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Change pg_test_timing to measure in nanoseconds not microseconds.

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Most of our platforms have better-than-microsecond timing resolution,
so the original definition of this program is getting less and less
useful.  Make it report nanoseconds not microseconds.  Also, add a
second output table that reports the exact observed timing durations,
up to a limit of 1024 ns; and be sure to report the largest observed
duration.

The documentation for this program included a lot of system-specific
details that now seem largely obsolete.  Move all that text to the
PG wiki, where perhaps it will be easier to maintain and update.

Also, improve the TAP test so that it actually runs a short standard
run, allowing most of the code to be exercised; its coverage before
was abysmal.

Author: Hannu Krosing <hannuk@google.com>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/be0339cc-1ae1-4892-9445-8e6d8995a44d@eisentraut.org
This commit is contained in:
Tom Lane
2025-07-08 11:23:15 -04:00
parent a27893df45
commit 0b096e379e
3 changed files with 258 additions and 224 deletions
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293
doc/src/sgml/ref/pgtesttiming.sgml
View File

@ -30,11 +30,23 @@ PostgreSQL documentation
<title>Description</title> <title>Description</title>
<para> <para>
<application>pg_test_timing</application> is a tool to measure the timing overhead <application>pg_test_timing</application> is a tool to measure the
on your system and confirm that the system time never moves backwards. timing overhead on your system and confirm that the system time never
moves backwards. It simply reads the system clock over and over again
as fast as it can for a specified length of time, and then prints
statistics about the observed differences in successive clock readings.
</para>
<para>
Smaller (but not zero) differences are better, since they imply both
more-precise clock hardware and less overhead to collect a clock reading.
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. <command>EXPLAIN ANALYZE</command> results.
</para> </para>
<para>
This tool is also helpful to determine if
the <varname>track_io_timing</varname> configuration parameter is likely
to produce useful results.
</para>
</refsect1> </refsect1>
<refsect1> <refsect1>
@ -59,6 +71,21 @@ PostgreSQL documentation
</listitem> </listitem>
</varlistentry> </varlistentry>
<varlistentry>
<term><option>-c <replaceable class="parameter">cutoff</replaceable></option></term>
<term><option>--cutoff=<replaceable class="parameter">cutoff</replaceable></option></term>
<listitem>
<para>
Specifies the cutoff percentage for the list of exact observed
timing durations (that is, the changes in the system clock value
from one reading to the next). The list will end once the running
percentage total reaches or exceeds this value, except that the
largest observed duration will always be printed. The default
cutoff is 99.99.
</para>
</listitem>
</varlistentry>
<varlistentry> <varlistentry>
<term><option>-V</option></term> <term><option>-V</option></term>
<term><option>--version</option></term> <term><option>--version</option></term>
@ -92,204 +119,82 @@ PostgreSQL documentation
