gstreamer/markdown/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:
GST_TRACERS="log(events,buffers);stats(all)". 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;

``` c
  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 the will emit.

instance init Tracers attach handlers to one or more hooks using
gst_tracing_register_hook(). In case the 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.

One way would be to introspect the data from the plugin. This has the
disadvantage that the postprocessing app needs to load the plugins or
talk to the gstreamer registry. An alternative is to also log the format
description into the log. Right now we’re logging several nested
GstStructure from the `tracer_class_init()` function (except in the
log tracer).

```
gst_tracer_record_new ("thread-rusage.class",
    // value in the log record (order does not matter)
    // *thread-id* is a *key* to related 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);
```


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:m: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
    - the stats tracer logs *new-element* and *new-pad* messages
    - they add a unique *ix* to each instance as the memory ptr 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 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 and event flow
- send custom event on buffer flow at source elements
- catch events on event transfer at sink elements

## 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

- 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.

- If the GST_LEAKS_TRACER_STACK_TRACE env variable is defined log
  the creation stack trace of leaked objects. This may significantly
  increase memory consumption.

## 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

### 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