buffer is 8192 bytes, unless a different value of `BLCKSZ` was chosen when building the server. This setting must be at least 16, as well as at least twice the value of max_connections; however, settings significantly higher than the minimum are usually needed for good performance. Values of a few thousand are recommended for production installations. This option can only be set at server start.

  Increasing this parameter may cause PostgreSQL to request more System V shared memory than your operating system's default configuration allows. See Section 16.4.1, “Shared Memory and Semaphores” for information on how to adjust those parameters, if necessary.

`temp_buffers` (`integer`)

  Sets the maximum number of temporary buffers used by each database session. These are session-local buffers used only for access to temporary tables. The default is 1000. The setting can be changed within individual sessions, but only up until the first use of temporary tables within a session; subsequent attempts to change the value will have no effect on that session.

  A session will allocate temporary buffers as needed up to the limit given by `temp_buffers`. The cost of setting a large value in sessions that do not actually need a lot of temporary buffers is only a buffer descriptor, or about 64 bytes, per increment in `temp_buffers`. However if a buffer is actually used an additional 8192 bytes will be consumed for it (or in general, `BLCKSZ` bytes).

`max_prepared_transactions` (`integer`)

  Sets the maximum number of transactions that can be in the “prepared” state simultaneously (see PREPARE TRANSACTION). Setting this parameter to zero disables the prepared-transaction feature. The default is 5. This option can only be set at server start.

  If you are not using prepared transactions, this parameter may as well be set to zero. If you are using them, you will probably want `max_prepared_transactions` to be at least as large as max_connections, to avoid unwanted failures at the prepare step.

  Increasing this parameter may cause PostgreSQL to request more System V shared memory than your operating system's default configuration allows. See Section 16.4.1, “Shared Memory and Semaphores” for information on how to adjust those parameters, if necessary.

`work_mem` (`integer`)

  Specifies the amount of memory to be used by internal sort operations and hash tables before switching to temporary disk files. The value is specified in kilobytes, and defaults to 1024 kilobytes (1 MB). Note that for a complex query, several sort or hash operations might be running in parallel; each one will be allowed to use as much memory as this value specifies before it starts to put data into temporary files. Also, several running sessions could be doing such operations concurrently. So the total memory used could be many times the value of `work_mem`; it is necessary to keep this fact in mind when choosing the value. Sort operations are used for `ORDER BY`, `DISTINCT`, and merge joins. Hash tables are used in hash joins, hash-based aggregation, and hash-based processing of `IN` subqueries.

`maintenance_work_mem` (`integer`)

  Specifies the maximum amount of memory to be used in maintenance operations, such as `VACUUM`, `CREATE INDEX`, and `ALTER TABLE ADD FOREIGN KEY`. The value is specified in kilobytes, and defaults to 16384 kilobytes (16 MB). Since only one of these operations can be executed at a time by a database session, and an installation normally doesn't have very many of them happening concurrently, it's safe to set this value significantly larger than `work_mem`. Larger settings may improve performance for vacuuming and for restoring database dumps.

`max_stack_depth` (`integer`)

  Specifies the maximum safe depth of the server's execution stack. The ideal setting for this parameter is the actual stack size limit enforced by the kernel (as set by `ulimit -s` or local equivalent), less a safety margin of a megabyte or so. The safety margin is needed because the stack depth is not checked in every routine in the server, but only in key potentially-recursive routines such as expression evaluation. Setting the parameter higher than the actual kernel limit will mean that a runaway recursive function can crash an individual backend process. The default setting is 2048 KB (two megabytes), which is conservatively small and unlikely to risk crashes. However, it may be too small to allow execution of complex functions.

### 17.4.2. Free Space Map

These parameters control the size of the shared free space map, which tracks the locations of unused space in the database. An undersized free space map may cause the database to consume increasing amounts of disk space over time, because free space that is not in the map cannot be re-used; instead PostgreSQL will request more disk space from the operating system when it needs to store new data. The last few lines displayed by a database-wide `VACUUM VERBOSE` command can help in determining if the current settings are adequate. A `NOTICE` message is also printed during such an operation if the current settings are too low.