<title>Interpreting Results</title> <title>Interpreting Results</title>
<para> <para>
Good results will show most (>90%) individual timing calls take less than The first block of output has four columns, with rows showing a
one microsecond. Average per loop overhead will be even lower, below 100 shifted-by-one log2(ns) histogram of timing durations (that is, the
nanoseconds. This example from an Intel i7-860 system using a TSC clock differences between successive clock readings). This is not the
source shows excellent performance: classic log2(n+1) histogram as it counts zeros separately and then
switches to log2(ns) starting from value 1.
</para>
<para>
The columns are:
<itemizedlist spacing="compact">
<listitem>
<simpara>nanosecond value that is &gt;= the durations in this
bucket</simpara>
</listitem>
<listitem>
<simpara>percentage of durations in this bucket</simpara>
</listitem>
<listitem>
<simpara>running-sum percentage of durations in this and previous
buckets</simpara>
</listitem>
<listitem>
<simpara>count of durations in this bucket</simpara>
</listitem>
</itemizedlist>
</para>
<para>
The second block of output goes into more detail, showing the exact
timing differences observed. For brevity this list is cut off when the
running-sum percentage exceeds the user-selectable cutoff value.
However, the largest observed difference is always shown.
</para>
<para>
The example results below show that 99.99% of timing loops took between
8 and 31 nanoseconds, with the worst case somewhere between 32768 and
65535 nanoseconds. In the second block, we can see that typical loop
time is 16 nanoseconds, and the readings appear to have full nanosecond
precision.
</para>
<para>
<screen><![CDATA[ <screen><![CDATA[
Testing timing overhead for 3 seconds. Testing timing overhead for 3 seconds.
Per loop time including overhead: 35.96 ns Per loop time including overhead: 16.40 ns
Histogram of timing durations: Histogram of timing durations:
< us % of total count <= ns % of total running % count
1 96.40465 80435604 0 0.0000 0.0000 0
2 3.59518 2999652 1 0.0000 0.0000 0
4 0.00015 126 3 0.0000 0.0000 0
8 0.00002 13 7 0.0000 0.0000 0
16 0.00000 2 15 4.5452 4.5452 8313178
31 95.4527 99.9979 174581501
63 0.0001 99.9981 253
127 0.0001 99.9982 165
255 0.0000 99.9982 35
511 0.0000 99.9982 1
1023 0.0013 99.9994 2300
2047 0.0004 99.9998 690
4095 0.0000 99.9998 9
8191 0.0000 99.9998 8
16383 0.0002 100.0000 337
32767 0.0000 100.0000 2
65535 0.0000 100.0000 1
Observed timing durations up to 99.9900%:
ns % of total running % count
15 4.5452 4.5452 8313178
16 58.3785 62.9237 106773354
17 33.6840 96.6078 61607584
18 3.1151 99.7229 5697480
19 0.2638 99.9867 482570
20 0.0093 99.9960 17054
...
38051 0.0000 100.0000 1
]]></screen> ]]></screen>
</para> </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> </refsect2>
</refsect1> </refsect1>
@ -298,6 +203,8 @@ Histogram of timing durations:
<simplelist type="inline"> <simplelist type="inline">
<member><xref linkend="sql-explain"/></member> <member><xref linkend="sql-explain"/></member>
<member><ulink url="https://wiki.postgresql.org/wiki/Pg_test_timing">Wiki
discussion about timing</ulink></member>
</simplelist> </simplelist>
</refsect1> </refsect1>
</refentry> </refentry>

