| 1 | lttng-enable-channel(1) |
| 2 | ======================= |
| 3 | |
| 4 | |
| 5 | NAME |
| 6 | ---- |
| 7 | lttng-enable-channel - Create or enable LTTng channels |
| 8 | |
| 9 | |
| 10 | SYNOPSIS |
| 11 | -------- |
| 12 | Create a Linux kernel channel: |
| 13 | |
| 14 | [verse] |
| 15 | *lttng* ['GENERAL OPTIONS'] *enable-channel* option:--kernel |
| 16 | [option:--discard | option:--overwrite] [option:--output=(`mmap` | `splice`)] |
| 17 | [option:--subbuf-size='SIZE'] [option:--num-subbuf='COUNT'] |
| 18 | [option:--switch-timer='PERIODUS'] [option:--read-timer='PERIODUS'] |
| 19 | [option:--tracefile-size='SIZE'] [option:--tracefile-count='COUNT'] |
| 20 | [option:--session='SESSION'] 'CHANNEL' |
| 21 | |
| 22 | Create a user space channel: |
| 23 | |
| 24 | [verse] |
| 25 | *lttng* ['GENERAL OPTIONS'] *enable-channel* option:--userspace |
| 26 | [option:--discard | option:--overwrite] [option:--buffers-pid] |
| 27 | [option:--subbuf-size='SIZE'] [option:--num-subbuf='COUNT'] |
| 28 | [option:--switch-timer='PERIODUS'] [option:--read-timer='PERIODUS'] |
| 29 | [option:--tracefile-size='SIZE'] [option:--tracefile-count='COUNT'] |
| 30 | [option:--session='SESSION'] 'CHANNEL' |
| 31 | |
| 32 | Enable existing channel(s): |
| 33 | |
| 34 | [verse] |
| 35 | *lttng* ['GENERAL OPTIONS'] *enable-channel* (option:--userspace | option:--kernel) |
| 36 | [option:--session='SESSION'] 'CHANNEL'[,'CHANNEL']... |
| 37 | |
| 38 | |
| 39 | DESCRIPTION |
| 40 | ----------- |
| 41 | The `lttng enable-channel` command can create a new channel, or enable |
| 42 | one or more existing and disabled ones. |
| 43 | |
| 44 | A channel is the owner of sub-buffers holding recorded events. Event, |
| 45 | rules, when created using linklttng:lttng-enable-event(1), are always |
| 46 | assigned to a channel. When creating a new channel, many parameters |
| 47 | related to those sub-buffers can be fine-tuned. They are described in |
| 48 | the subsections below. |
| 49 | |
| 50 | When 'CHANNEL' does not name an existing channel, a channel named |
| 51 | 'CHANNEL' is created. Otherwise, the disabled channel named 'CHANNEL' |
| 52 | is enabled. |
| 53 | |
| 54 | Note that the linklttng:lttng-enable-event(1) command can automatically |
| 55 | create default channels when no channel exist. |
| 56 | |
| 57 | A channel is always contained in a tracing session |
| 58 | (see linklttng:lttng-create(1) for creating a tracing session). The |
| 59 | session in which a channel is created using `lttng enable-channel` can |
| 60 | be specified using the option:--session option. If the option:--session |
| 61 | option is omitted, the current tracing session is targeted. |
| 62 | |
| 63 | Existing enabled channels can be disabled using |
| 64 | linklttng:lttng-disable-channel(1). Channels of a given session can be |
| 65 | listed using linklttng:lttng-list(1). |
| 66 | |
| 67 | As of this version of LTTng, it is not possible to: |
| 68 | |
| 69 | * Reconfigure a channel once it is created. |
| 70 | * Re-enable a disabled channel once its tracing session has been active |
| 71 | at least once. |
| 72 | * Create a channel once its tracing session has been active |
| 73 | at least once. |
| 74 | * Create a user space channel with a given buffering scheme |
| 75 | (option:--buffers-uid or option:--buffers-pid options) and create |
| 76 | a second user space channel with a different buffering scheme in the |
| 77 | same tracing session. |
| 78 | |
| 79 | |
| 80 | Event loss modes |
| 81 | ~~~~~~~~~~~~~~~~ |
| 82 | LTTng tracers are non-blocking: when no empty sub-buffer exists, |
| 83 | losing events is acceptable when the alternative would be to cause |
| 84 | substantial delays in the instrumented application's execution. |
| 85 | |
| 86 | LTTng privileges performance over integrity, aiming at perturbing the |
| 87 | traced system as little as possible in order to make tracing of subtle |
| 88 | race conditions and rare interrupt cascades possible. |
