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:

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.

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.

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: [email protected]

• IRC channel <irc://irc.oftc.net/lttng>: #lttng on irc.oftc.net

COPYRIGHTS

This program is part of the LTTng-tools project.

LTTng-tools is distributed under the GNU General Public License version 2 <http://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html>. See the LICENSE <https://github.com/lttng/lttng-tools/blob/master/LICENSE> file for details.

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.

AUTHORS

LTTng-tools was originally written by Mathieu Desnoyers, Julien Desfossez, and David Goulet. More people have since contributed to it.

LTTng-tools is currently maintained by Jérémie Galarneau <mailto:[email protected]>.