Chapter 2
LTTng Architecture and Internals
Peering beneath LTTng's command-line simplicity reveals a powerful choreography of daemons, data streams, and tight kernel integration that makes modern Linux tracing possible. In this chapter, we crack open the architecture and internals of LTTng-demystifying its components, tracing pipeline, and the innovative engineering behind its efficiency and scalability. By understanding these foundations, you'll be equipped not only to use LTTng, but to wield it to full effect in complex, high-concurrency environments.
2.1 Core Components: Session Daemon, Consumers, Agents
The LTTng (Linux Trace Toolkit Next Generation) tracing stack architecture is composed of three fundamental operational units: the session daemon, consumers, and agents. Each unit fulfills a distinct role within the tracing framework, collectively enabling efficient trace session management, data acquisition, and integration with diverse trace sources. Understanding the decomposition of these units, their interactions, and the control flow underpins the deployment and troubleshooting of LTTng in complex tracing environments.
Session Daemon: Orchestrating Trace Sessions
The session daemon (lttng-sessiond) acts as the central coordinator of trace sessions. It manages session lifecycle events including creation, start, stop, and destruction. Upon initialization, the daemon listens for client requests via a Unix domain socket, establishing a command interface for session control.
Each trace session under the daemon's supervision maintains metadata defining tracing parameters: tracepoints enabled, buffer configurations, and consumer channel assignments. This metadata persists on disk to ensure session state restoration following unexpected terminations or daemon restarts.
Internally, the session daemon holds session context structures that track constituent consumers and agents. It is responsible for global synchronization during trace lifecycle transitions; for example, a request to start tracing triggers the daemon to sequentially coordinate the activation of all configured consumers and agents, ensuring a consistent tracing state across all components.
The control flow of the session daemon can be summarized as follows:
- Session Creation: The daemon creates a session context, allocating identifiers and persisting configuration.
- Consumer and Agent Initialization: It launches the necessary consumer daemon processes and spawns agents as per session specification.
- Tracing Control: The daemon receives start/stop commands and propagates status updates and control commands to consumers and agents synchronously.
- Shutdown and Cleanup: Upon session destruction, it gracefully signals all child processes to halt tracing, finalize buffers, and release resources.
Failure recovery in the session daemon involves detecting consumer or agent termination and attempting automatic restarts or notifying the controlling client. Persistent state files and heartbeat mechanisms contribute to robust fault detection.
Consumers: Data Collection and Storage
Consumers are independent daemon processes responsible for collecting trace data from the instrumentation sources and writing it to persistent storage, typically on disk. They handle the transport and buffering aspects of trace data, optimizing for performance and resource utilization.
Two primary consumer types are distinguished in LTTng:
- Kernel Consumers: These capture kernel-space trace events delivered via relay channels.
- User-Space Consumers: Handling user-space instrumentation streams via the LTTng-UST (User-Space Tracer) framework.
Consumers communicate with the session daemon using a dedicated control protocol over Unix domain sockets, exchanging status and operational commands. Upon receipt of the tracing start command, consumers transition into a data acquisition mode, attaching to the configured relay channels and continuously reading event data.
The data flow within consumers involves circular buffers in shared memory regions, minimizing latency and lock contention. The consumers dequeue events from these buffers and perform file system writes in a batch-oriented manner to optimize throughput.
During runtime, consumers maintain synchronization with the session daemon by sending periodic heartbeat signals. If a consumer process crashes or becomes unresponsive, the session daemon's fault management subsystem detects the anomaly, triggers cleanup, and attempts restarts if possible.
Consumers handle graceful shutdown by flushing all buffered data, closing output streams, and releasing shared resources. This orderly procedure prevents data loss and ensures trace coherency.
Agents: Bridging External Trace Sources
Agents serve as intermediaries between the session daemon and third-party trace sources that do not natively integrate with LTTng core components. Examples include dynamic instrumentation frameworks or hardware trace units.
The agent processes are responsible for converting external trace events into LTTng's internal tracepoint format and feeding them into the session's consumer pipelines. They execute configuration directives issued by the session daemon, such as enabling or disabling specific tracepoints or adjusting buffering policies.
Agents register with the session daemon during session setup and maintain active communication channels for command and status propagation. They implement fault-tolerance by monitoring the validity of their external data sources and, if a failure condition is encountered (e.g., loss of hardware connection), signaling the session daemon to initiate recovery procedures.
Interaction and Synchronization Dynamics
The interaction among these core components follows a strict orchestration protocol managed by the session daemon, ensuring consistent tracing across the system. Figure illustrates the control and data flow during key lifecycle phases: startup, runtime, and shutdown.
Control synchronization is critical when transitioning between tracing states. For instance, at runtime start, the session daemon issues a START command to all consumers and agents. Consumers respond with acknowledgment signals once ready, ensuring that trace buffers are prepared before kernel or user-space instrumentation generates events. Similarly, agents synchronize their external source states before enabling event forwarding.
Failure recovery mechanisms involve timeout monitoring and process watchdog functionality embedded within the daemon. If a consumer or agent fails to respond within a predetermined window, the session daemon attempts an automated restart sequence:
Procedure AttemptRecovery(component) Terminate component process if still running Reinitialize component with last known state Wait for component heartbeats with timeout If heartbeat received then Resume normal operation Else Notify user and log error End Procedure
This recovery protocol minimizes service interruption and preserves trace integrity.
Practical Startup and Shutdown Sequence
The following simplified pseudocode depicts the startup sequence managed by the session daemon:
void sessiond_start(Session *session) { // Initialize session resources load_config(session); launch_consumers(session->consumer_config); launch_agents(session->agent_config); ...
