projects / lttng-tools.git / blame
| Commit | Line | Data |
|---|---|---|
| 24bc0841 JD |
1 | LTTng Live trace reading how-to |
| 2 | ||
| 3 | Julien Desfossez | |
| 4 | September 27th, 2013 | |
| 5 | ||
| 6 | This document presents a summary on how to use the live trace reading feature | |
| 7 | introduced in LTTng 2.4. For the details about the protocol, please refer to | |
| 8 | the live-reading-protocol.txt document. | |
| 9 | ||
| 10 | Live trace reading refers to the feature of reading the trace while it is being | |
| 11 | recorded. In order to do that, the trace must be streamed a relay even if the | |
| 12 | viewer is running on the same machine as the tracer. | |
| 13 | ||
| 14 | So, the first thing to do is to start a lttng-relayd process. It can be | |
| 15 | anywhere on the network (including localhost) as long as the sessiond/consumerd | |
| 16 | and the viewer can communicate with it over TCP/IP. | |
| 17 | ||
| 18 | $ lttng-relayd -d | |
| 19 | ||
| 20 | Then, we can create a session configured for streaming with the new --live | |
| 21 | parameter. | |
| 22 | ||
| 23 | $ lttng create --live 1000000 -U net://localhost | |
| 24 | ||
| 25 | The --live parameter activates a session-wide timer (usec) that is responsible | |
| 26 | for checking at a user-defined rate if new data is available. If there is new | |
| 27 | data, it is flushed automatically, otherwise a beacon is sent to the relayd to | |
| 28 | inform it that the stream is currently empty and the viewer can ignore this | |
| 29 | stream up to a certain point in time. | |
| 30 | ||
| 31 | Once the session is created, the user can activate events as usual. | |
| 32 | ||
| 33 | In order to view the live trace, the viewer must implement the live-reading | |
| 34 | protocol. | |
| 35 | ||
| 36 | For now, a basic client is available in the branch index2013 of the git | |
| 37 | repository : | |
| 38 | https://github.com/jdesfossez/babeltrace-dev.git | |
| 39 | ||
| 40 | This client is still in heavy development and the branch will be rebased, it is | |
| 41 | only provided as a proof-of-concept and an exemple on how to use the protocol. | |
| 42 | Once compiled and installed, just run : | |
| 43 | $ test-live hostname 2>/dev/null | |
| 44 | ||
| 45 | If you want to see all the debug, just get rid of "2>/dev/null". | |
| 46 | Once again, it is a client in development, the name is purposely bad and the | |
| 47 | debug output is relatively heavy. | |
| 48 | ||
| 49 | Known viewer issues : | |
| 50 | - adding metadata on the fly (enabling events when a viewer is connected) | |
| 51 | - destroy not clean | |
| 52 | - aggressive polling when all the streams are inactive (after a lttng stop) | |
| 53 | - restart reading from the beginning (SEEK_LAST not implemented yet) |