Fuchsia tracing system

The purpose of tracing in Fuchsia is to provide a means to collect, aggregate, and visualize diagnostic tracing information from Fuchsia user space processes and the Zircon kernel.

For more information on using tracing in Fuchsia, see the following guides:

By default, tracing in Fuchsia is turned off. When an event occurs, such as a system problem or the initiation of a debugging sessions, tracing can be turned on for an arbitrary period. Not only can tracing be turned on or off, but specific tracing categories can be individually selected.

Parts of the Fuchsia tracing system

In the Fuchsia tracing system, the following components work in a distributed manner:

Trace providers

When you write a component that creates tracing data, the component becomes a tracing provider. A system can have multiple trace providers.

Fuchsia uses a distributed implementation for its tracing system, which lets the trace provider write tracing data directly into a Zircon Virtual Memory Object (VMO), which is a shared memory segment. The tracing data is stored in memory as it is generated and isn't copied to any other location. For more information on VMOs, see Virtual Memory Object.

When you specify trace statements in your code, categories give you control over what tracing data is recorded. Then, when you run a trace client and specify certain categories, the trace client only records the specified categories from the trace providers.

Components, which can be traced or offer tracing information to the system, implement the TraceProvider FIDL protocol and register the trace provider with the Registry. Once registered, trace providers receive messages whenever tracing is started or stopped and have the opportunity to provide trace data encoded in the Fuchsia trace format. See fuchsia.tracing.provider.

Trace providers (components being traced) only write to trace buffers. If a trace provider's trace buffer becomes full while a trace is running, the buffering mode determines the behavior for the trace provider. For more information on buffering modes, see tracing provider buffering modes.

Trace client

The trace client is a component that processes the data that trace providers generate. A system can have multiple trace providers and trace clients. A trace client contacts the trace manager to request that tracing should either start or stop. A trace client can also request to save collected trace data. The trace manager gathers the data and sends it over a socket to the trace client.

The trace client cannot directly affect the operation of the trace provider. In the Fuchsia tracing system, trace providers write data to a VMO, then the trace manager reads data from the VMO, and finally the trace client reads tracing data from a socket.

Fuchsia uses a single trace client that consists of these utilities:

  • trace: This utility runs on the Fuchsia target. trace controls tracing and sends the commands to the trace manager to start and stop tracing, and it gathers the trace data. It also supports converting Fuchsia trace archives into other formats, such as Catapult JSON records.
  • ffx trace: This utility runs on the development host. ffx trace communicates with the trace utility. Trace data can then be streamed from trace to the development host as an output file.

You can collect trace information directly with the Controller FIDL protocol. See fuchsia.tracing.controller.

Trace manager

The trace manager is a system service that coordinates registration of trace providers and locates the data from each trace provider. There is only one trace manager in Fuchsia. It ensures that tracing proceeds in an orderly manner and isolates components that offer trace providers from trace clients. The trace manager assigns a VMO for every registered trace provider.

The trace manager implements the following FIDL protocols:

  • Controller: Provides trace clients with the ability to enumerate trace providers and collect trace data. See fuchsia.tracing.controller.
  • Registry: Provides trace providers with the ability to register themselves at runtime so that they can be discovered by the tracing system. See Registry.

A first in, first out (FIFO) method notifies the trace manager of the start and shutdown of a trace provider. The FIFO handle is passed from the trace manager to each trace provider as part of the initial start tracing request. For more information on the FIFO packets that the Fuchsia tracing system uses, see FIFO packets.

Cross-process communication

The following are important characteristics of the cross-process communication for Fuchsia tracing:

  • Trace providers and the trace manager do not have synchronization points other than starting or stopping collection.
  • Trace providers (components being traced) only write to trace buffers, they never read from them.
  • The trace manager only reads from trace buffers, it never writes to them.
  • Trace clients receive trace archives over a socket from the trace manager and never read the original trace buffers. This protects trace providers from manipulation by trace clients.

When you initiate tracing, the trace manager asks all relevant trace providers to start tracing and provides each provider with a trace buffer VMO into which they should write their trace records. While a trace is running, the trace manager watches for newly registered trace providers and activates them as needed.

When tracing finishes, the trace manager makes all of the active trace providers stop tracing. Then the trace manager waits for each trace provider to acknowledge that they have finished writing out their trace events. The trace manager then reads and validates trace data written into the trace buffer VMOs and creates a trace archive.

The trace manager can often recover partial data even when trace providers terminate abnormally as long as they managed to store some data into their trace buffers. If a trace provider's trace buffer becomes full while a trace is running, the buffering mode determines the behavior for the trace provider. For more information, see Tracing provider buffering modes.

The trace manager delivers the resulting trace archive to its client through a socket. This data is formed according to the Fuchsia trace format, but it may contain nonsensical data if trace providers deliberately emit garbage data.

Figure 1 shows the components of the Fuchsia tracing system and the way how the components communicate with one another.

This figure shows the Fuchsia tracing system.

Figure 1. The Fuchsia tracing system