172
src/bin/pg_test_timing/pg_test_timing.c
View File

@ -9,19 +9,30 @@
#include <limits.h> #include <limits.h>
#include "getopt_long.h" #include "getopt_long.h"
#include "port/pg_bitutils.h"
#include "portability/instr_time.h" #include "portability/instr_time.h"
static const char *progname; static const char *progname;
static unsigned int test_duration = 3; static unsigned int test_duration = 3;
static double max_rprct = 99.99;
/* record duration in powers of 2 nanoseconds */
static long long int histogram[32];
/* record counts of first 1024 durations directly */
#define NUM_DIRECT 1024
static long long int direct_histogram[NUM_DIRECT];
/* separately record highest observed duration */
static int32 largest_diff;
static long long int largest_diff_count;
static void handle_args(int argc, char *argv[]); static void handle_args(int argc, char *argv[]);
static uint64 test_timing(unsigned int duration); static uint64 test_timing(unsigned int duration);
static void output(uint64 loop_count); static void output(uint64 loop_count);
/* record duration in powers of 2 microseconds */
static long long int histogram[32];
int int
main(int argc, char *argv[]) main(int argc, char *argv[])
{ {
@ -44,6 +55,7 @@ handle_args(int argc, char *argv[])
{ {
static struct option long_options[] = { static struct option long_options[] = {
{"duration", required_argument, NULL, 'd'}, {"duration", required_argument, NULL, 'd'},
{"cutoff", required_argument, NULL, 'c'},
{NULL, 0, NULL, 0} {NULL, 0, NULL, 0}
}; };
@ -56,7 +68,7 @@ handle_args(int argc, char *argv[])
{ {
if (strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-?") == 0) if (strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-?") == 0)
{ {
printf(_("Usage: %s [-d DURATION]\n"), progname); printf(_("Usage: %s [-d DURATION] [-c CUTOFF]\n"), progname);
exit(0); exit(0);
} }
if (strcmp(argv[1], "--version") == 0 || strcmp(argv[1], "-V") == 0) if (strcmp(argv[1], "--version") == 0 || strcmp(argv[1], "-V") == 0)
@ -66,7 +78,7 @@ handle_args(int argc, char *argv[])
} }
} }
while ((option = getopt_long(argc, argv, "d:", while ((option = getopt_long(argc, argv, "d:c:",
long_options, &optindex)) != -1) long_options, &optindex)) != -1)
{ {
switch (option) switch (option)
@ -93,6 +105,26 @@ handle_args(int argc, char *argv[])
} }
break; break;
case 'c':
errno = 0;
max_rprct = strtod(optarg, &endptr);
if (endptr == optarg || *endptr != '\0' || errno != 0)
{
fprintf(stderr, _("%s: invalid argument for option %s\n"),
progname, "--cutoff");
fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
exit(1);
}
if (max_rprct < 0 || max_rprct > 100)
{
fprintf(stderr, _("%s: %s must be in range %u..%u\n"),
progname, "--cutoff", 0, 100);
exit(1);
}
break;
default: default:
fprintf(stderr, _("Try \"%s --help\" for more information.\n"), fprintf(stderr, _("Try \"%s --help\" for more information.\n"),
progname); progname);
@ -111,7 +143,6 @@ handle_args(int argc, char *argv[])
exit(1); exit(1);
} }
printf(ngettext("Testing timing overhead for %u second.\n", printf(ngettext("Testing timing overhead for %u second.\n",
"Testing timing overhead for %u seconds.\n", "Testing timing overhead for %u seconds.\n",
test_duration), test_duration),
@ -130,19 +161,19 @@ test_timing(unsigned int duration)
end_time, end_time,
temp; temp;
total_time = duration > 0 ? duration * INT64CONST(1000000) : 0; total_time = duration > 0 ? duration * INT64CONST(1000000000) : 0;
INSTR_TIME_SET_CURRENT(start_time); INSTR_TIME_SET_CURRENT(start_time);
cur = INSTR_TIME_GET_MICROSEC(start_time); cur = INSTR_TIME_GET_NANOSEC(start_time);
while (time_elapsed < total_time) while (time_elapsed < total_time)
{ {
int32 diff, int32 diff,
bits = 0; bits;
prev = cur; prev = cur;
INSTR_TIME_SET_CURRENT(temp); INSTR_TIME_SET_CURRENT(temp);
cur = INSTR_TIME_GET_MICROSEC(temp); cur = INSTR_TIME_GET_NANOSEC(temp);
diff = cur - prev; diff = cur - prev;
/* Did time go backwards? */ /* Did time go backwards? */
@ -154,18 +185,30 @@ test_timing(unsigned int duration)
} }
/* What is the highest bit in the time diff? */ /* What is the highest bit in the time diff? */
while (diff) if (diff > 0)
{ bits = pg_leftmost_one_pos32(diff) + 1;
diff >>= 1; else
bits++; bits = 0;
}
/* Update appropriate duration bucket */ /* Update appropriate duration bucket */