| 89 | |
| 90 | When it comes to losing events because no empty sub-buffer is available, |
| 91 | the channel's event loss mode, specified by one of the option:--discard |
| 92 | and option:--overwrite options, determines what to do amongst: |
| 93 | |
| 94 | Discard:: |
| 95 | Drop the newest events until a sub-buffer is released. |
| 96 | |
| 97 | Overwrite:: |
| 98 | Clear the sub-buffer containing the oldest recorded events and start |
| 99 | recording the newest events there. This mode is sometimes called |
| 100 | _flight recorder mode_ because it behaves like a flight recorder: |
| 101 | always keep a fixed amount of the latest data. |
| 102 | |
| 103 | Which mechanism to choose depends on the context: prioritize the newest |
| 104 | or the oldest events in the ring buffer? |
| 105 | |
| 106 | Beware that, in overwrite mode (option:--overwrite option), a whole |
| 107 | sub-buffer is abandoned as soon as a new event doesn't find an empty |
| 108 | sub-buffer, whereas in discard mode (option:--discard option), only the |
| 109 | event that doesn't fit is discarded. |
| 110 | |
| 111 | Also note that a count of lost events is incremented and saved in the |
| 112 | trace itself when an event is lost in discard mode, whereas no |
| 113 | information is kept when a sub-buffer gets overwritten before being |
| 114 | committed. |
| 115 | |
| 116 | The probability of losing events, if it is experience in a given |
| 117 | context, can be reduced by fine-tuning the sub-buffers count and size |
| 118 | (see next subsection). |
| 119 | |
| 120 | |
| 121 | Sub-buffers count and size |
| 122 | ~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| 123 | The option:--num-subbuf and option:--subbuf-size options respectively |
| 124 | set the number of sub-buffers and their individual size when creating |
| 125 | a new channel. |
| 126 | |
| 127 | Note that there is a noticeable tracer's CPU overhead introduced when |
| 128 | switching sub-buffers (marking a full one as consumable and switching |
| 129 | to an empty one for the following events to be recorded). Knowing this, |
| 130 | the following list presents a few practical situations along with how |
| 131 | to configure sub-buffers for them when creating a channel in overwrite |
| 132 | mode (option:--overwrite option): |
| 133 | |
| 134 | High event throughput:: |
| 135 | In general, prefer bigger sub-buffers to lower the risk of losing |
| 136 | events. Having bigger sub-buffers also ensures a lower sub-buffer |
| 137 | switching frequency. The number of sub-buffers is only meaningful |
| 138 | if the channel is enabled in overwrite mode: in this case, if a |
| 139 | sub-buffer overwrite happens, the other sub-buffers |
| 140 | are left unaltered. |
| 141 | |
| 142 | Low event throughput:: |
| 143 | In general, prefer smaller sub-buffers since the risk of losing |
| 144 | events is already low. Since events happen less frequently, the |
| 145 | sub-buffer switching frequency should remain low and thus the |
| 146 | tracer's overhead should not be a problem. |
| 147 | |
| 148 | Low memory system:: |
| 149 | If the target system has a low memory limit, prefer fewer first, |
| 150 | then smaller sub-buffers. Even if the system is limited in memory, |
| 151 | it is recommended to keep the sub-buffers as big as possible to |
| 152 | avoid a high sub-buffer switching frequency. |
| 153 | |
| 154 | In discard mode (option:--discard option), the sub-buffers count |
| 155 | parameter is pointless: using two sub-buffers and setting their size |
| 156 | according to the requirements of the context is fine. |
| 157 | |
| 158 | |
| 159 | Switch and read timers |
| 160 | ~~~~~~~~~~~~~~~~~~~~~~ |
| 161 | When a channel's switch timer fires, a sub-buffer switch happens. This |
| 162 | timer may be used to ensure that event data is consumed and committed |
| 163 | to trace files periodically in case of a low event throughput. |
