v811_spc009/external/perfetto/docs/concepts/service-model.md

Service-based model

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).

NOTE: 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 Socket protocol).

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).

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