Events are collected by means of instrumentation added to the server source code. Instruments time events, which is how the Performance Schema provides an idea of how long events take. It is also possible to configure instruments not to collect timing information. This section discusses the available timers and their characteristics, and how timing values are represented in events.
Two Performance Schema tables provide timer information:
Timers vary in precision and amount of overhead. To see what
timers are available and their characteristics, check the
mysql> SELECT * FROM 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 | | TICK | 105 | 1 | 2416 | +-------------+-----------------+------------------+----------------+
The columns have these meanings:
TIMER_NAMEcolumn shows the names of the available timers.
CYCLErefers to the timer that is based on the CPU (processor) cycle counter. The timers in
setup_timersthat you can use are those that do not have
NULLin the other columns. If the values associated with a given timer name are
NULL, that timer is not supported on your platform.
TIMER_FREQUENCYindicates the number of timer units per second. For a cycle timer, the frequency is generally related to the CPU speed. The value shown was obtained on a system with a 2.4GHz processor. The other timers are based on fixed fractions of seconds. For
TICK, the frequency may vary by platform (for example, some use 100 ticks/second, others 1000 ticks/second).
TIMER_RESOLUTIONindicates the number of timer units by which timer values increase at a time. If a timer has a resolution of 10, its value increases by 10 each time.
TIMER_OVERHEADis the minimal number of cycles of overhead to obtain one timing with the given timer. The overhead per event is twice the value displayed because the timer is invoked at the beginning and end of the event.
To see which timer is in effect or to change the timer, access
mysql> SELECT * FROM setup_timers; +------+------------+ | NAME | TIMER_NAME | +------+------------+ | wait | CYCLE | +------+------------+ mysql> UPDATE setup_timers SET TIMER_NAME = 'MICROSECOND' WHERE NAME = 'wait'; mysql> SELECT * FROM setup_timers; +------+-------------+ | NAME | TIMER_NAME | +------+-------------+ | wait | MICROSECOND | +------+-------------+
By default, the Performance Schema uses the best timer
available for each instrument type, but you can select a
different one. Generally the best timer is
CYCLE, which uses the CPU cycle counter
whenever possible to provide high precision and low overhead.
The precision offered by the cycle counter depends on
processor speed. If the processor runs at 1 GHz (one billion
cycles/second) or higher, the cycle counter delivers
sub-nanosecond precision. Using the cycle counter is much
cheaper than getting the actual time of day. For example, the
gettimeofday() function can take
hundreds of cycles, which is an unacceptable overhead for data
gathering that may occur thousands or millions of times per
Cycle counters also have disadvantages:
End users expect to see timings in wall-clock units, such as fractions of a second. Converting from cycles to fractions of seconds can be expensive. For this reason, the conversion is a quick and fairly rough multiplication operation.
Processor cycle rate might change, such as when a laptop goes into power-saving mode or when a CPU slows down to reduce heat generation. If a processor's cycle rate fluctuates, conversion from cycles to real-time units is subject to error.
Cycle counters might be unreliable or unavailable depending on the processor or the operating system. For example, on Pentiums, the instruction is
RDTSC(an assembly-language rather than a C instruction) and it is theoretically possible for the operating system to prevent user-mode programs from using it.
Some processor details related to out-of-order execution or multiprocessor synchronization might cause the counter to seem fast or slow by up to 1000 cycles.
MySQL works with cycle counters on x386 (Windows, OS X, Linux, Solaris, and other Unix flavors), PowerPC, and IA-64.
Rows in Performance Schema tables that store current events
and historical events have three columns to represent timing
TIMER_END indicate when the event started
and finished, and
TIMER_WAIT indicates the
setup_instruments table has
ENABLED column to indicate the
instruments for which to collect events. The table also has a
TIMED column to indicate which instruments
are timed. If an instrument is not enabled, it produces no
events. If an enabled instrument is not timed, events produced
by the instrument have
NULL for the
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.
Within events, times are stored in picoseconds (trillionths of a second) to normalize them to a standard unit, regardless of which timer is selected. The timer used for an event is the one in effect when event timing begins. This timer is used to convert start and end values to picoseconds for storage in the event.
Modifications to the
table affect monitoring immediately. Events already measured
are stored using the original timer unit, and events in
progress may use the original timer for the begin time and the
new timer for the end time. To avoid unpredictable results if
you make timer changes, use
TABLE to reset Performance Schema statistics.
The timer baseline (“time zero”) occurs at
Performance Schema initialization during server startup.
TIMER_END values in events represent
picoseconds since the baseline.
values are durations in picoseconds.
Picosecond values in events are approximate. Their accuracy is
subject to the usual forms of error associated with conversion
from one unit to another. If the
timer is used and the processor rate varies, there might be
drift. For these reasons, it is not reasonable to look at the
TIMER_START value for an event as an
accurate measure of time elapsed since server startup. On the
other hand, it is reasonable to use
TIMER_WAIT values in
BY clauses to order events by start time or
The choice of picoseconds in events rather than a value such
as microseconds has a performance basis. One implementation
goal was to show results in a uniform time unit, regardless of
the timer. In an ideal world this time unit would look like a
wall-clock unit and be reasonably precise; in other words,
microseconds. But to convert cycles or nanoseconds to
microseconds, it would be necessary to perform a division for
every instrumentation. Division is expensive on many
platforms. Multiplication is not expensive, so that is what is
used. Therefore, the time unit is an integer multiple of the
using a multiplier large enough to ensure that there is no
major precision loss. The result is that the time unit is
“picoseconds.” This precision is spurious, but
the decision enables overhead to be minimized.