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Only a few months after LTTng 2.14, this release delivers an impressive number of features considering the short time frame, in particular: • CTF 2 (Common Trace Format) as the default trace format. • Reduced memory footprint for user space tracing. • Improved resiliency of shared LTTng-UST buffers. • Redesigned lttng-list(1) command with memory usage reporting. • Extended automation capabilities for the lttng-add-trigger(1) command. Read on for the details. WHAT'S NEW IN LTTng 2.15? ━━━━━━━━━━━━━━━━━━━━━━━━━ New features and changes in LTTng 2.15: • Choose the trace format for each recording session with the new `--trace-format` option of the lttng-create(1) command: $ lttng create my-session --trace-format=ctf-1.8 Select between CTF 1.8 [1] and CTF 2 [2]. CTF 2 now is the default trace format for all newly created recording sessions. Babeltrace 2 [3] supports CTF 2 since version 2.1 (January 2025). To keep the default set to CTF 1.8 for your application, use the new `--default-trace-format` option of lttng-sessiond(8). The LTTng project adopts CTF 2 because it has many benefits over CTF 1.8: Easier decoding: CTF 1.8 requires parsing TSDL metadata, a CTF-specific language, which increases implementation time and limits tool integration. CTF 2 replaces this with JSON Text Sequences, readable by standard JSON parsers, significantly reducing new decoding code. Cleaner trace streaming: CTF 1.8 allows partial metadata items, forcing parsers to detect item boundaries manually, increasing complexity and bugs. CTF 2 separates metadata fragments with an RS byte, enabling simple chunk identification without decoding. Better trace readability (not in LTTng yet): CTF 1.8 lacks semantic annotations, so readers rely on external domain knowledge to interpret event record data. CTF 2 introduces user-defined attributes to describe semantics, enabling automated analysis (recognizing IP addresses, for example). Broader type support (not in LTTng yet): CTF 2 adds native field classes that can improve readability, performance, and trace size, unlike CTF 1.8's more limited types. Examples include a BLOB type for binary data, richer string encodings, bit maps, optionals, and LEB128 integers. This per-recording session configuration replaces the mechanism introduced in LTTng 2.14 where setting the `LTTNG_EXPERIMENTAL_FORCE_CTF_2` environment variable to `1` at `lttng-sessiond` startup forced _all_ recording sessions to produce CTF 2 traces. • A new channel memory reclaim operation, exclusively available for user space channels, can free memory used by eligible sub-buffers by deallocating their backing memory when no longer needed. The two ways to reclaim memory are: Automatic reclaim: Configure a per-channel, automatic reclaim policy which periodically tries to reclaim memory for sub-buffers older than a given age, or as soon as they're consumed, with the new `--auto-reclaim-memory` option of the lttng-enable-channel(1) command. This new option is only available in discard mode (without the `--overwrite` option). Immediate reclaim: The new lttng-reclaim-memory(1) command reclaims memory immediately for eligible sub-buffers of one or more channels. Only reclaim the sub-buffers older than a given age with the `--older-than` option: $ lttng reclaim-memory --older-than=2m Both memory reclaim modes are possible because channel ring buffers are now backed by sparse files [4] when the file system supports them. • Choose the buffer _preallocation_ policy of a user space channel with the new `--buffer-preallocation` option of the lttng-enable-channel(1) command: $ lttng enable-channel my-channel --userspace \ --buffer-preallocation=on-demand LTTng can either preallocate all ring buffer memory up front (pre-2.15 behaviour; still the default) or allocate as needed. Choose preallocation to ensure predictable memory usage and avoid run-time allocation latency; choose on demand to minimize the initial footprint for low traffic, for short-lived recording sessions, or in conjunction with memory reclaims. This feature is also enabled by sparse file-backed ring buffers. • Before this release, an application terminated while writing to a user space ring-buffer could leave a sub-buffer stuck in a partially-written state, making it unusable. A new health check mechanism, the _watchdog timer_, now periodically monitors user space ring-buffers and makes stalled sub-buffers consumable