Propagate default values from configure.ac to asciidoc.conf
[lttng-tools.git] / doc / man / lttng-calibrate.1.txt
1 lttng-calibrate(1)
2 ==================
3
4
5 NAME
6 ----
7 lttng-calibrate - Quantify LTTng overhead
8
9
10 SYNOPSIS
11 --------
12 [verse]
13 *lttng* ['linkgenoptions:(GENERAL OPTIONS)'] *calibrate*
14
15
16 DESCRIPTION
17 -----------
18 The `lttng calibrate` commands quantifies the overhead of LTTng tracers.
19
20 The `lttng calibrate` command can be used to find out the combined
21 average overhead of the LTTng tracers and the instrumentation mechanisms
22 used. This overhead can be calibrated in terms of time or using any of
23 the PMU performance counter available on the system.
24
25 For now, the only implemented calibration is the Linux kernel function
26 instrumentation (_kretprobes_).
27
28
29 Calibrate Linux kernel function instrumentation
30 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
31
32 As an example, we use an i7 processor with 4 general-purpose PMU
33 registers. This information is available by issuing `dmesg`, looking
34 for `generic registers`.
35
36 The following sequence of commands gathers a trace executing a kretprobe
37 hooked on an empty function, gathering PMU counters LLC
38 (Last Level Cache) misses information (use `lttng add-context --list` to
39 get the list of available PMU counters).
40
41 [role="term"]
42 ------------------------------------------------------------------------
43 lttng create calibrate-function
44 lttng enable-event calibrate --kernel \
45 --function=lttng_calibrate_kretprobe
46 lttng add-context --kernel --type=perf:cpu:LLC-load-misses \
47 --type=perf:cpu:LLC-store-misses \
48 --type=perf:cpu:LLC-prefetch-misses
49 lttng start
50
51 for a in $(seq 1 10); do
52 lttng calibrate --kernel --function
53 done
54
55 lttng destroy
56 babeltrace $(ls -1drt ~/lttng-traces/calibrate-function-* | tail -n 1)
57 ------------------------------------------------------------------------
58
59 The output from man:babeltrace(1) can be saved to a text file and
60 opened in a spreadsheet (for example, in LibreOffice) to focus on the
61 per-PMU counter delta between consecutive `calibrate_entry` and
62 `calibrate_return` events. Note that these counters are per-CPU, so
63 scheduling events would need to be present to account for migration
64 between CPUs. Therefore, for calibration purposes, only events staying
65 on the same CPU must be considered.
66
67 Here's an example of the average result, for the i7, on 10 samples:
68
69 [width="40%",options="header"]
70 |=============================================================
71 | PMU counter | Average | Standard deviation
72 | `perf_LLC_load_misses` | 5.0 | 0.577
73 | `perf_LLC_store_misses` | 1.6 | 0.516
74 | `perf_LLC_prefetch_misses` | 9.0 | 14.742
75 |=============================================================
76
77 As we can notice, the load and store misses are relatively stable across
78 runs (their standard deviation is relatively low) compared to the
79 prefetch misses. We could conclude from this information that LLC load
80 and store misses can be accounted for quite precisely, but prefetches
81 within a function seems to behave too erratically (not much causality
82 link between the code executed and the CPU prefetch activity) to be
83 accounted for.
84
85
86 include::common-cmd-options-head.txt[]
87
88
89 Domain
90 ~~~~~~
91 One of:
92
93 option:-k, option:--kernel::
94 Quantify LTTng overhead in the Linux kernel domain.
95
96 option:-u, option:--userspace::
97 Quantify LTTng overhead in the user space domain.
98
99
100 Calibration
101 ~~~~~~~~~~~
102 option:--function::
103 Use dynamic function entry/return probes to calibrate (default).
104 +
105 This option requires the option:--kernel option.
106
107
108 include::common-cmd-help-options.txt[]
109
110
111 include::common-cmd-footer.txt[]
112
113
114 SEE ALSO
115 --------
116 man:lttng(1)
This page took 0.034439 seconds and 5 git commands to generate.