pg_wait_sampling – sampling based statistics of wait events

Introduction

PostgreSQL provides information about current wait event of particular process. However, in order to gather descriptive statistics of server behavior user have to sample current wait event multiple times. pg_wait_sampling is an extension for collecting sampling statistics of wait events.

The module must be loaded by adding pg_wait_sampling to shared_preload_libraries in postgresql.conf, because it requires additional shared memory and launches background worker. This means that a server restart is needed to add or remove the module.

When used with pg_stat_statements it is recommended to put pg_stat_statements before pg_wait_sampling in shared_preload_libraries so queryIds of utility statements are not rewritten by the former.

When pg_wait_sampling is enabled, it collects two kinds of statistics.

• History of waits events. It's implemented as in-memory ring buffer where samples of each process wait events are written with given (configurable) period. Therefore, for each running process user can see some number of recent samples depending on history size (configurable). Assuming there is a client who periodically read this history and dump it somewhere, user can have continuous history.
• Waits profile. It's implemented as in-memory hash table where count of samples are accumulated per each process and each wait event (and each query with pg_stat_statements). This hash table can be reset by user request. Assuming there is a client who periodically dumps profile and resets it, user can have statistics of intensivity of wait events among time.

In combination with pg_stat_statements this extension can also provide per query statistics.

pg_wait_sampling launches special background worker for gathering the statistics above.

Availability

pg_wait_sampling is implemented as an extension and not available in default PostgreSQL installation. It is available from github under the same license as PostgreSQL and supports PostgreSQL 13+.

Installation

Pre-built pg_wait_sampling packages are provided in official PostgreSQL repository: https://download.postgresql.org/pub/repos/

Manual build

pg_wait_sampling is PostgreSQL extension which requires PostgreSQL 13 or higher. Before build and install you should ensure following:

• PostgreSQL version is 13 or higher.
• You have development package of PostgreSQL installed or you built PostgreSQL from source.
• Your PATH variable is configured so that pg_config command available, or set PG_CONFIG variable.

Typical installation procedure may look like this:

$ git clone https://github.com/postgrespro/pg_wait_sampling.git
$ cd pg_wait_sampling
$ make USE_PGXS=1
$ sudo make USE_PGXS=1 install

Then add shared_preload_libraries = pg_wait_sampling to postgresql.conf and restart the server.

To test your installation:

$ make USE_PGXS=1 installcheck

To create the extension in the target database:

CREATE EXTENSION pg_wait_sampling;

Compilation on Windows is not supported, since the extension uses symbols from PostgreSQL that are not exported.

Usage

pg_wait_sampling interacts with user by set of views and functions.

pg_wait_sampling_current view – information about current wait events for all processed including background workers.

Column name	Column type	Description
pid	int4	Id of process
event_type	text	Name of wait event type
event	text	Name of wait event
queryid	int8	Id of query

pg_wait_sampling_get_current(pid int4) returns the same table for single given process.

pg_wait_sampling_history view – history of wait events obtained by sampling into in-memory ring buffer.

Column name	Column type	Description
pid	int4	Id of process
ts	timestamptz	Sample timestamp
event_type	text	Name of wait event type
event	text	Name of wait event
queryid	int8	Id of query

pg_wait_sampling_profile view – profile of wait events obtained by sampling into in-memory hash table.

Column name	Column type	Description
pid	int4	Id of process
event_type	text	Name of wait event type
event	text	Name of wait event
queryid	int8	Id of query
count	text	Count of samples

pg_wait_sampling_reset_profile() function resets the profile.

The work of wait event statistics collector worker is controlled by following GUCs.

Parameter name	Data type	Description	Default value
pg_wait_sampling.history_size	int4	Size of history in-memory ring buffer	5000
pg_wait_sampling.history_period	int4	Period for history sampling in milliseconds	10
pg_wait_sampling.profile_period	int4	Period for profile sampling in milliseconds	10
pg_wait_sampling.profile_pid	bool	Whether profile should be per pid	true
pg_wait_sampling.profile_queries	enum	Whether profile should be per query	top
pg_wait_sampling.sample_cpu	bool	Whether on CPU backends should be sampled	true

If pg_wait_sampling.profile_pid is set to false, sampling profile wouldn't be collected in per-process manner. In this case the value of pid could would be always zero and corresponding row contain samples among all the processes.

If pg_wait_sampling.profile_queries is set to none, queryid field in views will be zero. If it is set to top, queryIds only of top level statements are recorded. If it is set to all, queryIds of nested statements are recorded.

If pg_wait_sampling.sample_cpu is set to true then processes that are not waiting on anything are also sampled. The wait event columns for such processes will be NULL.

Values of these GUC variables can be changed only in config file or with ALTER SYSTEM. Then you need to reload server's configuration (such as with pg_reload_conf function) for changes to take effect.

See PostgreSQL documentation for list of possible wait events.

Contribution

Please, notice, that pg_wait_sampling is still under development and while it's stable and tested, it may contains some bugs. Don't hesitate to raise issues at github with your bug reports.

If you're lacking of some functionality in pg_wait_sampling and feeling power to implement it then you're welcome to make pull requests.

Authors

• Alexander Korotkov a.korotkov@postgrespro.ru, Postgres Professional, Moscow, Russia
• Ildus Kurbangaliev i.kurbangaliev@gmail.com, Postgres Professional, Moscow, Russia