As with all components in Perfetto, the trace processor can be built in several build systems:
The trace processor is exposed as a static library //:trace_processor to Bazel and src/trace_processor:trace_processor in GN; it is not exposed to Android (but patches to add support for this are welcome).
The trace processor is also built as a WASM target src/trace_processor:trace_processor_wasm for the Perfetto UI; patches for adding support for other supported build systems are welcome.
The trace processor is also built as a shell binary, trace_processor_shell which backs the trace_processor tool described in other parts of the documentation. This is exposed as the trace_processor_shell target to Android, //:trace_processor_shell to Bazel and src/trace_processor:trace_processor_shell in GN.
The trace processor library is structured around the TraceProcessor class; all API methods exposed by trace processor are member functions on this class.
The C++ header for this class is split between two files: include/perfetto/trace_processor/trace_processor_storage.h and include/perfetto/trace_processor/trace_processor.h.
To ingest a trace into trace processor, the Parse function can be called multiple times to with chunks of the trace and NotifyEndOfFile can be called at the end.
As this is a common task, a helper function ReadTrace is provided in include/perfetto/trace_processor/read_trace.h. This will read a trace file directly from the filesystem and calls into appropriate TraceProcessorfunctions to perform parsing.
The ExecuteQuery function can be called with an SQL statement to execute. This will return an iterator which can be used to retrieve rows in a streaming fashion.
WARNING: embedders should ensure that the iterator is forwarded using Next before any other functions are called on the iterator.
WARNING: embedders should ensure that the status of the iterator is checked after every row and at the end of iteration to verify that the query was successful.
Any registered metrics can be computed using using the ComputeMetric function. Any metric in src/trace_processor/metrics is built-in to trace processor so can be called without any other steps.
Metrics can also be registered at run time using the RegisterMetric and ExtendMetricsProto functions. These can subsequently be executed with ComputeMetric.
WARNING: embedders should ensure that the path of any registered metric is consistent with the name used to execute the metric and output view in the SQL.
The DescribeSlice function is exposed to SQL through the describe_slice table. This table has the following schema:
| Name | Type | Meaning |
|---|---|---|
| description | string | Provides the description for the given slice |
| doc_link | string | Provides a hyperlink to documentation which gives more context for the slice |
The table also has a hidden column slice_id which needs to be set equal to the id of the slice for which to get the description. For example, to get the description and doc link for slice with id 5:
select description, doc_link from describe_slice where slice_id = 5
The describe_slice table can also be joined with the slice table to obtain descriptions for more than one slice. For example, to get the ts, dur and description for all measure slices:
select ts, dur, description from slice s join desribe_slice d on s.id = d.slice_id where name = 'measure'
As creating derived events is tied to the metrics subsystem, the ComputeMetrics function in the trace processor API should be called with the appropriate metrics. This will create the <metric_name>_event table/view which can then be queried using the ExectueQuery function.
NOTE: At some point, there are plans to add an API which does not create the metrics proto but just executes the queries in the metric.