Increasing these parameters may cause PostgreSQL to request more System V shared memory than your operating system's default configuration allows. See Section 16.4.1, “Shared Memory and Semaphores” for information on how to adjust those parameters, if necessary.

`max_fsm_pages` (`integer`)

  Sets the maximum number of disk pages for which free space will be tracked in the shared free-space map. Six bytes of shared memory are consumed for each page slot. This setting must be more than 16 * `max_fsm_relations`. The default is 20000. This option can only be set at server start.

`max_fsm_relations` (`integer`)

  Sets the maximum number of relations (tables and indexes) for which free space will be tracked in the shared free-space map. Roughly seventy bytes of shared memory are consumed for each slot. The default is 1000. This option can only be set at server start.

### 17.4.3. Kernel Resource Usage

`max_files_per_process` (`integer`)

  Sets the maximum number of simultaneously open files allowed to each server subprocess. The default is 1000. If the kernel is enforcing a safe per-process limit, you don't need to worry about this setting. But on some platforms (notably, most BSD systems), the kernel will allow individual processes to open many more files than the system can really support when a large number of processes all try to open that many files. If you find yourself seeing “Too many open files” failures, try reducing this setting. This option can only be set at server start.

`preload_libraries` (`string`)

  This variable specifies one or more shared libraries that are to be preloaded at server start. A parameterless initialization function can optionally be called for each library. To specify that, add a colon and the name of the initialization function after the library name. For example `'$libdir/mylib:mylib_init'` would cause `mylib` to be preloaded and `mylib_init` to be executed. If more than one library is to be loaded, separate their names with commas.

  If a specified library or initialization function is not found, the server will fail to start.

  PostgreSQL procedural language libraries may be preloaded in this way, typically by using the syntax `'$libdir/plXXX:plXXX_init'` where `XXX` is `pgsql`, `perl`, `tcl`, or `python`.

  By preloading a shared library (and initializing it if applicable), the library startup time is avoided when the library is first used. However, the time to start each new server process may increase slightly, even if that process never uses the library. So this option is recommended only for libraries that will be used in most sessions.

### 17.4.4. Cost-Based Vacuum Delay

During the execution of VACUUM and ANALYZE commands, the system maintains an internal counter that keeps track of the estimated cost of the various I/O operations that are performed. When the accumulated cost reaches a limit (specified by `vacuum_cost_limit`), the process performing the operation will sleep for a while (specified by `vacuum_cost_delay`). Then it will reset the counter and continue execution.

The intent of this feature is to allow administrators to reduce the I/O impact of these commands on concurrent database activity. There are many situations in which it is not very important that maintenance commands like `VACUUM` and `ANALYZE` finish quickly; however, it is usually very important that these commands do not significantly interfere with the ability of the system to perform other database operations. Cost-based vacuum delay provides a way for administrators to achieve this.

This feature is disabled by default. To enable it, set the `vacuum_cost_delay` variable to a nonzero value.

`vacuum_cost_delay` (`integer`)

  The length of time, in milliseconds, that the process will sleep when the cost limit has been exceeded. The default value is 0, which disables the cost-based vacuum delay feature. Positive values enable cost-based vacuuming. Note that on many systems, the effective resolution of sleep delays is 10 milliseconds; setting `vacuum_cost_delay` to a value that is not a multiple of 10 may have the same results as setting it to the next higher multiple of 10.

`vacuum_cost_page_hit` (`integer`)

  The estimated cost for vacuuming a buffer found in the shared buffer cache. It represents the cost to lock the buffer pool, lookup the shared hash table and scan the content of the page. The default value is 1.

`vacuum_cost_page_miss` (`integer`)

  The estimated cost for vacuuming a buffer that has to be read from disk. This represents the effort to lock the buffer pool, lookup the shared hash table, read the desired block in from the disk and scan its content. The default value is 10.

