Memory not freed after gstreamer pipeline set to GST_STATE_NULL

Question

My application requires gstreamer pipeline to be restarted multiple times. But after setting the pipeline to GST_STATE_NULL and calling unref on the pipeline, memory appears to be not freed. After every restart, the memory associated with the process keeps increasing.

I was able to reproduce the problem with just videotestsrc-fakesink elements as given below :

//g++ -Wall testpage_Simple.cpp -o testpage_Simple $(pkg-config --cflags --libs gstreamer-1.0)

#include <gst/gst.h> 
GstElement *pipeline;
GstElement *src;
GstElement *sink;
void clearPipeline () {
    // g_print ("clearPipeline    ");
    gst_element_set_state (pipeline, GST_STATE_NULL); 
    gst_object_unref (pipeline); 
}
void createPipeline () {
    pipeline = gst_pipeline_new ("pipelinePlay");
    src = gst_element_factory_make ("videotestsrc", "source");
    sink = gst_element_factory_make ("fakesink", "sink");

    gst_bin_add_many (GST_BIN (pipeline), src, sink, NULL);

    if (gst_element_link (src, sink)!= TRUE) {
        g_printerr ("src, sink could not be linked.\n");
    }
    gst_element_set_state (pipeline, GST_STATE_PLAYING);
}

gint main (gint argc, gchar * argv[]) 
{ 
    gst_init (NULL, NULL);
    system("gst-launch-1.0 --gst-version");
    g_print ("Start Test - ");
 //   for (int i=1; i<=10; i++) {
        system ("top -b -n 1 | grep testpage | awk '{print $6}'");
        createPipeline();
        clearPipeline(); 
 //   }
    g_print ("End of test !! ");
    system ("top -b -n 1 | grep testpage | awk '{print $6}'");
    gst_deinit();
    return 0; 
}

Sample output on Ubuntu 19.04 (showing only RES column value from top command for this process):
GStreamer Core Library version 1.16.1
Start Test - 7140
End of test !! 8504

We observed similar memory trend on Ubuntu 18.04 with gstreamer 1.12.4 as well.

Is this the expected behavior or did I miss something in cleaning up the pipeline ??

I checked the below link. It appears to be a similar problem, but this question is unanswered
GStreamer memory leak after the pipeline restart

I tried the disk cache suggestion in the below link, with a similar problem. But that did not help as well.
http://gstreamer-devel.966125.n4.nabble.com/Properly-freeing-resources-td4658631.html

Answer 1

Just a guess, maybe the problem is related to possible state transitions for elements

according https://gstreamer.freedesktop.org/documentation/additional/design/states.html?gi-language=c# the following state changes are possible:

NULL -> READY:
    The element must check if the resources it needs are available. Device sinks and sources typically try to probe the device to constrain their caps.
    The element opens the device, this is needed if the previous step requires the device to be opened.

READY -> PAUSED:
    The element pads are activated in order to receive data in PAUSED. Streaming threads are started.
    Some elements might need to return ASYNC and complete the state change when they have enough information. It is a requirement for sinks to return ASYNC and complete the state change when they receive the first buffer or EOS event (preroll). Sinks also block the dataflow when in PAUSED.
    A pipeline resets the running_time to 0.
    Live sources return NO_PREROLL and don't generate data.

PAUSED -> PLAYING:
    Most elements ignore this state change.
    The pipeline selects a clock and distributes this to all the children before setting them to PLAYING. This means that it is only allowed to synchronize on the clock in the PLAYING state.
    The pipeline uses the clock and the running_time to calculate the base_time. This base_time is distributed to all children when performing the state change.
    Sink elements stop blocking on the preroll buffer or event and start rendering the data.
    Sinks can post the EOS message in the PLAYING state. It is not allowed to post EOS when not in the PLAYING state.
    While streaming in PAUSED or PLAYING elements can create and remove sometimes pads.
    Live sources start generating data and return SUCCESS.

PLAYING -> PAUSED:
    Most elements ignore this state change.
    The pipeline calculates the running_time based on the last selected clock and the base_time. It stores this information to continue playback when going back to the PLAYING state.
    Sinks unblock any clock wait calls.
    When a sink does not have a pending buffer to play, it returns ASYNC from this state change and completes the state change when it receives a new buffer or an EOS event.
    Any queued EOS messages are removed since they will be reposted when going back to the PLAYING state. The EOS messages are queued in GstBins.
    Live sources stop generating data and return NO_PREROLL.

PAUSED -> READY:
    Sinks unblock any waits in the preroll.
    Elements unblock any waits on devices
    Chain or get_range() functions return FLUSHING.
    The element pads are deactivated so that streaming becomes impossible and all streaming threads are stopped.
    The sink forgets all negotiated formats
    Elements remove all sometimes pads

READY -> NULL:
    Elements close devices
    Elements reset any internal state.

if the current state is playing then st. like

gst_element_set_state (pipeline, GST_STATE_PAUSED);
gst_element_set_state (pipeline, GST_STATE_READY);
gst_element_set_state (pipeline, GST_STATE_NULL);

might help

Checking the return value of gst_element_set_state(...) might not be a bad idea either :)