gstreamer/subprojects/gst-docs/markdown/additional/design/tracing.md

Tracing

This subsystem will provide a mechanism to get structured tracing info from GStreamer applications. This can be used for post-run analysis as well as for live introspection.

Use cases

• I’d like to get statistics from a running application.

• I’d like to to understand which parts of my pipeline use how many resources.

• I’d like to know which parts of the pipeline use how much memory.

• I’d like to know about ref-counts of parts in the pipeline to find ref-count issues.

Non use-cases

• Some element in the pipeline does not play along the rules, find out which one. This could be done with generic tests.

Design

The system brings the following new items: core hooks: probes in the core api, that will expose internal state when tracing is in use tracers: plugin features that can process data from the hooks and emit a log tracing front-ends: applications that consume logs from tracers

Like the logging, the tracer hooks can be compiled out and if not use a local condition to check if active.

Certain GStreamer core function (such as gst_pad_push() or gst_element_add_pad()) will call into the tracer subsystem to dispatch into active tracing modules. Developers will be able to select a list of plugins by setting an environment variable, such as GST_TRACERS="meminfo;dbus". One can also pass parameters to plugins, e.g:

GST_TRACERS='leaks(filters="GstEvent,GstMessage",stack-traces-flags=none);latency(flags=pipeline+element+reported)'

When then plugins are loaded, we’ll add them to certain hooks according to which they are interested in.

Right now tracing info is logged as GstStructures to the TRACE level. Idea: Another env var GST_TRACE_CHANNEL could be used to send the tracing to a file or a socket. See https://bugzilla.gnome.org/show_bug.cgi?id=733188 for discussion on these environment variables.

Hook api

We’ll wrap interesting api calls with two macros, e.g. gst_pad_push():

GstFlowReturn gst_pad_push (GstPad * pad, GstBuffer * buffer) {
  GstFlowReturn res;

  g_return_val_if_fail (GST_IS_PAD (pad), GST_FLOW_ERROR);
  g_return_val_if_fail (GST_PAD_IS_SRC (pad), GST_FLOW_ERROR);
  g_return_val_if_fail (GST_IS_BUFFER (buffer), GST_FLOW_ERROR);

  GST_TRACER_PAD_PUSH_PRE (pad, buffer);
  res = gst_pad_push_data (pad,
      GST_PAD_PROBE_TYPE_BUFFER | GST_PAD_PROBE_TYPE_PUSH, buffer);
  GST_TRACER_PAD_PUSH_POST (pad, res);
  return res;
}

TODO(ensonic): gcc has some magic for wrapping functions - http://gcc.gnu.org/onlinedocs/gcc/Constructing-Calls.html - http://www.clifford.at/cfun/gccfeat/#gccfeat05.c

TODO(ensonic): we should eval if we can use something like jump_label in the kernel - http://lwn.net/Articles/412072/ + http://lwn.net/Articles/435215/ - http://lxr.free-electrons.com/source/kernel/jump_label.c - http://lxr.free-electrons.com/source/include/linux/jump_label.h - http://lxr.free-electrons.com/source/arch/x86/kernel/jump_label.c TODO(ensonic): liblttng-ust provides such a mechanism for user-space - but this is mostly about logging traces - it is linux specific :/

In addition to api hooks we should also provide timer hooks. Interval timers are useful to get e.g. resource usage snapshots. Also absolute timers might make sense. All this could be implemented with a clock thread. We can use another env-var GST_TRACE_TIMERS="100ms,75ms" to configure timers and then pass them to the tracers like, GST_TRACERS="rusage(timer=100ms);meminfo(timer=75ms)". Maybe we can create them ad-hoc and avoid the GST_TRACE_TIMERS var.

