Perfetto key concepts and architecture

Producer <> Service <> Consumer model

Perfetto Stack

Service
The tracing service is a long-lived entity (a system daemon on Linux/Android, a service in Chrome) that has the following responsibilities:

  • Maintains a registry of active producers and their data sources.
  • Owns the trace buffers.
  • Handles multiplexing of several tracing sessions.
  • Routes the trace config from the consumers to the corresponding producers.
  • Tells the Producers when and what to trace.
  • Moves data from the Producer's shared memory buffer to the central non-shared trace buffers.

Producer
A producer is an untrusted entity that offers the ability to contribute to the trace. In a multiprocess model, a producer almost always corresponds to a client process of the tracing service. It advertises its ability to contribute to the trace with one or more data sources. Each producer has exactly:

  • One shared memory buffer, shared exclusively with the tracing service.
  • One IPC channel with the tracing service.

A producer is completely decoupled (both technically and conceptually) from consumer(s). A producer knows nothing about:

  • How many consumer(s) are connected to the service.
  • How many tracing sessions are active.
  • How many other producer(s) are registered or active.
  • Trace data written by other producer(s).

*** aside In rare circumstances a process can host more than one producer and hence more than one shared memory buffer. This can be the case for a process bundling third-party libraries that in turn include the Perfetto client library.
Concrete example: at some point in the future Chrome might expose one Producer for tracing within the main project, one for V8 and one for Skia (for each child process).

Consumer
A consumer is a trusted entity (a cmdline client on Linux/Android, an interface of the Browser process in Chrome) that controls (non-exclusively) the tracing service and reads back (destructively) the trace buffers. A consumer has the ability to:

  • Send a trace config to the service, determining:
  • How many trace buffers to create.
  • How big the trace buffers should be.
  • The policy for each buffer (ring-buffer or stop-when-full).
  • Which data sources to enable.
  • The configuration for each data source.
  • The target buffer for the data produced by each data source configured.
  • Enable and disable tracing.
  • Read back the trace buffers:
    • Streaming data over the IPC channel.
    • Passing a file descriptor to the service and instructing it to periodically save the trace buffers into the file.

Data source
A data source is a capability, exposed by a Producer, of providing some tracing data. A data source almost always defines its own schema (a protobuf) consisting of:

  • At most one DataSourceConfig sub-message (example)
  • One or more TracePacket sub-messages (example)

Different producers may expose the same data source. Concrete example: *** aside At some point in the near future we might offer, as part of Perfetto, a library for in-process heap profiling. In such case more than one producer, linking against the updated Perfetto library, will expose the heap profiler data source, for its own process.

IPC channel
In a multiprocess scenario, each producer and each consumer interact with the service using an IPC channel. IPC is used only in non-fast-path interactions, mostly handshakes such as enabling/disabling trace (consumer), (un)registering and starting/stopping data sources (producer). The IPC is typically NOT employed to transport the protobufs for the trace. Perfetto provides a POSIX-friendly IPC implementation, based on protobufs over a UNIX socket (see ipc.md). That IPC implementation is not mandated. Perfetto allows the embedder:

  • Wrap its own IPC subsystem (e.g., Perfetto in Chromium uses Mojo)
  • Not use an IPC mechanism at all and just short circuit the Producer <> Service <> Consumer interaction via PostTask(s).

See embedder-guide.md for more details.

Shared memory buffer
Producer(s) write tracing data, in the form of protobuf-encoded binary blobs, directly into its shared memory buffer, using a special library called ProtoZero. The shared memory buffer:

  • Has a fixed and typically small size (configurable, default: 128 KB).
  • Is an ABI and must maintain backwards compatibility.
  • Is shared by all data sources of the producer.
  • Is independent of the number and the size of the trace buffers.
  • Is independent of the number of Consumer(s).
  • Is partitioned in chunks of variable size.

Each chunk:

  • Is owned exclusively by one Producer thread (or shared through a mutex).
  • Contains a linear sequence of TracePacket(s), or fragments of that. A TracePacket can span across several chunks, the fragmentation is not exposed to the consumers (consumers always see whole packets as if they were never fragmented).
  • Can be owned and written by exactly one TraceWriter.
  • Is part of a reliable and ordered sequence, identified by the WriterID: packets in a sequence are guaranteed to be read back in order, without gaps and without repetitions (see trace-format.md for more).

See the comments in shared_memory_abi.h for more details about the binary format of this buffer.

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