pg_test_timing — measure timing overhead
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.
pg_test_timing accepts the following command-line options:
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.
Print the pg_test_timing version and exit.
Show help about pg_test_timing command line arguments, and exit.
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
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.