Hooks (* already implemented)

* gst_bin_add
* gst_bin_remove
* gst_element_add_pad
* gst_element_post_message
* gst_element_query
* gst_element_remove_pad
* gst_element_factory_make
* gst_pad_link
* gst_pad_pull_range
* gst_pad_push
* gst_pad_push_list
* gst_pad_push_event
* gst_pad_unlink

Tracer api

Tracers are plugin features. They have a simple api:

class init Here the tracers describe the data they will emit.

instance init Tracers attach handlers to one or more hooks using gst_tracing_register_hook(). In case they are configurable, they can read the options from the params property. This is the extra detail from the environment var.

hook functions Hooks marshal the parameters given to a trace hook into varargs and also add some extra into such as a timestamp. Hooks will be called from misc threads. The trace plugins should only consume (=read) the provided data. Expensive computation should be avoided to not affect the execution too much. Most trace plugins will log data to a trace channel.

instance destruction Tracers can output results and release data. This would ideally be done at the end of the applications, but gst_deinit() is not mandatory. gst_tracelib was using a gcc_destructor. Ideally tracer modules log data as they have them and leave aggregation to a tool that processes the log.

tracer event classes

Most tracers will log some kind of events : a data transfer, an event, a message, a query or a measurement. Every tracer should describe the data format. This way tools that process tracer logs can show the data in a meaningful way without having to know about the tracer plugin.

Tracers can use gst_tracer_record_new in their tracer_class_init() to describe their format:

fmt = gst_tracer_record_new ("thread-rusage.class",
    // value in the log record (order does not matter)
    // *thread-id* is a *key* to relate the record to something as indicated
    // by *scope* substructure
    "thread-id", GST_TYPE_STRUCTURE, gst_structure_new ("scope",
        "type", G_TYPE_GTYPE, G_TYPE_GUINT64,
        "related-to", GST_TYPE_TRACER_VALUE_SCOPE, GST_TRACER_VALUE_SCOPE_THREAD,
        NULL),
    // next value in the record
    // *average-cpuload* is a measurement as indicated by the *value*
    // substructure
    "average-cpuload", GST_TYPE_STRUCTURE, gst_structure_new ("value",
        // value type
        "type", G_TYPE_GTYPE, G_TYPE_UINT,
        // human readable description, that can be used as a graph label
        "description", G_TYPE_STRING, "average cpu usage per thread",
        // flags that help to use the right graph type
        // flags { aggregated, windowed, cumulative, … }
        "flags", GST_TYPE_TRACER_VALUE_FLAGS, GST_TRACER_VALUE_FLAGS_AGGREGATED,
        // value range
        "min", G_TYPE_UINT, 0,
        "max", G_TYPE_UINT, 100,
        NULL),
    …
    NULL);

Later tracers can use the GstTracerRecord instance to log values efficiently:

gst_tracer_record_log (fmt, (guint64) (guintptr) thread_id, avg_cpuload);

Below a few more examples for parts of tracer classes:

An optional value. Since the PTS can be GST_CLOCK_TIME_NONE and that is (-1), we don't want to log this.

"buffer-pts", GST_TYPE_STRUCTURE, gst_structure_new ("value",
    "type", G_TYPE_GTYPE, G_TYPE_UINT64,
    "description", G_TYPE_STRING, "presentation timestamp of the buffer in ns",
    "flags", GST_TYPE_TRACER_VALUE_FLAGS, GST_TRACER_VALUE_FLAGS_OPTIONAL,
    "min", G_TYPE_UINT64, G_GUINT64_CONSTANT (0),
    "max", G_TYPE_UINT64, G_MAXUINT64,
    NULL),

In the tracer code an optional value is logged using a boolean + the value, where the boolean indicated the presence:

GstClockTime pts = GST_BUFFER_PTS (buf);

gst_tracer_record_log (fmt, ..., GST_CLOCK_TIME_IS_VALID (pts), pts, ...);

A few ideas that are not yet in the above spec:

• it would be nice to describe the unit of values

  • putting it into the description is not flexible though, e.g. time would be a guint64 but a ui would reformat it to e.g. hⓂ️s.ms
  • other units are e.g.: percent, per-mille, or kbit/s

• we’d like to have some metadata on scopes

  • e.g. we’d like to log the thread-names, so that a UI can show that instead of thread-ids
  • ordering: e.g. in the latency tracer we'd like to order by 'sink' and then by 'src'

• unique instance ids

  • the stats tracer logs new-element and new-pad messages
  • they add a unique ix to each instance as the memory ptr or the object name can be reused for new instances, the data is attached to the objects as qdata
  • the latency tracer would like to also reference this metadata (right now, it relies on unique element names)
  • the relationship between a scope 'element-ix' ('related-to'=_ELEMENT) and an earlier message 'new-element' that has scope 'ix' ('related-to'=_ELEMENT) and value 'name' is not obvious

Right now we log the classes as structures, this is important so that the log is self contained. It would be nice to add them to the registry, so that gst-inspect can show them. We could also consider to add each value as a READONLY gobject property. The property has name/description. We could use qdata for scope and flags (or have some new property flags). We would also need a new "notify" signal, so that value-change notifications would include a time-stamp. This way the tracers would not needs to be aware of the logging. The core tracer would register the notify handlers and emit the log. Or we just add a gst_tracer_class_install_event() and that mimics the g_object_class_install_property().

