Assuming that the Performance Schema is available, it is enabled by default as of MySQL 5.6.6. Before 5.6.6, it is disabled by default. To enable or disable it explicitly, start the server with the performance_schema variable set to an appropriate value. For example, use these lines in your my.cnf file:

[mysqld]
performance_schema=ON

If the server is unable to allocate any internal buffer during Performance Schema initialization, the Performance Schema disables itself and sets performance_schema to OFF, and the server runs without instrumentation.

As of MySQL 5.6.4, the Performance Schema permits instrument and consumer configuration at server startup, which previously was possible only at runtime using UPDATE statements for the setup_instruments and setup_consumers tables. This change was made because configuration at runtime is too late to disable instruments that have already been initialized during server startup. For example, the wait/synch/mutex/sql/LOCK_open mutex is initialized once during server startup, so attempts to disable the corresponding instrument at runtime have no effect.

To control an instrument at server startup, use an option of this form:

--performance-schema-instrument='instrument_name=value'

Here, instrument_name is an instrument name such as wait/synch/mutex/sql/LOCK_open, and value is one of these values:

Each --performance-schema-instrument option can specify only one instrument name, but multiple instances of the option can be given to configure multiple instruments. In addition, patterns are permitted in instrument names to configure instruments that match the pattern. To configure all condition synchronization instruments as enabled and counted, use this option:

--performance-schema-instrument='wait/synch/cond/%=COUNTED'

To disable all instruments, use this option:

--performance-schema-instrument='%=OFF'

Longer instrument name strings take precedence over shorter pattern names, regardless of order. For information about specifying patterns to select instruments, see Section 22.3.9, “Naming Instruments or Consumers for Filtering Operations”.

An unrecognized instrument name is ignored. It is possible that a plugin installed later may create the instrument, at which time the name is recognized and configured.

To control a consumer at server startup, use an option of this form:

--performance-schema-consumer-consumer_name=value

Here, consumer_name is a consumer name such as events_waits_history, and value is one of these values:

For example, to enable the events_waits_history consumer, use this option:

--performance-schema-consumer-events-waits-history=ON

The permitted consumer names can be found by examining the setup_consumers table. Patterns are not permitted. Consumer names in the setup_consumers table use underscores, but for consumers set at startup, dashes and underscores within the name are equivalent.

The Performance Schema includes several system variables that provide configuration information:

mysql> SHOW VARIABLES LIKE 'perf%';
+--------------------------------------------------------+---------+
| Variable_name                                          | Value   |
+--------------------------------------------------------+---------+
| performance_schema                                     | ON      |
| performance_schema_accounts_size                       | 100     |
| performance_schema_digests_size                        | 200     |
| performance_schema_events_stages_history_long_size     | 10000   |
| performance_schema_events_stages_history_size          | 10      |
| performance_schema_events_statements_history_long_size | 10000   |
| performance_schema_events_statements_history_size      | 10      |
| performance_schema_events_waits_history_long_size      | 10000   |
| performance_schema_events_waits_history_size           | 10      |
| performance_schema_hosts_size                          | 100     |
| performance_schema_max_cond_classes                    | 80      |
| performance_schema_max_cond_instances                  | 1000    |
...

The performance_schema variable is ON or OFF to indicate whether the Performance Schema is enabled or disabled. The other variables indicate table sizes (number of rows) or memory allocation values.

To change the value of Performance Schema system variables, set them at server startup. For example, put the following lines in a my.cnf file to change the sizes of the history tables for wait events:

[mysqld]
performance_schema
performance_schema_events_waits_history_size=20
performance_schema_events_waits_history_long_size=15000

As of MySQL 5.6.6, the Performance Schema automatically sizes the values of several of its parameters at server startup if they are not set explicitly. For example, it sizes the parameters that control the sizes of the events waits tables this way. To see which parameters are autosized under this policy, use mysqld --verbose --help and look for those with a default value of −1, or see Section 22.13, “Performance Schema System Variables”.

For each autosized parameter that is not set at server startup (or is set to −1), the Performance Schema determines how to set its value based on the value of the following system values, which are considered as “hints” about how you have configured your MySQL server:

max_connections
open_files_limit
table_definition_cache
table_open_cache

To override autosizing for a given parameter, set it to a value other than −1 at startup. In this case, the Performance Schema assigns it the specified value.

At runtime, SHOW VARIABLES displays the actual values that autosized parameters were set to.

If the Performance Schema is disabled, its autosized parameters remain set to −1 and SHOW VARIABLES displays −1.