# lttng-calibrate

## NAME



           lttng-calibrate - Quantify LTTng overhead
    


## SYNOPSIS



           **lttng**[_GENERAL OPTIONS_] **calibrate**
    


## DESCRIPTION



           The **lttng calibrate** commands quantifies the overhead of LTTng
           tracers.
    
           The **lttng calibrate** command can be used to find out the combined
           average overhead of the LTTng tracers and the instrumentation
           mechanisms used. This overhead can be calibrated in terms of time
           or using any of the PMU performance counter available on the
           system.
    
           For now, the only implemented calibration is the Linux kernel
           function instrumentation (_kretprobes_).
    
       **Calibrate Linux kernel function instrumentation**
           As an example, we use an i7 processor with 4 general-purpose PMU
           registers. This information is available by issuing **dmesg** , looking
           for **generic registers**.
    
           The following sequence of commands gathers a trace executing a
           kretprobe hooked on an empty function, gathering PMU counters LLC
           (Last Level Cache) misses information (use **lttng add-context**
           **--list** to get the list of available PMU counters).
    
               lttng create calibrate-function
               lttng enable-event calibrate --kernel \
                                            --function=lttng_calibrate_kretprobe
               lttng add-context --kernel --type=perf:cpu:LLC-load-misses \
                                          --type=perf:cpu:LLC-store-misses \
                                          --type=perf:cpu:LLC-prefetch-misses
               lttng start
    
               for a in $(seq 1 10); do
                   lttng calibrate --kernel --function
               done
    
               lttng destroy
               babeltrace $(ls -1drt ~/lttng-traces/calibrate-function-* | tail -n 1)
    
           The output from **babeltrace**(1) can be saved to a text file and
           opened in a spreadsheet (for example, in LibreOffice) to focus on
           the per-PMU counter delta between consecutive **calibrate_entry** and
           **calibrate_return** events. Note that these counters are per-CPU, so
           scheduling events would need to be present to account for
           migration between CPUs. Therefore, for calibration purposes, only
           events staying on the same CPU must be considered.
    
           Here’s an example of the average result, for the i7, on 10
           samples:
           ┌──────────────────────────┬─────────┬────────────────────┐
           │ **PMU counter** │ **Average** │ **Standard deviation** │
           ├──────────────────────────┼─────────┼────────────────────┤
           │                          │         │                    │
           │ **perf_LLC_load_misses** │ 5.0     │ 0.577              │
           ├──────────────────────────┼─────────┼────────────────────┤
           │                          │         │                    │
           │ **perf_LLC_store_misses** │ 1.6     │ 0.516              │
           ├──────────────────────────┼─────────┼────────────────────┤
           │                          │         │                    │
           │ **perf_LLC_prefetch_misses** │ 9.0     │ 14.742             │
           └──────────────────────────┴─────────┴────────────────────┘
    
           As we can notice, the load and store misses are relatively stable
           across runs (their standard deviation is relatively low) compared
           to the prefetch misses. We could conclude from this information
           that LLC load and store misses can be accounted for quite
           precisely, but prefetches within a function seems to behave too
           erratically (not much causality link between the code executed and
           the CPU prefetch activity) to be accounted for.
    


## OPTIONS



           General options are described in lttng(1).
    
       **Domain**
           One of:
    
           **-k** , **--kernel**
               Quantify LTTng overhead in the Linux kernel domain.
    
           **-u** , **--userspace**
               Quantify LTTng overhead in the user space domain.
    
       **Calibration**
           **--function**
               Use dynamic function entry/return probes to calibrate
               (default).
    
               This option requires the **--kernel** option.
    
       **Program information**
           **-h** , **--help**
               Show command help.
    
               This option, like lttng-help(1), attempts to launch
               **/usr/bin/man** to view the command’s man page. The path to the
               man pager can be overridden by the **LTTNG_MAN_BIN_PATH**
               environment variable.
    
           **--list-options**
               List available command options.
    


## ENVIRONMENT VARIABLES



           **LTTNG_ABORT_ON_ERROR**
               Set to 1 to abort the process after the first error is
               encountered.
    
           **LTTNG_HOME**
               Overrides the **$HOME** environment variable. Useful when the user
               running the commands has a non-writable home directory.
    
           **LTTNG_MAN_BIN_PATH**
               Absolute path to the man pager to use for viewing help
               information about LTTng commands (using lttng-help(1) or **lttng**
               **COMMAND --help**).
    
           **LTTNG_SESSION_CONFIG_XSD_PATH**
               Path in which the **session.xsd** session configuration XML schema
               may be found.
    
           **LTTNG_SESSIOND_PATH**
               Full session daemon binary path.
    
               The **--sessiond-path** option has precedence over this
               environment variable.
    
           Note that the lttng-create(1) command can spawn an LTTng session
           daemon automatically if none is running. See lttng-sessiond(8) for
           the environment variables influencing the execution of the session
           daemon.
    


## FILES



           **$LTTNG_HOME/.lttngrc**
               User LTTng runtime configuration.
    
               This is where the per-user current tracing session is stored
               between executions of lttng(1). The current tracing session
               can be set with lttng-set-session(1). See lttng-create(1) for
               more information about tracing sessions.
    
           **$LTTNG_HOME/lttng-traces**
               Default output directory of LTTng traces. This can be
               overridden with the **--output** option of the lttng-create(1)
               command.
    
           **$LTTNG_HOME/.lttng**
               User LTTng runtime and configuration directory.
    
           **$LTTNG_HOME/.lttng/sessions**
               Default location of saved user tracing sessions (see
               lttng-save(1) and lttng-load(1)).
    
           **/usr/local/etc/lttng/sessions**
               System-wide location of saved tracing sessions (see
               lttng-save(1) and lttng-load(1)).
    
               **Note**
    
               **$LTTNG_HOME** defaults to **$HOME** when not explicitly set.
    


## EXIT STATUS



           **0**
               Success
    
           **1**
               Command error
    
           **2**
               Undefined command
    
           **3**
               Fatal error
    
           **4**
               Command warning (something went wrong during the command)
    


## BUGS



           If you encounter any issue or usability problem, please report it
           on the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-
           tools>.
    


## RESOURCES



           •   LTTng project website <http://lttng.org>
    
           •   LTTng documentation <http://lttng.org/docs>
    
           •   Git repositories <http://git.lttng.org>
    
           •   GitHub organization <http://github.com/lttng>
    
           •   Continuous integration <http://ci.lttng.org/>
    
           •   Mailing list <http://lists.lttng.org> for support and
               development: **lttng-dev@lists.lttng.org**
    
           •   IRC channel <irc://irc.oftc.net/lttng>: **#lttng** on **irc.oftc.net**
    


## THANKS



           Special thanks to Michel Dagenais and the DORSAL laboratory
           <http://www.dorsal.polymtl.ca/> at École Polytechnique de Montréal
           for the LTTng journey.
    
           Also thanks to the Ericsson teams working on tracing which helped
           us greatly with detailed bug reports and unusual test cases.
    


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