Frontends can:

• do an events over time histogram
• plot curves of values over time or deltas
• show gauges
• collect statistics (min, max, avg, …)

latency

• register to buffer, event and query flow
• send custom event on buffer flow at source elements
• catch events on event transfer at sink pads

meminfo (not yet implemented)

• register to an interval-timer hook.
• call mallinfo() and log memory usage rusage
• register to an interval-timer hook.
• call getrusage() and log resource usage

dbus (not yet implemented)

• provide a dbus iface to announce applications that are traced
• tracing UIs can use the dbus iface to find the channels where logging and tracing is getting logged to
• one would start the tracing UI first and when the application is started with tracing activated, the dbus plugin will announce the new application, upon which the tracing UI can start reading from the log channels, this avoid missing some data

topology (not yet implemented)

• register to pipeline topology hooks
• tracing UIs can show a live pipeline graph

stats

• register to buffer, event, message and query flow
• tracing apps can do e.g. statistics

refcounts (not yet implemented)

• log ref-counts of objects
• just logging them outside of glib/gobject would still make it hard to detect issues though

opengl (not yet implemented)

• upload/download times
• there is not hardware agnostic way to get e.g. memory usage info (gl extensions)

memory (not yet implemented)

• trace live instance (and pointer to the memory)
• use an atexit handler to dump leaked instance https://bugzilla.gnome.org/show_bug.cgi?id=756760#c6

leaks

• track creation/destruction of GstObject and GstMiniObject

• log those which are still alive when app is exiting and raise an error if any

• The tracer takes several parameters in a GstStructure like syntax (without the structure name):

  • check-refs (boolean): Whether to also track object ref and unref operations example: GST_TRACERS=leaks(check-refs=true) COMMAND
  • stack-traces-flags: Flags to use when generating stack trace (does not generate stack trace if not set), valid values are “full” to retrieve as much information as possible in the backtrace, or “none” for a simple backtrace (usually does not contain line number or source files). This may significantly increase memory consumption. (You can also set the GST_LEAKS_TRACER_STACK_TRACE environment variable for that).
  • filters: (string): A comma separated list of object types to trace (make sure to enclose in quotation marks)

Run the leaks tracer on all GstProxyPad objects logging the references with full backtraces

GST_TRACERS=leaks(stack-traces-flags=full,filters=”GstProxyPad”,check-refs=true) COMMAND

Run the leaks tracer on all (mini)objects logging the references with less complete backtraces

GST_TRACERS=leaks(stack-traces-flags=fast,check-refs=true) COMMAND

• If the GST_LEAKS_TRACER_SIG env variable is defined the tracer will handle the following UNIX signals:

• SIGUSR1: log alive objects

• SIGUSR2: create a checkpoint and print a list of objects created and destroyed since the previous checkpoint.

gst-debug-viewer

gst-debug-viewer could be given the trace log in addition to the debug log (or a combined log). Alternatively it would show a dialog that shows all local apps (if the dbus plugin is loaded) and read the log streams from the sockets/files that are configured for the app.

gst-tracer

Counterpart of gst-tracelib-ui.

gst-stats

A terminal app that shows summary/running stats like the summary gst-tracelib shows at the end of a run. Currently only shows an aggregated status.

live-graphers

Maybe we can even feed the log into existing live graphers, with a little driver * https://github.com/dkogan/feedgnuplot

• should tracers log into the debug.log or into a separate log?

• separate log

• use a binary format?

• worse performance (we’re writing two logs at the same time)

• need to be careful when people to GST_DEBUG_CHANNEL=stderr and GST_TRACE_CHANNEL=stderr (use a shared channel, but what about the formats?)

