Performance Schema Atom and Molecule Events 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  by 200 to indicate that instances could not be created. A value of  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 . The bottom line is that if  SHOW STATUS LIKE ’perf%’  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  to inspect the internal operation of the Performance Schema code: 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” . For a table I/O event, there are usually two rows in  events_waits_current , not one. For example, a row fetch might result in rows like this: 27.8 Performance Schema Atom and Molecule Events 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 ... Row# EVENT_NAME TIMER_START TIMER_END ---- ---------- ----------- --------- 1 wait/io/file/myisam/dfile 10001 10002 4931