again to keep trace recording running smoothly. This mechanism is enabled by default and can be controlled with the new `--watchdog-timer` option of the lttng-enable-channel(1) command. As of LTTng 2.15, it only applies to user space channels with a per-user buffer ownership model. • We completely revamped the human-readable output of the lttng-list(1) command. The command now has a clean tree structure. Colors help improve the readability of objects and properties when the connected terminal supports it: the `LTTNG_TERM_COLOR` and `NO_COLOR` [5] environment variables provide further terminal color control. ✔ Channel `channel0` ❬42 event rules❭ 🞂 Auto. memory reclaim policy: None 🞂 Loss mode: Discard newest event record 🞂 Preallocation policy: On demand 🞂 Ring buffer configuration: 4 sub-buffers of 512.00 KiB per CPU, per Unix user 🞂 Timer periods: 🞂 Monitor timer: 1.00 s 🞂 Read timer: Inactive 🞂 Switch timer: Inactive 🞂 Watchdog timer: 2.00 s 🞂 Statistics: 🞂 Discarded event records: 23,182 By default, the command: ‣ Adds empty lines between blocks of related information. Remove those empty lines with the new `compact` setting of the new `--style` option. ‣ Truncates lines, adding an ellipsis, to fit the current terminal width. Avoid truncation with the new `--no-truncate` option. If you still prefer or need the previous (“legacy”) version, set the `LTTNG_LIST_LEGACY` environment variable to `1` before you run lttng-list(1). Please note, however, that the legacy output will never show anything related to features introduced after LTTng 2.14. • The lttng-list(1) command now shows the total memory usage of each channel: 🞂 Memory usage: ❲●●●●●●●●●●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 19.07 MiB / 40.12 MiB The new `--mem-usage` option controls the display mode of memory usage: `compact` shows the memory usage for each Unix user or process while `full` shows the memory usage for each CPU (if available): 🞂 Memory usage: ❲●●●●●●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 13.30 MiB / 40.12 MiB ┆ For UID 1000 (64-bit): ❲●●●●●┄┄┄┄┄┄┄┄┄┄┄❳ 13.30 MiB / 40.12 MiB ┆ CPU 0: ❲●●●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 540.00 KiB / 2.51 MiB ┆ CPU 1: ❲●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 360.00 KiB / 2.51 MiB ┆ CPU 2: ❲●●●●●●●●●●●●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 1.30 MiB / 2.51 MiB ┆ CPU 3: ❲●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 256.00 KiB / 2.51 MiB ┆ CPU 4: ❲●●●●●●●●●●●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 1.20 MiB / 2.51 MiB ┆ CPU 5: ❲●●●●●●●●●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 1.07 MiB / 2.51 MiB ┆ CPU 6: ❲●●●●●●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 768.00 KiB / 2.51 MiB ┆ CPU 7: ❲●●●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 524.00 KiB / 2.51 MiB ┆ CPU 8: ❲●●●●●●●●┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄❳ 792.00 KiB / 2.51 MiB • The lttng-status(1) command now accepts the same options as lttng-list(1): $ lttng status --mem-usage=compact --channel=my-channel lttng-status(1) forwards its options as is to lttng-list(1) for the current recording session. • The lttng(1) commands won't emit multi-byte UTF-8 sequences when the `LTTNG_NO_UTF_8` environment variable is set to `1`, making a best effort to find alternatives. • New trigger conditions are now available for the lttng-add-trigger(1) command: `channel-buffer-usage-ge` and `channel-buffer-usage-le`: Satisfied when the buffer usage of a given channel becomes greater/less than or equal to some threshold (size in bytes or ratio of the total channel buffer size): $ lttng add-trigger --condition=channel-buffer-usage-ge \ --session=my-session --channel=my-channel \ --domain=kernel --threshold-ratio=0.75 \ --action=notify `session-consumed-size-ge`: Satisfied when the total consumed size of the tracing data of all the channels of a recording session becomes greater than or equal to a threshold: $ lttng add-trigger --condition=session-consumed-size-ge \ --session=my-session --threshold-size=100M \ --action=snapshot-session other-session `session-rotation-starts`: `session-rotation-finishes`: Satisfied when the rotation operation of a recording session starts or finishes: $ lttng add-trigger --condition=session-rotation-finishes \ --session=my-session --action=notify • The `liblttng-ctl` C API is updated to support the new features above: ‣ Set the trace format of a recording session