• debug log

• the tracer subsystem would need to log the GST_TRACE at a level that is active

• should the tracer call gst_debug_category_set_threshold() to ensure things work, even though the levels don’t make a lot of sense here

• make logging a tracer (a hook in gst_debug_log_valist(), move gst_debug_log_default() to the tracer module)

• log all debug log to the tracer log, some of the current logging statements can be replaced by generic logging as shown in the log-tracer

• add tools/gst-debug to extract a human readable debug log from the trace log

• we could maintain a list of log functions, where gst_tracer_log_trace() is the default one. This way e.g. gst-validate could consume the traces directly.

• when hooking into a timer, should we just have some predefined intervals?

• can we add a tracer module that registers the timer hook? then we could do GST_TRACER="timer(10ms);rusage" right now the tracer hooks are defined as an enum though.

• when connecting to a running app, we can’t easily get the current state if logging is using a socket, as past events are not explicitly stored, we could determine the current topology and emit events with GST_CLOCK_TIME_NONE as ts to indicate that the events are synthetic.

• we need stable ids for scopes (threads, elements, pads)

• the address can be reused

• we can use gst_util_seqnum_next()

• something like gst_object_get_path_string() won’t work as objects are initially without parent

• right now the tracing-hooks are enabled/disabled from configure with --{enable,disable}-gst-tracer-hooks The tracer code and the plugins are still built though. We should add a --{enable,disable}-gst-tracer to disabled the whole system, allthough this is a bit confusing with the --{enable,disable}-trace option we have already.

Try it

Traces for buffer flow, events and messages in TRACE level:

GST_DEBUG="GST_TRACER:7,GST_BUFFER*:7,GST_EVENT:7,GST_MESSAGE:7" \
GST_TRACERS=log gst-launch-1.0 fakesrc num-buffers=10 ! fakesink

Print some pipeline stats on exit:

GST_DEBUG="GST_TRACER:7" GST_TRACERS="stats;rusage" \
GST_DEBUG_FILE=trace.log gst-launch-1.0 fakesrc num-buffers=10 \
sizetype=fixed ! queue ! fakesink && gst-stats-1.0 trace.log

get ts, average-cpuload, current-cpuload, time and plot

GST_DEBUG="GST_TRACER:7" GST_TRACERS="stats;rusage" \
GST_DEBUG_FILE=trace.log /usr/bin/gst-play-1.0 $HOME/Videos/movie.mp4 &&
./scripts/gst-plot-traces.sh --format=png | gnuplot eog trace.log.*.png

print processing latencies

GST_DEBUG="GST_TRACER:7" GST_TRACERS=latency gst-launch-1.0 \
audiotestsrc num-buffers=10 ! audioconvert ! volume volume=0.7 ! \
autoaudiosink

print processing latencies for each element

GST_DEBUG="GST_TRACER:7" GST_TRACERS="latency(flags=element)" gst-launch-1.0 \
audiotestsrc num-buffers=10 ! audioconvert ! volume volume=0.7 ! \
autoaudiosink

print reported latencies for each element

GST_DEBUG="GST_TRACER:7" GST_TRACERS="latency(flags=reported)" gst-launch-1.0 \
audiotestsrc num-buffers=10 ! audioconvert ! volume volume=0.7 ! \
autoaudiosink

print all type of latencies for a pipeline

GST_DEBUG="GST_TRACER:7" \
GST_TRACERS="latency(flags=pipeline+element+reported)" gst-launch-1.0 \
alsasrc num-buffers=20 ! flacenc ! identity ! \
fakesink

Raise a warning if a leak is detected

GST_TRACERS="leaks" gst-launch-1.0 videotestsrc num-buffers=10 ! \
fakesink

check if any GstEvent or GstMessage is leaked and raise a warning

GST_DEBUG="GST_TRACER:7" GST_TRACERS="leaks(GstEvent,GstMessage)" \
gst-launch-1.0 videotestsrc num-buffers=10 ! fakesink

Performance

run ./tests/benchmarks/tracing.sh <tracer(s)> <media>

egrep -c "(proc|thread)-rusage" trace.log 658618 grep -c
"gst_tracer_log_trace" trace.log 823351

• we can optimize most of it by using quarks in structures or eventually avoid structures totally