histogram[bits]++; histogram[bits]++;
/* Update direct histogram of time diffs */
if (diff < NUM_DIRECT)
direct_histogram[diff]++;
/* Also track the largest observed duration, even if >= NUM_DIRECT */
if (diff > largest_diff)
{
largest_diff = diff;
largest_diff_count = 1;
}
else if (diff == largest_diff)
largest_diff_count++;
loop_count++; loop_count++;
INSTR_TIME_SUBTRACT(temp, start_time); INSTR_TIME_SUBTRACT(temp, start_time);
time_elapsed = INSTR_TIME_GET_MICROSEC(temp); time_elapsed = INSTR_TIME_GET_NANOSEC(temp);
} }
INSTR_TIME_SET_CURRENT(end_time); INSTR_TIME_SET_CURRENT(end_time);
@ -181,28 +224,95 @@ test_timing(unsigned int duration)
static void static void
output(uint64 loop_count) output(uint64 loop_count)
{ {
int64 max_bit = 31, int max_bit = 31;
i; const char *header1 = _("<= ns");
char *header1 = _("< us"); const char *header1b = _("ns");
char *header2 = /* xgettext:no-c-format */ _("% of total"); const char *header2 = /* xgettext:no-c-format */ _("% of total");
char *header3 = _("count"); const char *header3 = /* xgettext:no-c-format */ _("running %");
const char *header4 = _("count");
int len1 = strlen(header1); int len1 = strlen(header1);
int len2 = strlen(header2); int len2 = strlen(header2);
int len3 = strlen(header3); int len3 = strlen(header3);
int len4 = strlen(header4);
double rprct;
bool stopped = false;
/* find highest bit value */ /* find highest bit value */
while (max_bit > 0 && histogram[max_bit] == 0) while (max_bit > 0 && histogram[max_bit] == 0)
max_bit--; max_bit--;
printf(_("Histogram of timing durations:\n")); /* set minimum column widths */
printf("%*s %*s %*s\n", len1 = Max(8, len1);
Max(6, len1), header1, len2 = Max(10, len2);
Max(10, len2), header2, len3 = Max(10, len3);
Max(10, len3), header3); len4 = Max(10, len4);
for (i = 0; i <= max_bit; i++) printf(_("Histogram of timing durations:\n"));
printf("%*ld %*.5f %*lld\n", printf("%*s %*s %*s %*s\n",
Max(6, len1), 1l << i, len1, header1,
Max(10, len2) - 1, (double) histogram[i] * 100 / loop_count, len2, header2,
Max(10, len3), histogram[i]); len3, header3,
len4, header4);
rprct = 0;
for (int i = 0; i <= max_bit; i++)
{
double prct = (double) histogram[i] * 100 / loop_count;
rprct += prct;
printf("%*ld %*.4f %*.4f %*lld\n",
len1, (1L << i) - 1,
len2, prct,
len3, rprct,
len4, histogram[i]);
}
printf(_("\nObserved timing durations up to %.4f%%:\n"), max_rprct);
printf("%*s %*s %*s %*s\n",
len1, header1b,
len2, header2,
len3, header3,
len4, header4);
rprct = 0;
for (int i = 0; i < NUM_DIRECT; i++)
{
if (direct_histogram[i])
{
double prct = (double) direct_histogram[i] * 100 / loop_count;
bool print_it = !stopped;
rprct += prct;
/* if largest diff is < NUM_DIRECT, be sure we print it */
if (i == largest_diff)
{
if (stopped)
printf("...\n");
print_it = true;
}
if (print_it)
printf("%*d %*.4f %*.4f %*lld\n",
len1, i,
len2, prct,
len3, rprct,
len4, direct_histogram[i]);
if (rprct >= max_rprct)
stopped = true;
}
}
/* print largest diff when it's outside the array range */
if (largest_diff >= NUM_DIRECT)
{
double prct = (double) largest_diff_count * 100 / loop_count;
printf("...\n");
printf("%*d %*.4f %*.4f %*lld\n",
len1, largest_diff,
len2, prct,
len3, 100.0,
len4, largest_diff_count);
}
} }

17
src/bin/pg_test_timing/t/001_basic.pl
View File

@ -25,5 +25,22 @@ command_fails_like(
[ 'pg_test_timing', '--duration' => '0' ], [ 'pg_test_timing', '--duration' => '0' ],
qr/\Qpg_test_timing: --duration must be in range 1..4294967295\E/, qr/\Qpg_test_timing: --duration must be in range 1..4294967295\E/,
'pg_test_timing: --duration must be in range'); 'pg_test_timing: --duration must be in range');
command_fails_like(
[ 'pg_test_timing', '--cutoff' => '101' ],
qr/\Qpg_test_timing: --cutoff must be in range 0..100\E/,
'pg_test_timing: --cutoff must be in range');
#########################################
# We obviously can't check for specific output, but we can
# do a simple run and make sure it produces something.
command_like(
[ 'pg_test_timing', '--duration' => '1' ],
qr/
\QTesting timing overhead for 1 second.\E.*
\QHistogram of timing durations:\E.*
\QObserved timing durations up to 99.9900%:\E
/sx,
'pg_test_timing: sanity check');
done_testing(); done_testing();