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diff --git a/doc/src/sgml/ref/pgtesttiming.sgml b/doc/src/sgml/ref/pgtesttiming.sgml new file mode 100644 index 0000000..a5eb3aa --- /dev/null +++ b/doc/src/sgml/ref/pgtesttiming.sgml @@ -0,0 +1,303 @@ +<!-- +doc/src/sgml/ref/pgtesttiming.sgml +PostgreSQL documentation +--> + +<refentry id="pgtesttiming"> + <indexterm zone="pgtesttiming"> + <primary>pg_test_timing</primary> + </indexterm> + + <refmeta> + <refentrytitle><application>pg_test_timing</application></refentrytitle> + <manvolnum>1</manvolnum> + <refmiscinfo>Application</refmiscinfo> + </refmeta> + + <refnamediv> + <refname>pg_test_timing</refname> + <refpurpose>measure timing overhead</refpurpose> + </refnamediv> + + <refsynopsisdiv> + <cmdsynopsis> + <command>pg_test_timing</command> + <arg rep="repeat"><replaceable>option</replaceable></arg> + </cmdsynopsis> + </refsynopsisdiv> + + <refsect1> + <title>Description</title> + + <para> + <application>pg_test_timing</application> 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 + <command>EXPLAIN ANALYZE</command> results. + </para> + </refsect1> + + <refsect1> + <title>Options</title> + + <para> + <application>pg_test_timing</application> accepts the following + command-line options: + + <variablelist> + + <varlistentry> + <term><option>-d <replaceable class="parameter">duration</replaceable></option></term> + <term><option>--duration=<replaceable class="parameter">duration</replaceable></option></term> + <listitem> + <para> + 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. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><option>-V</option></term> + <term><option>--version</option></term> + <listitem> + <para> + Print the <application>pg_test_timing</application> version and exit. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><option>-?</option></term> + <term><option>--help</option></term> + <listitem> + <para> + Show help about <application>pg_test_timing</application> command line + arguments, and exit. + </para> + </listitem> + </varlistentry> + + </variablelist> + </para> + + </refsect1> + + <refsect1> + <title>Usage</title> + + <refsect2> + <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: + +<screen><![CDATA[ +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 +]]></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 (ns), + while the individual timing calls can only resolve down to one microsecond + (us). + </para> + + </refsect2> + <refsect2> + <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 <application>psql</application> program: + +<screen> +CREATE TABLE t AS SELECT * FROM generate_series(1,100000); +\timing +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. + </para> + + </refsect2> + + <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: + +<screen><![CDATA[ +# 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 +]]></screen> + </para> + + <para> + In this configuration, the sample <command>EXPLAIN ANALYZE</command> 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 <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" +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 +</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: + +<screen><![CDATA[ +$ 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 +]]></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. + </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. + </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 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. + </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. + </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. + </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. + </para> + + <para> + 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. + </para> + </refsect2> + </refsect1> + + <refsect1> + <title>See Also</title> + + <simplelist type="inline"> + <member><xref linkend="sql-explain"/></member> + </simplelist> + </refsect1> +</refentry> |