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26.12.4.1 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 26.9, “Performance Schema Tables for Current and Historical Events”.
For information about configuring whether to collect wait events, see Section 26.12.4, “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
andEVENT_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 thesetup_instruments
table. Instrument names may have multiple parts and form a hierarchy, as discussed in Section 26.6, “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
andTIMER_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 andTIMER_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, andTIMER_START
,TIMER_END
, andTIMER_WAIT
are allNULL
.For discussion of picoseconds as the unit for event times and factors that affect time values, see Section 26.4.1, “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
, andOBJECT_TYPE
areNULL
.OBJECT_INSTANCE_BEGIN
is the address of the synchronization object in memory.
For a file I/O object:
OBJECT_SCHEMA
isNULL
.OBJECT_NAME
is the file name.OBJECT_TYPE
isFILE
.OBJECT_INSTANCE_BEGIN
is an address in memory.
For a socket object:
OBJECT_NAME
is theIP: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
isTABLE
for a persistent base table orTEMPORARY 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 withGROUP 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
, orWAIT
.OPERATION
The type of operation performed, such as
lock
,read
, orwrite
.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
: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 fromt1
, 20 rows fromt2
per row fromt1
, and 30 rows fromt3
per row of tablet2
. 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
andt2
). With batch I/O reporting, the Performance Schema produces an event for each scan of the innermost tablet3
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.
The events_waits_current
table
has these indexes:
Primary key on (
THREAD_ID
,EVENT_ID
)
TRUNCATE TABLE
is permitted for
the events_waits_current
table.
It removes the rows.
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