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)