| 164 | |
| 165 | It's also convenient when big sub-buffers are used to cope with sporadic |
| 166 | high event throughput, even if the throughput is normally lower. |
| 167 | |
| 168 | By default, a notification mechanism is used to signal a full sub-buffer |
| 169 | so that it can be consumed. When such notifications must be avoided, |
| 170 | for example in real-time applications, the channel's read timer can be |
| 171 | used instead. When the read timer fires, sub-buffers are checked for |
| 172 | consumption when they are full. |
| 173 | |
| 174 | |
| 175 | Buffering scheme |
| 176 | ~~~~~~~~~~~~~~~~ |
| 177 | In the user space tracing domain, two buffering schemes are available |
| 178 | when creating a channel: |
| 179 | |
| 180 | Per-process buffering (option:--buffers-pid option):: |
| 181 | Keep one ring buffer per process. |
| 182 | |
| 183 | Per-user buffering (option:--buffers-uid option):: |
| 184 | Keep one ring buffer for all the processes of a single user. |
| 185 | |
| 186 | The per-process buffering scheme consumes more memory than the per-user |
| 187 | option if more than one process is instrumented for LTTng-UST. |
| 188 | However, per-process buffering ensures that one process having a high |
| 189 | event throughput won't fill all the shared sub-buffers, only its own. |
| 190 | |
| 191 | The Linux kernel tracing domain only has one available buffering scheme |
| 192 | which is to use a single ring buffer for the whole system |
| 193 | (option:--buffers-global option). |
| 194 | |
| 195 | |
| 196 | Trace files limit and size |
| 197 | ~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| 198 | By default, trace files can grow as large as needed. The maximum size |
| 199 | of each trace file written by a channel can be set on creation using the |
| 200 | option:--tracefile-size option. When such a trace file's size reaches |
| 201 | the channel's fixed maximum size, another trace file is created to hold |
| 202 | the next recorded events. A file count is appended to each trace file |
| 203 | name in this case. |
| 204 | |
| 205 | If the option:--tracefile-size option is used, the maximum number of |
| 206 | created trace files is unlimited. To limit them, the |
| 207 | option:--tracefile-count option can be used. This option is always used |
| 208 | in conjunction with the option:--tracefile-size option. |
| 209 | |
| 210 | For example, consider this command: |
| 211 | |
| 212 | ----------------------------------------------------- |
| 213 | lttng enable-channel --kernel --tracefile-size=4096 \ |
| 214 | --tracefile-count=32 my-channel |
| 215 | ----------------------------------------------------- |
| 216 | |
| 217 | Here, for each stream, the maximum size of each trace file is |
| 218 | 4 kiB and there can be a maximum of 32 different files. When there is |
| 219 | no space left in the last file, _trace file rotation_ happens: the first |
| 220 | file is cleared and new sub-buffers containing events are written there. |
| 221 | |
| 222 | |
| 223 | include::common-cmd-options-head.txt[] |
| 224 | |
| 225 | |
| 226 | Domain |
| 227 | ~~~~~~ |
| 228 | One of: |
| 229 | |
| 230 | option:-k, option:--kernel:: |
| 231 | Enable channel in the Linux kernel domain. |
| 232 | |
| 233 | option:-u, option:--userspace:: |
| 234 | Enable channel in the user space domain. |
| 235 | |
| 236 | |
| 237 | Target |
| 238 | ~~~~~~ |
| 239 | option:-s, option:--session='SESSION':: |
| 240 | Create or enable channel in the tracing session named 'SESSION' |
| 241 | instead of the current tracing session. |
| 242 | |
| 243 | |
| 244 | Event loss mode |
| 245 | ~~~~~~~~~~~~~~~ |
| 246 | One of: |
| 247 | |
| 248 | option:--discard:: |
| 249 | Discard events when sub-buffers are full (default). |
| 250 | |
| 251 | option:--overwrite:: |
| 252 | Flight recorder mode: always keep a fixed amount of the latest |
| 253 | data. |
| 254 | |
| 255 | |
| 256 | Sub-buffers |
| 257 | ~~~~~~~~~~~ |