`vacuum_cost_page_dirty` (`integer`)

  The estimated cost charged when vacuum modifies a block that was previously clean. It represents the extra I/O required to flush the dirty block out to disk again. The default value is 20.

`vacuum_cost_limit` (`integer`)

  The accumulated cost that will cause the vacuuming process to sleep. The default value is 200.

### Note

There are certain operations that hold critical locks and should therefore complete as quickly as possible. Cost-based vacuum delays do not occur during such operations. Therefore it is possible that the cost accumulates far higher than the specified limit. To avoid uselessly long delays in such cases, the actual delay is calculated as `vacuum_cost_delay` * `accumulated_balance` / `vacuum_cost_limit` with a maximum of `vacuum_cost_delay` * 4.

### 17.4.5. Background Writer

Beginning in PostgreSQL 8.0, there is a separate server process called the background writer, whose sole function is to issue writes of “dirty” shared buffers. The intent is that server processes handling user queries should seldom or never have to wait for a write to occur, because the background writer will do it. This arrangement also reduces the performance penalty associated with checkpoints. The background writer will continuously trickle out dirty pages to disk, so that only a few pages will need to be forced out when checkpoint time arrives, instead of the storm of dirty-buffer writes that formerly occurred at each checkpoint. However there is a net overall increase in I/O load, because where a repeatedly-dirtied page might before have been written only once per checkpoint interval, the background writer might write it several times in the same interval. In most situations a continuous low load is preferable to periodic spikes, but the parameters discussed in this subsection can be used to tune the behavior for local needs.

`bgwriter_delay` (`integer`)

  Specifies the delay between activity rounds for the background writer. In each round the writer issues writes for some number of dirty buffers (controllable by the following parameters). It then sleeps for `bgwriter_delay` milliseconds, and repeats. The default value is 200. Note that on many systems, the effective resolution of sleep delays is 10 milliseconds; setting `bgwriter_delay` to a value that is not a multiple of 10 may have the same results as setting it to the next higher multiple of 10. This option can be set at server start or in the `postgresql.conf` file.

`bgwriter_lru_percent` (`floating point`)

  To reduce the probability that server processes will need to issue their own writes, the background writer tries to write buffers that are likely to be recycled soon. In each round, it examines up to `bgwriter_lru_percent` of the buffers that are nearest to being recycled, and writes any that are dirty. The default value is 1.0 (this is a percentage of the total number of shared buffers). This option can be set at server start or in the `postgresql.conf` file.

`bgwriter_lru_maxpages` (`integer`)

  In each round, no more than this many buffers will be written as a result of scanning soon-to-be-recycled buffers. The default value is 5. This option can be set at server start or in the `postgresql.conf` file.

`bgwriter_all_percent` (`floating point`)

  To reduce the amount of work that will be needed at checkpoint time, the background writer also does a circular scan through the entire buffer pool, writing buffers that are found to be dirty. In each round, it examines up to `bgwriter_all_percent` of the buffers for this purpose. The default value is 0.333 (this is a percentage of the total number of shared buffers). With the default `bgwriter_delay` setting, this will allow the entire shared buffer pool to be scanned about once per minute. This option can be set at server start or in the `postgresql.conf` file.

`bgwriter_all_maxpages` (`integer`)

  In each round, no more than this many buffers will be written as a result of the scan of the entire buffer pool. (If this limit is reached, the scan stops, and resumes at the next buffer during the next round.) The default value is 5. This option can be set at server start or in the `postgresql.conf` file.

Smaller values of `bgwriter_all_percent` and `bgwriter_all_maxpages` reduce the extra I/O load caused by the background writer, but leave more work to be done at checkpoint time. To reduce load spikes at checkpoints, increase these two values. Similarly, smaller values of `bgwriter_lru_percent` and `bgwriter_lru_maxpages` reduce the extra I/O load caused by the background writer, but make it more likely that server processes will have to issue writes for themselves, delaying interactive queries. To disable background writing entirely, set both `maxpages` values and/or both `percent` values to zero.