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The performance_timers  table shows which event timers are available: 

mysql> SELECT * FROM performance_schema.performance_timers; +-------------+-----------------+------------------+----------------+ | TIMER_NAME | TIMER_FREQUENCY | TIMER_RESOLUTION | TIMER_OVERHEAD | +-------------+-----------------+------------------+----------------+ | CYCLE | 2389029850 | 1 | 72 | | NANOSECOND | 1000000000 | 1 | 112 | | MICROSECOND | 1000000 | 1 | 136 | | MILLISECOND | 1036 | 1 | 168 | +-------------+-----------------+------------------+----------------+ 

If the values associated with a given timer name are NULL , that timer is not supported on your platform. For an explanation of how event timing occurs, see Section 27.4.1, “Performance Schema Event Timing” . 

The performance_timers  table has these columns: 

• TIMER_NAME 

The timer name. 

• TIMER_FREQUENCY 

The number of timer units per second. For a cycle timer, the frequency is generally related to the CPU speed. For example, on a system with a 2.4GHz processor, the CYCLE  may be close to 2400000000. 

• TIMER_RESOLUTION 

Indicates the number of timer units by which timer values increase. If a timer has a resolution of 10, its value increases by 10 each time. 

• TIMER_OVERHEAD 

The minimal number of cycles of overhead to obtain one timing with the given timer. The Performance Schema determines this value by invoking the timer 20 times during initialization and picking the smallest value. The total overhead really is twice this amount because the instrumentation invokes the timer at the start and end of each event. The timer code is called only for timed events, so this overhead does not apply for nontimed events. 

## 27.12.21.5 The processlist Table 

The performance_timers  table has no indexes. 

TRUNCATE TABLE  is not permitted for the performance_timers  table. 

The MySQL process list indicates the operations currently being performed by the set of threads executing within the server. The processlist  table is one source of process information. For a comparison of this table with other sources, see Sources of Process Information. 

The processlist  table can be queried directly. If you have the PROCESS  privilege, you can see all threads, even those belonging to other users. Otherwise (without the PROCESS  privilege), nonanonymous users have access to information about their own threads but not threads for other users, and anonymous users have no access to thread information. 

<img src='content_image/842117.jpg'>

## Note 

If the performance_schema_show_processlist  system variable is enabled, the processlist  table also serves as the basis for an alternative implementation underlying the SHOW PROCESSLIST  statement. For details, see later in this section. 

