postgrespro / Pg_wait_sampling
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pg_wait_sampling – sampling based statistics of wait events
Introduction
PostgreSQL 9.6+ provides an 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 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 9.6+.
Installation
pg_wait_sampling is PostgreSQL extension which requires PostgreSQL 9.6 or
higher. Before build and install you should ensure following:
- PostgreSQL version is 9.6 or higher.
- You have development package of PostgreSQL installed or you built PostgreSQL from source.
- Your PATH variable is configured so that
pg_configcommand 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
$ make USE_PGXS=1 installcheck
$ psql DB -c "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 | bool | Whether profile should be per query | false |
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.
While pg_wait_sampling.profile_queries is set to false queryid field in
views will be zero.
These GUCs are allowed to be changed by superuser. Also, they are placed into shared memory. Thus, they could be changed from any backend and affects worker runtime.
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.
Releases
New features are developed in feature-branches and then merged into master. To make a new release:
- Bump
PGXNversion in theMETA.json. - Merge master into stable.
- Tag new release in the stable with
git tag -a v1.1.X, where the last digit is used for indicating compatible shared library changes and bugfixes. Second digit is used to indicate extension schema change, i.e. whenALTER EXTENSION pg_wait_sampling UPDATE;is required. - Merge stable into debian. This separate branch is used to independently support
Debianpackaging and @anayrat with @df7cb have an access there.
Authors
- Alexander Korotkov [email protected], Postgres Professional, Moscow, Russia
- Ildus Kurbangaliev [email protected], Postgres Professional, Moscow, Russia