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pg_test_timing(1)        PostgreSQL 12.0 Documentation       pg_test_timing(1)




NAME

       pg_test_timing - measure timing overhead


SYNOPSIS

       pg_test_timing [option...]


DESCRIPTION

       pg_test_timing is a tool to measure the timing overhead on your system
       and confirm that the system time never moves backwards. Systems that
       are slow to collect timing data can give less accurate EXPLAIN ANALYZE
       results.


OPTIONS

       pg_test_timing accepts the following command-line options:

       -d duration
       --duration=duration
           Specifies the test duration, in seconds. Longer durations give
           slightly better accuracy, and are more likely to discover problems
           with the system clock moving backwards. The default test duration
           is 3 seconds.

       -V
       --version
           Print the pg_test_timing version and exit.

       -?
       --help
           Show help about pg_test_timing command line arguments, and exit.


USAGE

   Interpreting Results
       Good results will show most (>90%) individual timing calls take less
       than one microsecond. Average per loop overhead will be even lower,
       below 100 nanoseconds. This example from an Intel i7-860 system using a
       TSC clock source shows excellent performance:

           Testing timing overhead for 3 seconds.
           Per loop time including overhead: 35.96 ns
           Histogram of timing durations:
             < us   % of total      count
                1     96.40465   80435604
                2      3.59518    2999652
                4      0.00015        126
                8      0.00002         13
               16      0.00000          2

       Note that different units are used for the per loop time than the
       histogram. The loop can have resolution within a few nanoseconds (ns),
       while the individual timing calls can only resolve down to one
       microsecond (us).

   Measuring Executor Timing Overhead
       When the query executor is running a statement using EXPLAIN ANALYZE,
       individual operations are timed as well as showing a summary. The
       overhead of your system can be checked by counting rows with the psql
       program:

           CREATE TABLE t AS SELECT * FROM generate_series(1,100000);
           \timing
           SELECT COUNT(*) FROM t;
           EXPLAIN ANALYZE SELECT COUNT(*) FROM t;

       The i7-860 system measured runs the count query in 9.8 ms while the
       EXPLAIN ANALYZE version takes 16.6 ms, each processing just over
       100,000 rows. That 6.8 ms difference means the timing overhead per row
       is 68 ns, about twice what pg_test_timing estimated it would be. Even
       that relatively small amount of overhead is making the fully timed
       count statement take almost 70% longer. On more substantial queries,
       the timing overhead would be less problematic.

   Changing Time Sources
       On some newer Linux systems, it's possible to change the clock source
       used to collect timing data at any time. A second example shows the
       slowdown possible from switching to the slower acpi_pm time source, on
       the same system used for the fast results above:

           # cat /sys/devices/system/clocksource/clocksource0/available_clocksource
           tsc hpet acpi_pm
           # echo acpi_pm > /sys/devices/system/clocksource/clocksource0/current_clocksource
           # pg_test_timing
           Per loop time including overhead: 722.92 ns
           Histogram of timing durations:
             < us   % of total      count
                1     27.84870    1155682
                2     72.05956    2990371
                4      0.07810       3241
                8      0.01357        563
               16      0.00007          3

       In this configuration, the sample EXPLAIN ANALYZE above takes 115.9 ms.
       That's 1061 ns of timing overhead, again a small multiple of what's
       measured directly by this utility. That much timing overhead means the
       actual query itself is only taking a tiny fraction of the accounted for
       time, most of it is being consumed in overhead instead. In this
       configuration, any EXPLAIN ANALYZE totals involving many timed
       operations would be inflated significantly by timing overhead.

       FreeBSD also allows changing the time source on the fly, and it logs
       information about the timer selected during boot:

           # dmesg | grep "Timecounter"
           Timecounter "ACPI-fast" frequency 3579545 Hz quality 900
           Timecounter "i8254" frequency 1193182 Hz quality 0
           Timecounters tick every 10.000 msec
           Timecounter "TSC" frequency 2531787134 Hz quality 800
           # sysctl kern.timecounter.hardware=TSC
           kern.timecounter.hardware: ACPI-fast -> TSC

       Other systems may only allow setting the time source on boot. On older
       Linux systems the "clock" kernel setting is the only way to make this
       sort of change. And even on some more recent ones, the only option
       you'll see for a clock source is "jiffies". Jiffies are the older Linux
       software clock implementation, which can have good resolution when it's
       backed by fast enough timing hardware, as in this example:

           $ cat /sys/devices/system/clocksource/clocksource0/available_clocksource
           jiffies
           $ dmesg | grep time.c
           time.c: Using 3.579545 MHz WALL PM GTOD PIT/TSC timer.
           time.c: Detected 2400.153 MHz processor.
           $ pg_test_timing
           Testing timing overhead for 3 seconds.
           Per timing duration including loop overhead: 97.75 ns
           Histogram of timing durations:
             < us   % of total      count
                1     90.23734   27694571
                2      9.75277    2993204
                4      0.00981       3010
                8      0.00007         22
               16      0.00000          1
               32      0.00000          1

   Clock Hardware and Timing Accuracy
       Collecting accurate timing information is normally done on computers
       using hardware clocks with various levels of accuracy. With some
       hardware the operating systems can pass the system clock time almost
       directly to programs. A system clock can also be derived from a chip
       that simply provides timing interrupts, periodic ticks at some known
       time interval. In either case, operating system kernels provide a clock
       source that hides these details. But the accuracy of that clock source
       and how quickly it can return results varies based on the underlying
       hardware.

       Inaccurate time keeping can result in system instability. Test any
       change to the clock source very carefully. Operating system defaults
       are sometimes made to favor reliability over best accuracy. And if you
       are using a virtual machine, look into the recommended time sources
       compatible with it. Virtual hardware faces additional difficulties when
       emulating timers, and there are often per operating system settings
       suggested by vendors.

       The Time Stamp Counter (TSC) clock source is the most accurate one
       available on current generation CPUs. It's the preferred way to track
       the system time when it's supported by the operating system and the TSC
       clock is reliable. There are several ways that TSC can fail to provide
       an accurate timing source, making it unreliable. Older systems can have
       a TSC clock that varies based on the CPU temperature, making it
       unusable for timing. Trying to use TSC on some older multicore CPUs can
       give a reported time that's inconsistent among multiple cores. This can
       result in the time going backwards, a problem this program checks for.
       And even the newest systems can fail to provide accurate TSC timing
       with very aggressive power saving configurations.

       Newer operating systems may check for the known TSC problems and switch
       to a slower, more stable clock source when they are seen. If your
       system supports TSC time but doesn't default to that, it may be
       disabled for a good reason. And some operating systems may not detect
       all the possible problems correctly, or will allow using TSC even in
       situations where it's known to be inaccurate.

       The High Precision Event Timer (HPET) is the preferred timer on systems
       where it's available and TSC is not accurate. The timer chip itself is
       programmable to allow up to 100 nanosecond resolution, but you may not
       see that much accuracy in your system clock.

       Advanced Configuration and Power Interface (ACPI) provides a Power
       Management (PM) Timer, which Linux refers to as the acpi_pm. The clock
       derived from acpi_pm will at best provide 300 nanosecond resolution.

       Timers used on older PC hardware include the 8254 Programmable Interval
       Timer (PIT), the real-time clock (RTC), the Advanced Programmable
       Interrupt Controller (APIC) timer, and the Cyclone timer. These timers
       aim for millisecond resolution.


SEE ALSO

       EXPLAIN(7)



PostgreSQL 12.0                      2019                    pg_test_timing(1)

postgresql 12.0 - Generated Mon Oct 28 18:21:19 CDT 2019
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