MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0
The setup_instruments
table lists
classes of instrumented objects for which events can be
collected:
mysql> SELECT * FROM performance_schema.setup_instruments\G
*************************** 1. row ***************************
NAME: wait/synch/mutex/pfs/LOCK_pfs_share_list
ENABLED: NO
TIMED: NO
PROPERTIES: singleton
VOLATILITY: 1
DOCUMENTATION: Components can provide their own performance_schema tables.
This lock protects the list of such tables definitions.
...
*************************** 369. row ***************************
NAME: stage/sql/executing
ENABLED: NO
TIMED: NO
PROPERTIES:
VOLATILITY: 0
DOCUMENTATION: NULL
...
*************************** 687. row ***************************
NAME: statement/abstract/Query
ENABLED: YES
TIMED: YES
PROPERTIES: mutable
VOLATILITY: 0
DOCUMENTATION: SQL query just received from the network. At this point,
the real statement type is unknown, the type will be
refined after SQL parsing.
...
*************************** 696. row ***************************
NAME: memory/performance_schema/metadata_locks
ENABLED: YES
TIMED: NULL
PROPERTIES: global_statistics
VOLATILITY: 1
DOCUMENTATION: Memory used for table performance_schema.metadata_locks
...
Each instrument added to the source code provides a row for
the setup_instruments
table, even
when the instrumented code is not executed. When an instrument
is enabled and executed, instrumented instances are created,
which are visible in the
tables, such as xxx
_instancesfile_instances
or
rwlock_instances
.
Modifications to most
setup_instruments
rows affect
monitoring immediately. For some instruments, modifications
are effective only at server startup; changing them at runtime
has no effect. This affects primarily mutexes, conditions, and
rwlocks in the server, although there may be other instruments
for which this is true.
For more information about the role of the
setup_instruments
table in event
filtering, see
Section 27.4.3, “Event Pre-Filtering”.
The setup_instruments
table has
these columns:
NAME
The instrument name. Instrument names may have multiple
parts and form a hierarchy, as discussed in
Section 27.6, “Performance Schema Instrument Naming Conventions”.
Events produced from execution of an instrument have an
EVENT_NAME
value that is taken from the
instrument NAME
value. (Events do not
really have a “name,” but this provides a way
to associate events with instruments.)
ENABLED
Whether the instrument is enabled. The value is
YES
or NO
. A
disabled instrument produces no events. This column can be
modified, although setting ENABLED
has
no effect for instruments that have already been created.
TIMED
Whether the instrument is timed. The value is
YES
, NO
, or
NULL
. This column can be modified,
although setting TIMED
has no effect
for instruments that have already been created.
A TIMED
value of
NULL
indicates that the instrument does
not support timing. For example, memory operations are not
timed, so their TIMED
column is
NULL
.
Setting TIMED
to
NULL
for an instrument that supports
timing has no effect, as does setting
TIMED
to non-NULL
for an instrument that does not support timing.
If an enabled instrument is not timed, the instrument code
is enabled, but the timer is not. Events produced by the
instrument have NULL
for the
TIMER_START
,
TIMER_END
, and
TIMER_WAIT
timer values. This in turn
causes those values to be ignored when calculating the
sum, minimum, maximum, and average time values in summary
tables.
PROPERTIES
The instrument properties. This column uses the
SET
data type, so multiple
flags from the following list can be set per instrument:
global_statistics
: The instrument
produces only global summaries. Summaries for finer
levels are unavailable, such as per thread, account,
user, or host. For example, most memory instruments
produce only global summaries.
mutable
: The instrument can
“mutate” into a more specific one. This
property applies only to statement instruments.
progress
: The instrument is capable
of reporting progress data. This property applies only
to stage instruments.
singleton
: The instrument has a
single instance. For example, most global mutex locks
in the server are singletons, so the corresponding
instruments are as well.
user
: The instrument is directly
related to user workload (as opposed to system
workload). One such instrument is
wait/io/socket/sql/client_connection
.
VOLATILITY
The instrument volatility. Volatility values range from
low to high. The values correspond to the
PSI_VOLATILITY_
constants defined in the
xxx
mysql/psi/psi_base.h
header file:
#define PSI_VOLATILITY_UNKNOWN 0 #define PSI_VOLATILITY_PERMANENT 1 #define PSI_VOLATILITY_PROVISIONING 2 #define PSI_VOLATILITY_DDL 3 #define PSI_VOLATILITY_CACHE 4 #define PSI_VOLATILITY_SESSION 5 #define PSI_VOLATILITY_TRANSACTION 6 #define PSI_VOLATILITY_QUERY 7 #define PSI_VOLATILITY_INTRA_QUERY 8
The VOLATILITY
column is purely
informational, to provide users (and the Performance
Schema code) some hint about the instrument runtime
behavior.
Instruments with a low volatility index (PERMANENT = 1) are created once at server startup, and never destroyed or re-created during normal server operation. They are destroyed only during server shutdown.
For example, the
wait/synch/mutex/pfs/LOCK_pfs_share_list
mutex is defined with a volatility of 1, which means it is
created once. Possible overhead from the instrumentation
itself (namely, mutex initialization) has no effect for
this instrument then. Runtime overhead occurs only when
locking or unlocking the mutex.
Instruments with a higher volatility index (for example,
SESSION = 5) are created and destroyed for every user
session. For example, the
wait/synch/mutex/sql/THD::LOCK_query_plan
mutex is created each time a session connects, and
destroyed when the session disconnects.
This mutex is more sensitive to Performance Schema overhead, because overhead comes not only from the lock and unlock instrumentation, but also from mutex create and destroy instrumentation, which is executed more often.
Another aspect of volatility concerns whether and when an
update to the ENABLED
column actually
has some effect:
An update to ENABLED
affects
instrumented objects created subsequently, but has no
effect on instruments already created.
Instruments that are more “volatile” use
new settings from the
setup_instruments
table
sooner.
For example, this statement does not affect the
LOCK_query_plan
mutex for existing
sessions, but does have an effect on new sessions created
subsequent to the update:
UPDATE performance_schema.setup_instruments
SET ENABLED=value
WHERE NAME = 'wait/synch/mutex/sql/THD::LOCK_query_plan';
This statement actually has no effect at all:
UPDATE performance_schema.setup_instruments
SET ENABLED=value
WHERE NAME = 'wait/synch/mutex/pfs/LOCK_pfs_share_list';
This mutex is permanent, and was created already before
the update is executed. The mutex is never created again,
so the ENABLED
value in
setup_instruments
is never
used. To enable or disable this mutex, use the
mutex_instances
table
instead.
DOCUMENTATION
A string describing the instrument purpose. The value is
NULL
if no description is available.
The setup_instruments
table has
these indexes:
Primary key on (NAME
)
TRUNCATE TABLE
is not permitted
for the setup_instruments
table.