projects / lttng-tools.git / blame
| Commit | Line | Data |
|---|---|---|
| 89dbeff8 PP |
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) |