определите накладные расходы LTTng (Quantify LTTng overhead)
Имя (Name)
lttng-calibrate - Quantify LTTng overhead
Синопсис (Synopsis)
lttng [GENERAL OPTIONS] calibrate
Описание (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.
Параметры (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.
Переменные окружения (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.
Файлы (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.
Статус выхода (Exit)
0
Success
1
Command error
2
Undefined command
3
Fatal error
4
Command warning (something went wrong during the command)
Ошибки (баги) (Bugs)
If you encounter any issue or usability problem, please report it
on the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-
tools>.
Ресурсы (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
