1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
|
'\" t
.\" Title: pg_test_timing
.\" Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\" Date: 2024
.\" Manual: PostgreSQL 16.3 Documentation
.\" Source: PostgreSQL 16.3
.\" Language: English
.\"
.TH "PG_TEST_TIMING" "1" "2024" "PostgreSQL 16.3" "PostgreSQL 16.3 Documentation"
.\" -----------------------------------------------------------------
.\" * Define some portability stuff
.\" -----------------------------------------------------------------
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.\" http://bugs.debian.org/507673
.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.ie \n(.g .ds Aq \(aq
.el .ds Aq '
.\" -----------------------------------------------------------------
.\" * set default formatting
.\" -----------------------------------------------------------------
.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.\" -----------------------------------------------------------------
.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.SH "NAME"
pg_test_timing \- measure timing overhead
.SH "SYNOPSIS"
.HP \w'\fBpg_test_timing\fR\ 'u
\fBpg_test_timing\fR [\fIoption\fR...]
.SH "DESCRIPTION"
.PP
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
\fBEXPLAIN ANALYZE\fR
results\&.
.SH "OPTIONS"
.PP
pg_test_timing
accepts the following command\-line options:
.PP
\fB\-d \fR\fB\fIduration\fR\fR
.br
\fB\-\-duration=\fR\fB\fIduration\fR\fR
.RS 4
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\&.
.RE
.PP
\fB\-V\fR
.br
\fB\-\-version\fR
.RS 4
Print the
pg_test_timing
version and exit\&.
.RE
.PP
\fB\-?\fR
.br
\fB\-\-help\fR
.RS 4
Show help about
pg_test_timing
command line arguments, and exit\&.
.RE
.SH "USAGE"
.SS "Interpreting Results"
.PP
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:
.sp
.if n \{\
.RS 4
.\}
.nf
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
.fi
.if n \{\
.RE
.\}
.PP
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)\&.
.SS "Measuring Executor Timing Overhead"
.PP
When the query executor is running a statement using
\fBEXPLAIN ANALYZE\fR, 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:
.sp
.if n \{\
.RS 4
.\}
.nf
CREATE TABLE t AS SELECT * FROM generate_series(1,100000);
\etiming
SELECT COUNT(*) FROM t;
EXPLAIN ANALYZE SELECT COUNT(*) FROM t;
.fi
.if n \{\
.RE
.\}
.PP
The i7\-860 system measured runs the count query in 9\&.8 ms while the
\fBEXPLAIN ANALYZE\fR
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\&.
.SS "Changing Time Sources"
.PP
On some newer Linux systems, it\*(Aqs 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:
.sp
.if n \{\
.RS 4
.\}
.nf
# 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
.fi
.if n \{\
.RE
.\}
.PP
In this configuration, the sample
\fBEXPLAIN ANALYZE\fR
above takes 115\&.9 ms\&. That\*(Aqs 1061 ns of timing overhead, again a small multiple of what\*(Aqs 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
\fBEXPLAIN ANALYZE\fR
totals involving many timed operations would be inflated significantly by timing overhead\&.
.PP
FreeBSD also allows changing the time source on the fly, and it logs information about the timer selected during boot:
.sp
.if n \{\
.RS 4
.\}
.nf
# 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
.fi
.if n \{\
.RE
.\}
.PP
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\*(Aqll see for a clock source is "jiffies"\&. Jiffies are the older Linux software clock implementation, which can have good resolution when it\*(Aqs backed by fast enough timing hardware, as in this example:
.sp
.if n \{\
.RS 4
.\}
.nf
$ 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
.fi
.if n \{\
.RE
.\}
.SS "Clock Hardware and Timing Accuracy"
.PP
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\&.
.PP
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\&.
.PP
The Time Stamp Counter (TSC) clock source is the most accurate one available on current generation CPUs\&. It\*(Aqs the preferred way to track the system time when it\*(Aqs 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\*(Aqs 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\&.
.PP
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\*(Aqt 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\*(Aqs known to be inaccurate\&.
.PP
The High Precision Event Timer (HPET) is the preferred timer on systems where it\*(Aqs 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\&.
.PP
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\&.
.PP
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\&.
.SH "SEE ALSO"
\fBEXPLAIN\fR(7)
|
