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* ['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 ------------------------------------------------------------------------
  42 lttng create calibrate-function
  43 lttng enable-event calibrate --kernel \
  44                              --function=lttng_calibrate_kretprobe
  45 lttng add-context --kernel --type=perf:cpu:LLC-load-misses \
  46                            --type=perf:cpu:LLC-store-misses \
  47                            --type=perf:cpu:LLC-prefetch-misses
  48 lttng start
  49
  50 for a in $(seq 1 10); do
  51     lttng calibrate --kernel --function
  52 done
  53
  54 lttng destroy
  55 babeltrace $(ls -1drt ~/lttng-traces/calibrate-function-* | tail -n 1)
  56 ------------------------------------------------------------------------
  57
  58 The output from linklttng:babeltrace(1) can be saved to a text file and
  59 opened in a spreadsheet (for example, in LibreOffice) to focus on the
  60 per-PMU counter delta between consecutive `calibrate_entry` and
  61 `calibrate_return` events. Note that these counters are per-CPU, so
  62 scheduling events would need to be present to account for migration
  63 between CPUs. Therefore, for calibration purposes, only events staying
  64 on the same CPU must be considered.
  65
  66 Here's an example of the average result, for the i7, on 10 samples:
  67
  68 [width="40%",options="header"]
  69 |=============================================================
  70 | PMU counter                | Average   | Standard deviation
  71 | `perf_LLC_load_misses`     | 5.0       | 0.577
  72 | `perf_LLC_store_misses`    | 1.6       | 0.516
  73 | `perf_LLC_prefetch_misses` | 9.0       | 14.742
  74 |=============================================================
  75
  76 As we can notice, the load and store misses are relatively stable across
  77 runs (their standard deviation is relatively low) compared to the
  78 prefetch misses. We could conclude from this information that LLC load
  79 and store misses can be accounted for quite precisely, but prefetches
  80 within a function seems to behave too erratically (not much causality
  81 link between the code executed and the CPU prefetch activity) to be
  82 accounted for.
  83
  84
  85 include::common-cmd-options-head.txt[]
  86
  87
  88 Domain
  89 ~~~~~~
  90 One of:
  91
  92 option:-k, option:--kernel::
  93     Quantify LTTng overhead in the Linux kernel domain.
  94
  95 option:-u, option:--userspace::
  96     Quantify LTTng overhead in the user space domain.
  97
  98
  99 Calibration
 100 ~~~~~~~~~~~
 101 option:--function::
 102     Use dynamic function entry/return probes to calibrate (default).
 103 +
 104 This option requires the option:--kernel option.
 105
 106
 107 include::common-cmd-help-options.txt[]
 108
 109
 110 include::common-cmd-footer.txt[]
 111
 112
 113 SEE ALSO
 114 --------
 115 linklttng:lttng(1)