25.12.4.1 The events_waits_current Table

The events_waits_current table contains current wait events. The table stores one row per thread showing the current status of the thread's most recent monitored wait event, so there is no system variable for configuring the table size.

Of the tables that contain wait event rows, events_waits_current is the most fundamental. Other tables that contain wait event rows are logically derived from the current events. For example, the events_waits_history and events_waits_history_long tables are collections of the most recent wait events that have ended, up to a maximum number of rows per thread and globally across all threads, respectively.

For more information about the relationship between the three wait event tables, see Section 25.9, “Performance Schema Tables for Current and Historical Events”.

For information about configuring whether to collect wait events, see Section 25.12.4, “Performance Schema Wait Event Tables”.

The events_waits_current table has these columns:

• THREAD_ID, EVENT_ID

  The thread associated with the event and the thread current event number when the event starts. The THREAD_ID and EVENT_ID values taken together uniquely identify the row. No two rows have the same pair of values.

• END_EVENT_ID

  This column is set to NULL when the event starts and updated to the thread current event number when the event ends.

• EVENT_NAME

  The name of the instrument that produced the event. This is a NAME value from the setup_instruments table. Instrument names may have multiple parts and form a hierarchy, as discussed in Section 25.6, “Performance Schema Instrument Naming Conventions”.

• SOURCE

  The name of the source file containing the instrumented code that produced the event and the line number in the file at which the instrumentation occurs. This enables you to check the source to determine exactly what code is involved. For example, if a mutex or lock is being blocked, you can check the context in which this occurs.

• TIMER_START, TIMER_END, TIMER_WAIT

  Timing information for the event. The unit for these values is picoseconds (trillionths of a second). The TIMER_START and TIMER_END values indicate when event timing started and ended. TIMER_WAIT is the event elapsed time (duration).

  If an event has not finished, TIMER_END is the current timer value and TIMER_WAIT is the time elapsed so far (TIMER_END − TIMER_START).

  If an event is produced from an instrument that has TIMED = NO, timing information is not collected, and TIMER_START, TIMER_END, and TIMER_WAIT are all NULL.

  For discussion of picoseconds as the unit for event times and factors that affect time values, see Section 25.4.1, “Performance Schema Event Timing”.

• SPINS

  For a mutex, the number of spin rounds. If the value is NULL, the code does not use spin rounds or spinning is not instrumented.

• OBJECT_SCHEMA, OBJECT_NAME, OBJECT_TYPE, OBJECT_INSTANCE_BEGIN

  These columns identify the object “being acted on.” What that means depends on the object type.

  For a synchronization object (cond, mutex, rwlock):

  • OBJECT_SCHEMA, OBJECT_NAME, and OBJECT_TYPE are NULL.

  • OBJECT_INSTANCE_BEGIN is the address of the synchronization object in memory.

  For a file I/O object:

  • OBJECT_SCHEMA is NULL.

  • OBJECT_NAME is the file name.

  • OBJECT_TYPE is FILE.

  • OBJECT_INSTANCE_BEGIN is an address in memory.

  For a socket object:

  • OBJECT_NAME is the IP:PORT value for the socket.

  • OBJECT_INSTANCE_BEGIN is an address in memory.

  For a table I/O object:

  • OBJECT_SCHEMA is the name of the schema that contains the table.

  • OBJECT_NAME is the table name.

  • OBJECT_TYPE is TABLE for a persistent base table or TEMPORARY TABLE for a temporary table.

  • OBJECT_INSTANCE_BEGIN is an address in memory.

  An OBJECT_INSTANCE_BEGIN value itself has no meaning, except that different values indicate different objects. OBJECT_INSTANCE_BEGIN can be used for debugging. For example, it can be used with GROUP BY OBJECT_INSTANCE_BEGIN to see whether the load on 1,000 mutexes (that protect, say, 1,000 pages or blocks of data) is spread evenly or just hitting a few bottlenecks. This can help you correlate with other sources of information if you see the same object address in a log file or another debugging or performance tool.

• INDEX_NAME

  The name of the index used. PRIMARY indicates the table primary index. NULL means that no index was used.

• NESTING_EVENT_ID

  The EVENT_ID value of the event within which this event is nested.

• NESTING_EVENT_TYPE

  The nesting event type. The value is TRANSACTION, STATEMENT, STAGE, or WAIT.

• OPERATION

  The type of operation performed, such as lock, read, or write.

• NUMBER_OF_BYTES

  The number of bytes read or written by the operation. For table I/O waits (events for the wait/io/table/sql/handler instrument), NUMBER_OF_BYTES indicates the number of rows. If the value is greater than 1, the event is for a batch I/O operation. The following discussion describes the difference between exclusively single-row reporting and reporting that reflects batch I/O.

  MySQL executes joins using a nested-loop implementation. The job of the Performance Schema instrumentation is to provide row count and accumulated execution time per table in the join. Assume a join query of the following form that is executed using a table join order of t1, t2, t3:

  SELECT ... FROM t1 JOIN t2 ON ... JOIN t3 ON ...

  Table “fanout” is the increase or decrease in number of rows from adding a table during join processing. If the fanout for table t3 is greater than 1, the majority of row-fetch operations are for that table. Suppose that the join accesses 10 rows from t1, 20 rows from t2 per row from t1, and 30 rows from t3 per row of table t2. With single-row reporting, the total number of instrumented operations is:

  10 + (10 * 20) + (10 * 20 * 30) = 6210

  A significant reduction in the number of instrumented operations is achievable by aggregating them per scan (that is, per unique combination of rows from t1 and t2). With batch I/O reporting, the Performance Schema produces an event for each scan of the innermost table t3 rather than for each row, and the number of instrumented row operations reduces to:

  10 + (10 * 20) + (10 * 20) = 410

  That is a reduction of 93%, illustrating how the batch-reporting strategy significantly reduces Performance Schema overhead for table I/O by reducing the number of reporting calls. The tradeoff is lesser accuracy for event timing. Rather than time for an individual row operation as in per-row reporting, timing for batch I/O includes time spent for operations such as join buffering, aggregation, and returning rows to the client.

  For batch I/O reporting to occur, these conditions must be true:

  • Query execution accesses the innermost table of a query block (for a single-table query, that table counts as innermost)

  • Query execution does not request a single row from the table (so, for example, eq_ref access prevents use of batch reporting)

  • Query execution does not evaluate a subquery containing table access for the table

• FLAGS

  Reserved for future use.

TRUNCATE TABLE is permitted for the events_waits_current table. It removes the rows.