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. We are going to introduce a GstTracer object. There will be only a single instance per process or none if tracing is off (not enabled via envvar or compiled out). Certain GStreamer core function (such as gst_pad_push or gst_element_add_pad) will call into the tracer. The tracer will dispatch into loaded tracing plugins. Developers will be able to select a list of plugins by setting an environment variable, such as GST_TRACE="meminfo,dbus". One can also pass parameters to plugins: GST_TRACE="log(events;buffers),stats(all)". When then plugins are loaded, we'll add them to certain hooks according to that they are interested in. Another env var GST_TRACE_CHANNEL can be used to send the tracing to a file or a socket (Do the same for GST_DEBUG_CHANNEL). The syntax could be GST_XXX_CHANNEL=file:///path/to/file or GST_XXX_CHANNEL=tcp://:. If no channel is set, the tracing goes to stderr like the debug logging. TODO(ensonic): we might want to have GST_{DEBUG|TRACE)_FORMAT envars as well. These could be raw, ansi-color, binary, suitable for babeltrace (see lttng), ... With these we can deprecate GST_DEBUG_FILE and GST_DEBUG_NO_COLOR. Hook api -------- e.g. gst_pad_push() would become: #ifndef GST_DISABLE_GST_DEBUG static inline GstFlowReturn __gst_pad_push (GstPad * pad, GstBuffer * buffer); #endif GstFlowReturn gst_pad_push (GstPad * pad, GstBuffer * buffer) #ifndef GST_DISABLE_GST_DEBUG { if (__tracer_enabled && __tracer_hook_is_used) gst_tracer_push_buffer_pre (pad, buffer); GstFlowReturn res = __gst_pad_push (GstPad * pad, GstBuffer * buffer); if (__tracer_enabled && __tracer_hook_is_used) gst_tracer_push_buffer_post (pad, res); return res; } static inline GstFlowReturn __gst_pad_push (GstPad * pad, GstBuffer * buffer) #endif { 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); return gst_pad_push_data (pad, GST_PAD_PROBE_TYPE_BUFFER | GST_PAD_PROBE_TYPE_PUSH, buffer); } 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. Hooks ----- - gst_bin_add - gst_bin_remove - gst_element_add_pad - gst_element_post_message - gst_element_query - gst_element_remove_pad - gst_pad_link - gst_pad_pull_range * gst_pad_push * gst_pad_push_list - gst_pad_push_event - gst_pad_unlink Plugin api ---------- TracerPlugins are plugin features. They have a simple api: 'instance creation' Plugins can attach handlers to one or more hooks. They use a HookMask to tell which events they are interested in. The params are the extra detail from the environment var. void invoke(GstTracerHookId id, GstStructure *s); Hooks marshall the parameters given to a trace hook into a GstStructure and also add some extra into such as a timestamp. The hooks will receive this structure. Hooks will be called from misc threads. The trace plugins should only consume (=read) the provided data. Most trace plugins will log data to a trace channel. 'instance destruction' Plugins 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 Plugins ideas ============= We can have some under gstreamer/plugins/tracers/ meminfo ------- - 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 ---- - 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 -------- - 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 UI == gst-debug-viewer ---------------- gst-debug-viewer could be given the tracelog in addition to the debug 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 Problems / Open items ===================== - when hooking into a timer, should we just have some predefined intervals? - how to trigger the shutdown processing? gst_deinit() (apps can call it from atexit()) - how to share the quark table from gst/gstquark? use a separate table in gst/gsttracers or add a getter to gst/gstquark - how to chain tracers (use the rank?) e.g. how can the stats tracer get data from rusage/mallinfo tracers or should all tracers just log and we would use a gst-stats tool to collect the data from the log and process it then (that would also solve the deinit() issue) - 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 stored Try it ====== GST_DEBUG="GST_REG*:4,GST_TRACER:4,log:7" GST_TRACE=log gst-launch-1.0 fakesrc num-buffers=10 ! fakesink - traces for buffer flow in TRACE level and default category