| 258 | option:--num-subbuf='COUNT':: |
| 259 | Use 'COUNT' sub-buffers. Rounded up to the next power of two. |
| 260 | + |
| 261 | Default values: |
| 262 | + |
| 263 | * `metadata` channel: 2 |
| 264 | * Everything else: 4 |
| 265 | |
| 266 | option:--subbuf-size='SIZE':: |
| 267 | Set the individual size of sub-buffers to 'SIZE' bytes. |
| 268 | The `k` (kiB), `M` (MiB), and `G` (GiB) suffixes are supported. |
| 269 | Rounded up to the next power of two. |
| 270 | + |
| 271 | The minimum sub-buffer size, for each tracer, is the maximum value |
| 272 | between the default below and the system's page size. The following |
| 273 | command shows the current system's page size: `getconf PAGE_SIZE`. |
| 274 | + |
| 275 | Default values: |
| 276 | + |
| 277 | * option:--userspace and option:--buffers-uid options: `128k` |
| 278 | * option:--userspace and option:--buffers-pid options: `4k` |
| 279 | * option:--kernel option: `256k` |
| 280 | * `metadata` channel: `4k` |
| 281 | |
| 282 | option:--output='TYPE':: |
| 283 | Set channel's output type to 'TYPE'. |
| 284 | + |
| 285 | Available types: `mmap` (always available) and `splice` (only available |
| 286 | with the option:--kernel option). |
| 287 | + |
| 288 | Default values: |
| 289 | + |
| 290 | * option:--userspace and option:--buffers-uid options: `mmap` |
| 291 | * option:--userspace and option:--buffers-pid options: `mmap` |
| 292 | * option:--kernel option: `splice` |
| 293 | * `metadata` channel: `mmap` |
| 294 | |
| 295 | Buffering scheme |
| 296 | ~~~~~~~~~~~~~~~~ |
| 297 | One of: |
| 298 | |
| 299 | option:--buffers-global:: |
| 300 | Use shared sub-buffers for the whole system (only available with the |
| 301 | option:--kernel option). |
| 302 | |
| 303 | option:--buffers-pid:: |
| 304 | Use different sub-buffers for each traced process (only available |
| 305 | with the the option:--userspace option). This is the default |
| 306 | buffering scheme for user space channels. |
| 307 | |
| 308 | option:--buffers-uid:: |
| 309 | Use shared sub-buffers for all the processes of the user running |
| 310 | the command (only available with the option:--userspace option). |
| 311 | |
| 312 | |
| 313 | Trace files |
| 314 | ~~~~~~~~~~~ |
| 315 | option:--tracefile-count='COUNT':: |
| 316 | Limit the number of trace files created by this channel to |
| 317 | 'COUNT'. 0 means unlimited. Default: 0. |
| 318 | + |
| 319 | Use this option in conjunction with the option:--tracefile-size option. |
| 320 | + |
| 321 | The file count within a stream is appended to each created trace |
| 322 | file. If 'COUNT' files are created and more events need to be recorded, |
| 323 | the first trace file of the stream is cleared and used again. |
| 324 | |
| 325 | option:--tracefile-size='SIZE':: |
| 326 | Set the maximum size of each trace file written by |
| 327 | this channel within a stream to 'SIZE' bytes. 0 means unlimited. |
| 328 | Default: 0. |
| 329 | + |
| 330 | Note: traces generated with this option may inaccurately report |
| 331 | discarded events as of CTF 1.8. |
| 332 | |
| 333 | |
| 334 | Timers |
| 335 | ~~~~~~ |
| 336 | option:--read-timer:: |
| 337 | Set the channel's read timer's period to 'PERIODUS' µs. 0 means a |
| 338 | disabled read timer. |
| 339 | + |
| 340 | Default values: |
| 341 | + |
| 342 | * option:--userspace and option:--buffers-uid options: 0 |
| 343 | * option:--userspace and option:--buffers-pid options: 0 |
| 344 | * option:--kernel option: 200000 |
| 345 | * `metadata` channel: 0 |
| 346 | |
| 347 | option:--switch-timer='PERIODUS':: |
| 348 | Set the channel's switch timer's period to 'PERIODUS' µs. 0 means |
| 349 | a disabled switch timer. Default: 0. |
| 350 | |
| 351 | |
| 352 | include::common-cmd-help-options.txt[] |
| 353 | |
| 354 | |
| 355 | include::common-cmd-footer.txt[] |
| 356 | |
| 357 | |
| 358 | SEE ALSO |
| 359 | -------- |
| 360 | linklttng:lttng-disable-channel(1), |
| 361 | linklttng:lttng(1) |