## 27.7 Performance Schema Status Monitoring

 There are several status variables associated with the Performance Schema:

```
mysql> SHOW STATUS LIKE 'perf%';
+-----------------------------------------------+-------+
| Variable_name                                 | Value |
+-----------------------------------------------+-------+
| Performance_schema_accounts_lost              | 0     |
| Performance_schema_cond_classes_lost          | 0     |
| Performance_schema_cond_instances_lost        | 0     |
| Performance_schema_digest_lost                | 0     |
| Performance_schema_file_classes_lost          | 0     |
| Performance_schema_file_handles_lost          | 0     |
| Performance_schema_file_instances_lost        | 0     |
| Performance_schema_hosts_lost                 | 0     |
| Performance_schema_locker_lost                | 0     |
| Performance_schema_memory_classes_lost        | 0     |
| Performance_schema_metadata_lock_lost         | 0     |
| Performance_schema_mutex_classes_lost         | 0     |
| Performance_schema_mutex_instances_lost       | 0     |
| Performance_schema_nested_statement_lost      | 0     |
| Performance_schema_program_lost               | 0     |
| Performance_schema_rwlock_classes_lost        | 0     |
| Performance_schema_rwlock_instances_lost      | 0     |
| Performance_schema_session_connect_attrs_lost | 0     |
| Performance_schema_socket_classes_lost        | 0     |
| Performance_schema_socket_instances_lost      | 0     |
| Performance_schema_stage_classes_lost         | 0     |
| Performance_schema_statement_classes_lost     | 0     |
| Performance_schema_table_handles_lost         | 0     |
| Performance_schema_table_instances_lost       | 0     |
| Performance_schema_thread_classes_lost        | 0     |
| Performance_schema_thread_instances_lost      | 0     |
| Performance_schema_users_lost                 | 0     |
+-----------------------------------------------+-------+
```

 The Performance Schema status variables provide information about instrumentation that could not be loaded or created due to memory constraints. Names for these variables have several forms:

* `Performance_schema_*`xxx`*_classes_lost` indicates how many instruments of type *`xxx`* could not be loaded.

* `Performance_schema_*`xxx`*_instances_lost` indicates how many instances of object type *`xxx`* could not be created.

* `Performance_schema_*`xxx`*_handles_lost` indicates how many instances of object type *`xxx`* could not be opened.

* `Performance_schema_locker_lost` indicates how many events are “lost” or not recorded.

 For example, if a mutex is instrumented in the server source but the server cannot allocate memory for the instrumentation at runtime, it increments [`Performance_schema_mutex_classes_lost`](performance-schema-status-variables.html#statvar_Performance_schema_mutex_classes_lost). The mutex still functions as a synchronization object (that is, the server continues to function normally), but performance data for it is not collected. If the instrument can be allocated, it can be used for initializing instrumented mutex instances. For a singleton mutex such as a global mutex, there is only one instance. Other mutexes have an instance per connection, or per page in various caches and data buffers, so the number of instances varies over time. Increasing the maximum number of connections or the maximum size of some buffers increases the maximum number of instances that might be allocated at once. If the server cannot create a given instrumented mutex instance, it increments [`Performance_schema_mutex_instances_lost`](performance-schema-status-variables.html#statvar_Performance_schema_mutex_instances_lost).

 Suppose that the following conditions hold:

* The server was started with the [`--performance_schema_max_mutex_classes=200`](performance-schema-system-variables.html#sysvar_performance_schema_max_mutex_classes) option and thus has room for 200 mutex instruments.

* 150 mutex instruments have been loaded already.

* The plugin named `plugin_a` contains 40 mutex instruments.

* The plugin named `plugin_b` contains 20 mutex instruments.

 The server allocates mutex instruments for the plugins depending on how many they need and how many are available, as illustrated by the following sequence of statements:

```
INSTALL PLUGIN plugin_a
```

 The server now has 150+40 = 190 mutex instruments.

```
UNINSTALL PLUGIN plugin_a;
```

 The server still has 190 instruments. All the historical data generated by the plugin code is still available, but new events for the instruments are not collected.

```
INSTALL PLUGIN plugin_a;
```

 The server detects that the 40 instruments are already defined, so no new instruments are created, and previously assigned internal memory buffers are reused. The server still has 190 instruments.

```
INSTALL PLUGIN plugin_b;
```

 The server has room for 200-190 = 10 instruments (in this case, mutex classes), and sees that the plugin contains 20 new instruments. 10 instruments are loaded, and 10 are discarded or “lost.” The [`Performance_schema_mutex_classes_lost`](performance-schema-status-variables.html#statvar_Performance_schema_mutex_classes_lost) indicates the number of instruments (mutex classes) lost:

```
mysql> SHOW STATUS LIKE "perf%mutex_classes_lost";
+---------------------------------------+-------+
| Variable_name                         | Value |
+---------------------------------------+-------+
| Performance_schema_mutex_classes_lost | 10    |
+---------------------------------------+-------+
1 row in set (0.10 sec)
```

 The instrumentation still works and collects (partial) data for `plugin_b`.

