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#55 in Debugging

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This crate provides a bridge between gstreamer and the tracing ecosystem.

The goal is to allow Rust applications utilizing GStreamer to better integrate into application that otherwise use the tracing crate for their observability needs.



To output gstreamer log messages as tracing events, call the integrate_events function. Calling it before the call to any other gstreamer call (especially before the gstreamer::init) is most likely to correctly forward all of the messages:

// Set up the tracing subscriber.
// e.g. tracing_subscriber::fmt::init();


Keep in mind that both GST_DEBUG and tracing filters are in effect. The gstreamer side of filters can be relaxed from code via:


Similarly you can use tracing APIs to adjust the filters on the tracing side.


To provide tracing with more contextual information for some of the events, you can also enable support for generating spans via gstreamer's own tracing infrastructure.

This functionality can be enabled by calling the integrate_spans function. It must be called after gstreamer::init.


Subscriber showcase

This section demonstrates the results obtained with different kinds of subscribers.


This subscriber is a great replacement for the built-in gstreamer log handler. Here's an example of what the output might look like when using this subscriber:

$ env RUST_LOG=info cargo run --example videoenc
    Finished dev [unoptimized + debuginfo] target(s) in 0.04s
     Running `target/debug/examples/videoenc`
Jan 01 00:00:00.000  INFO gstreamer::GST_INIT: Initializing GStreamer Core Library version 1.18.4
Jan 01 00:00:00.000  INFO gstreamer::GST_INIT: initialized GStreamer successfully
Jan 01 00:00:00.000  INFO gstreamer::GST_PIPELINE: parsing pipeline description '
        videotestsrc num-buffers=120
        ! vp9enc
        ! webmmux name=mux
        ! fakesink sync=false

        audiotestsrc num-buffers=120
        ! opusenc
        ! mux.

Certain messages may also provide more information than the built-in logger. While builtin logger does present you with the type and address of the object being logged, tracing-gstreamer may provide more readily useful information such as the element name:

Jan 01 00:00:00.000  INFO gstreamer::GST_STATES: completed state change to READY gobject.address=94331150660528 gobject.type="GstAudioTestSrc" gstobject.name="audiotestsrc0" gstelement.state="ready" gstelement.pending_state="void-pending"

or provide additional context via spans, which may help to figure out which element is logging the message when there is no other way to tell this otherwise, such as in this example:

Jan 01 00:00:00.000  INFO pad_push{gstpad.state={NEED_PARENT} gstpad.parent.name="audiotestsrc0"}: gstreamer::structure: Expected field 'channel-mask' in structure: audio/x-raw, rate=(int)48000, channels=(int)1, format=(string)S16LE, layout=(string)interleaved;


Tracy is a profiler focused primarily on game development workloads, but works fairly well for general purpose code as well. Tracy features a sampling profiler, but works best with applications that have manually instrumented points of interest. tracing is a great source of such manual instrumentation points and tracing-tracy is the bridge between the two. The following video demonstrates the videoenc example from this repository adapted to utilize the tracing-tracy subscriber.

In this video there are a couple of highlights

  • We can quickly see the amount of concurrency our pipeline enables (2 threads; perhaps adding some queues would help?)
  • Overall thread utilization (low for the audio portion and high for the video portion);
  • Investigate the performance of the specific elements and quickly find out why some of them are slow. For example the opusenc0 element sometimes takes an unusually long time because the downstream muxer already has a buffer queued at the time.

Similar results can be achieved with some other subscribers as well.

The GStreamer tracers

Several GStreamer tracers are also available so the integration of GStreamer tracing and logging into the Rust tracing system is possible without modifying the application that uses GStreamer.

For GStreamer to find the tracer you need to ensure that the libtracing_gstreamer.so is installed as GStreamer plugin (you can also set GST_PLUGIN_PATH for example with export GST_PLUGIN_PATH=$PWD/target/debug/:$GST_PLUGIN_PATH).

Currently 2 tracers are available:

  • chrometracing: This tracer will output the tracing events in the Chrome json tracing format. This will create a trace-XXX.json in the current directory that can be opened in perfetto. This is useful to analyze GStreamer performance in a graphical way.

  • fmttracing: Use the tracing-subscriber::fmt subscriber to format the tracing events. This is useful to get a human readable output. To actually get output you also need to set RUST_LOG=<loglevel>

Note that only one of those tracer can be used at a time, and the application itself should never activate any other tracing_subscriber.

Tracers parameters

The tracer has the following parameters:

  • log-level: String in the same form as the GST_DEBUG environment variable defining which GStreamer log level and category should be logged into the tracing system. This implies that the usual GStreamer log system will be disabled and the rust one will be used instead.


You can, for example, profile a GStreamer pipeline using gst-launch-1.0 with the following command:

Using the chrometracing tracer
# Builds the tracer plugin and make sure GStreamer finds it.
# Enable the tracer with the chrome tracing output and activating GStreamer info logs
cargo build --features "tracing-chrome" && \
  GST_TRACERS="chrometracing(log-level=4)" \
  gst-launch-1.0 playbin3 uri="https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm"

A new trace-XXX.json file will be created in the current directory. You can then open it in perfetto to analyze them.

Using the fmttracing tracer
# Builds the tracer plugin and make sure GStreamer finds it.
# Enable the tracer with the fmt tracing output and activating GStreamer info logs
# Logs will be output on stderr in the `tracing-subscriber::fmt` format
cargo build && \
  RUST_LOG=debug GST_TRACERS="fmttracing(log-level=4)" \
  gst-launch-1.0 playbin3 uri="https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm"


~238K SLoC