descriptor with lttng_session_descriptor_set_trace_format() before creating the recording session. ‣ Immediately reclaim user space channel memory for eligible sub-buffers with lttng_reclaim_channel_memory(). Get the results with lttng_reclaim_handle_get_reclaimed_subbuffer_count() and lttng_reclaim_handle_get_pending_subbuffer_count(). Optionally wait for pending sub-buffers to be reclaimed with lttng_reclaim_handle_wait_for_completion(). When done, destroy the memory reclaim operation handle with lttng_reclaim_handle_destroy(). ‣ Set and get the automatic memory reclamation policy of a user space channel with lttng_channel_set_automatic_memory_reclamation_policy() and lttng_channel_get_automatic_memory_reclamation_policy(). ‣ Set and get the buffer preallocation policy of a user space channel with lttng_channel_set_preallocation_policy() and lttng_channel_get_preallocation_policy(). ‣ Set and get the watchdog timer period of a channel with lttng_channel_set_watchdog_timer_interval() and lttng_channel_get_watchdog_timer_interval(). ‣ Enumerate and query the CPU ID and memory usage of the data streams of a channel with the data stream info API: ⁃ lttng_channel_get_data_stream_info_sets() ⁃ lttng_data_stream_info_sets_get_count() ⁃ lttng_data_stream_info_sets_get_at_index() ⁃ lttng_data_stream_info_set_get_count() ⁃ lttng_data_stream_info_set_get_uid() ⁃ lttng_data_stream_info_set_get_pid() ⁃ lttng_data_stream_info_set_get_app_bitness() ⁃ lttng_data_stream_info_set_get_at_index() ⁃ lttng_data_stream_info_get_cpu_id() ⁃ lttng_data_stream_info_get_memory_usage() ⁃ lttng_data_stream_info_sets_destroy() • The session daemon and relay daemon now support systemd's sd_notify() protocol, enabling proper synchronization with dependent systemd units by signaling when the daemons are ready to receive commands and when they're stopping. • The internal v2.15 communication protocol between LTTng-tools and LTTng-UST is incompatible with the LTTng 2.14 one: you cannot use LTTng-tools 2.15 with LTTng-UST 2.14 and vice versa, for example. • The lttng(1) Zsh completion function now fully completes the `add-trigger` command. From now on, the Debian/Ubuntu packages of LTTng-tools will contain the Zsh completion functions. VERSION NAME ━━━━━━━━━━━━ This release is named after Péché Mortel, the iconic Imperial Stout from Montréal's Dieu du Ciel! and a long-time team favorite. Deep black and unapologetically intense, Péché Mortel pours with a dense, mocha-tinted head and an aroma that immediately announces freshly roasted coffee. Decadent and dangerously drinkable, it is the ideal companion for long, cold winter evenings. CHANGES SINCE 2.15.0-rc1 ━━━━━━━━━━━━━━━━━━━━━━━━ Notable bug fixes since the release candidate: • Fixed a hang of the user space consumer daemon's flush operation on stalled channels when the watchdog timer is disabled. When a channel was configured without a watchdog timer, a flush could hang indefinitely on stalled streams since no mechanism would recover stalled sub-buffers. The watchdog timer is now dynamically enabled when this situation is detected. • Fixed metadata corruption when combining the metadata regeneration and trace rotation commands, especially visible when recording to slow block devices. A race condition between the metadata cache consumption mechanism and rotation could lead to truncated or mixed-version metadata in trace archives. The consumer daemon now tracks metadata coherency and defers rotation until a coherent state is reached. • Fixed a race condition in the session daemon where a concurrent metadata transmission and a metadata regeneration command could corrupt the sent metadata count, resulting in a metadata file containing mostly zeroes. • Fixed unreadable CTF 1.8 traces when tracepoint event fields have empty identifiers. This is a regression from LTTng 2.14 where a strict identifier length check was introduced. IMPORTANT LINKS ━━━━━━━━━━━━━━━ LTTng tarball: • • • LTTng website: LTTng 2.15 documentation: Mailing list for support and development: IRC channel: `#lttng` on `irc.oftc.net` Bug tracker: GitHub organization: Continuous integration: • • • Code review: • • • REFERENCES ━━━━━━━━━━ [1]: https://diamon.org/ctf/v1.8.3/ (CTF v1.8.3 specification) [2]: https://diamon.org/ctf/ (CTF 2 specification) [3]: https://babeltrace.org/ [4]: https://en.wikipedia.org/wiki/Sparse_file [5]: https://no-color.org/ -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com