 When the server cannot create a mutex instrument, these results occur:

* No row for the instrument is inserted into the [`setup_instruments`](performance-schema-setup-instruments-table.html) table.

* [`Performance_schema_mutex_classes_lost`](performance-schema-status-variables.html#statvar_Performance_schema_mutex_classes_lost) increases by 1.

* [`Performance_schema_mutex_instances_lost`](performance-schema-status-variables.html#statvar_Performance_schema_mutex_instances_lost) does not change. (When the mutex instrument is not created, it cannot be used to create instrumented mutex instances later.)

 The pattern just described applies to all types of instruments, not just mutexes.

 A value of [`Performance_schema_mutex_classes_lost`](performance-schema-status-variables.html#statvar_Performance_schema_mutex_classes_lost) greater than 0 can happen in two cases:

* To save a few bytes of memory, you start the server with [`--performance_schema_max_mutex_classes=*`N`*`](performance-schema-system-variables.html#sysvar_performance_schema_max_mutex_classes), where *`N`* is less than the default value. The default value is chosen to be sufficient to load all the plugins provided in the MySQL distribution, but this can be reduced if some plugins are never loaded. For example, you might choose not to load some of the storage engines in the distribution.

* You load a third-party plugin that is instrumented for the Performance Schema but do not allow for the plugin's instrumentation memory requirements when you start the server. Because it comes from a third party, the instrument memory consumption of this engine is not accounted for in the default value chosen for [`performance_schema_max_mutex_classes`](performance-schema-system-variables.html#sysvar_performance_schema_max_mutex_classes).

   If the server has insufficient resources for the plugin's instruments and you do not explicitly allocate more using [`--performance_schema_max_mutex_classes=*`N`*`](performance-schema-system-variables.html#sysvar_performance_schema_max_mutex_classes), loading the plugin leads to starvation of instruments.

 If the value chosen for [`performance_schema_max_mutex_classes`](performance-schema-system-variables.html#sysvar_performance_schema_max_mutex_classes) is too small, no error is reported in the error log and there is no failure at runtime. However, the content of the tables in the `performance_schema` database misses events. The [`Performance_schema_mutex_classes_lost`](performance-schema-status-variables.html#statvar_Performance_schema_mutex_classes_lost) status variable is the only visible sign to indicate that some events were dropped internally due to failure to create instruments.

 If an instrument is not lost, it is known to the Performance Schema, and is used when instrumenting instances. For example, `wait/synch/mutex/sql/LOCK_delete` is the name of a mutex instrument in the [`setup_instruments`](performance-schema-setup-instruments-table.html) table. This single instrument is used when creating a mutex in the code (in `THD::LOCK_delete`) however many instances of the mutex are needed as the server runs. In this case, `LOCK_delete` is a mutex that is per connection (`THD`), so if a server has 1000 connections, there are 1000 threads, and 1000 instrumented `LOCK_delete` mutex instances (`THD::LOCK_delete`).

 If the server does not have room for all these 1000 instrumented mutexes (instances), some mutexes are created with instrumentation, and some are created without instrumentation. If the server can create only 800 instances, 200 instances are lost. The server continues to run, but increments [`Performance_schema_mutex_instances_lost`](performance-schema-status-variables.html#statvar_Performance_schema_mutex_instances_lost) by 200 to indicate that instances could not be created.

 A value of [`Performance_schema_mutex_instances_lost`](performance-schema-status-variables.html#statvar_Performance_schema_mutex_instances_lost) greater than 0 can happen when the code initializes more mutexes at runtime than were allocated for [`--performance_schema_max_mutex_instances=*`N`*`](performance-schema-system-variables.html#sysvar_performance_schema_max_mutex_instances).

 The bottom line is that if [`SHOW STATUS LIKE 'perf%'`](show-status.html) says that nothing was lost (all values are zero), the Performance Schema data is accurate and can be relied upon. If something was lost, the data is incomplete, and the Performance Schema could not record everything given the insufficient amount of memory it was given to use. In this case, the specific `Performance_schema_*`xxx`*_lost` variable indicates the problem area.

 It might be appropriate in some cases to cause deliberate instrument starvation. For example, if you do not care about performance data for file I/O, you can start the server with all Performance Schema parameters related to file I/O set to 0. No memory is allocated for file-related classes, instances, or handles, and all file events are lost.

 Use [`SHOW ENGINE PERFORMANCE_SCHEMA STATUS`](show-engine.html) to inspect the internal operation of the Performance Schema code:

```
mysql> SHOW ENGINE PERFORMANCE_SCHEMA STATUS\G
...
*************************** 3. row ***************************
  Type: performance_schema
  Name: events_waits_history.size
Status: 76
*************************** 4. row ***************************
  Type: performance_schema
  Name: events_waits_history.count
Status: 10000
*************************** 5. row ***************************
  Type: performance_schema
  Name: events_waits_history.memory
Status: 760000
...
*************************** 57. row ***************************
  Type: performance_schema
  Name: performance_schema.memory
Status: 26459600
...
```

 This statement is intended to help the DBA understand the effects that different Performance Schema options have on memory requirements. For a description of the field meanings, see [Section 13.7.7.15, “SHOW ENGINE Statement”](show-engine.html).

