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Peter Eisentraut 2012-05-10 19:53:41 +03:00
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doc/src/sgml/pgtesttiming.sgml
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@ -29,7 +29,7 @@
<para>
<application>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
Systems that are slow to collect timing data can give less accurate
<command>EXPLAIN ANALYZE</command> results.
</para>
</refsect1>
@ -68,11 +68,10 @@
<title>Interpreting results</title>
<para>
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:
</para>
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:
<screen>
Testing timing overhead for 3 seconds.
@ -85,12 +84,13 @@ Histogram of timing durations:
2: 2999652 3.59518%
1: 80435604 96.40465%
</screen>
</para>
<para>
Note that different units are used for the per loop time than the
histogram. The loop can have resolution within a few nanoseconds
(nsec), while the individual timing calls can only resolve down to
one microsecond (usec).
Note that different units are used for the per loop time than the
histogram. The loop can have resolution within a few nanoseconds (nsec),
while the individual timing calls can only resolve down to one microsecond
(usec).
</para>
</refsect2>
@ -98,11 +98,10 @@ Histogram of timing durations:
<title>Measuring executor timing overhead</title>
<para>
When the query executor is running a statement using
<command>EXPLAIN ANALYZE</command>, 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:
</para>
When the query executor is running a statement using
<command>EXPLAIN ANALYZE</command>, 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:
<screen>
CREATE TABLE t AS SELECT * FROM generate_series(1,100000);
@ -110,16 +109,16 @@ CREATE TABLE t AS SELECT * FROM generate_series(1,100000);
SELECT COUNT(*) FROM t;
EXPLAIN ANALYZE SELECT COUNT(*) FROM t;
</screen>
</para>
<para>
The i7-860 system measured runs the count query in 9.8 ms while
the <command>EXPLAIN ANALYZE</command> 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.
The i7-860 system measured runs the count query in 9.8 ms while
the <command>EXPLAIN ANALYZE</command> 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.
</para>
</refsect2>
@ -127,14 +126,13 @@ EXPLAIN ANALYZE SELECT COUNT(*) FROM t;
<refsect2>
<title>Changing time sources</title>
<para>
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:
</para>
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:
<screen>
# cat /sys/devices/system/clocksource/clocksource0/available_clocksource
# 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
@ -147,45 +145,43 @@ Histogram of timing durations:
2: 2990371 72.05956%
1: 1155682 27.84870%
</screen>
<para>
In this configuration, the sample <command>EXPLAIN ANALYZE</command>
above takes 115.9 ms. That's 1061 nsec 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 <command>EXPLAIN ANALYZE</command> totals involving many
timed operations would be inflated significantly by timing overhead.
</para>
<para>
FreeBSD also allows changing the time source on the fly, and
it logs information about the timer selected during boot:
In this configuration, the sample <command>EXPLAIN ANALYZE</command> above
takes 115.9 ms. That's 1061 nsec 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 <command>EXPLAIN ANALYZE</command> totals involving
many timed operations would be inflated significantly by timing overhead.
</para>
<para>
FreeBSD also allows changing the time source on the fly, and it logs
information about the timer selected during boot:
<screen>
dmesg | grep "Timecounter"
dmesg | grep "Timecounter"
sysctl kern.timecounter.hardware=TSC
</screen>
</para>
<para>
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:
</para>
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:
<screen>
$ cat /sys/devices/system/clocksource/clocksource0/available_clocksource
jiffies
$ 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
$ pg_test_timing
Testing timing overhead for 3 seconds.
Per timing duration including loop overhead: 97.75 ns
Histogram of timing durations:
@ -197,76 +193,74 @@ Histogram of timing durations:
2: 2993204 9.75277%
1: 27694571 90.23734%
</screen>
</para>
</refsect2>
<refsect2>
<title>Clock hardware and timing accuracy</title>
<para>
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.
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.
</para>
<para>
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.
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.
</para>
<para>
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 multi-core 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.
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 multi-core 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.
</para>
<para>
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.
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.
</para>
<para>
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.
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.
</para>
<para>
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.
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.
</para>
<para>
Timers used on older PC hardware including 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.
Timers used on older PC hardware including 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.
</para>
</refsect2>
</refsect1>