# How scheduling works

Scheduling is, in short, a method for making sure that every element gets called once in a while to process data and prepare data for the next element. Likewise, a kernel has a scheduler to for processes, and your brain is a very complex scheduler too in a way. Randomly calling elements' chain functions won't bring us far, however, so you'll understand that the schedulers in &GStreamer; are a bit more complex than this. However, as a start, it's a nice picture. &GStreamer; currently provides two schedulers: a basic scheduler and an optimal scheduler. As the name says, the basic scheduler (basic) is an unoptimized, but very complete and simple scheduler. The optimal scheduler (opt), on the other hand, is optimized for media processing, but therefore also more complex.

Note that schedulers only operate on one thread. If your pipeline contains multiple threads, each thread will run with a separate scheduler. That is the reason why two elements running in different threads need a queue-like element (a `DECOUPLED` element) in between them.

## The Basic Scheduler

The basic scheduler assumes that each element is its own process. We don't use UNIX processes or POSIX threads for this, however; instead, we use so-called co-threads. Co-threads are threads that run besides each other, but only one is active at a time. The advantage of co-threads over normal threads is that they're lightweight. The disadvantage is that UNIX or POSIX do not provide such a thing, so we need to include our own co-threads stack for this to run.

The task of the scheduler here is to control which co-thread runs at what time. A well-written scheduler based on co-threads will let an element run until it outputs one piece of data. Upon pushing one piece of data to the next element, it will let the next element run, and so on. Whenever a running element requires data from the previous element, the scheduler will switch to that previous element and run that element until it has provided data for use in the next element.

This method of running elements as needed has the disadvantage that a lot of data will often be queued in between two elements, as the one element has provided data but the other element hasn't actually used it yet. These storages of in-between-data are called bufpens, and they can be visualized as a light queue.

Note that since every element runs in its own (co-)thread, this scheduler is rather heavy on your system for larger pipelines.

## The Optimal Scheduler

The optimal scheduler takes advantage of the fact that several elements can be linked together in one thread, with one element controlling the other. This works as follows: in a series of chain-based elements, each element has a function that accepts one piece of data, and it calls a function that provides one piece of data to the next element. The optimal scheduler will make sure that the `gst_pad_push ()` function of the first element directly calls the chain-function of the second element. This significantly decreases the latency in a pipeline. It takes similar advantage of other possibilities of short-cutting the data path from one element to the next.

The disadvantage of the optimal scheduler is that it is not fully implemented. Also it is badly documented; for most developers, the opt scheduler is one big black box. Features that are not implemented include pad-unlinking within a group while running, pad-selecting (i.e. waiting for data to arrive on a list of pads), and it can't really cope with multi-input/-output elements (with the elements linked to each of these in-/outputs running in the same thread) right now.

Some of our developers are intending to write a new scheduler, similar to the optimal scheduler (but better documented and more completely implemented).

# How a loopfunc works

A `_loop ()` function is a function that is called by the scheduler, but without providing data to the element. Instead, the element will become responsible for acquiring its own data, and it will still be responsible of sending data over to its source pads. This method noticeably complicates scheduling; you should only write loop-based elements when you need to. Normally, chain-based elements are preferred. Examples of elements that have to be loop-based are elements with multiple sink pads. Since the scheduler will push data into the pads as it comes (and this might not be synchronous), you will easily get ascynronous data on both pads, which means that the data that arrives on the first pad has a different display timestamp then the data arriving on the second pad at the same time. To get over these issues, you should write such elements in a loop-based form. Other elements that are easier to write in a loop-based form than in a chain-based form are demuxers and parsers. It is not required to write such elements in a loop-based form, though.

Below is an example of the easiest loop-function that one can write:

    static void	gst_my_filter_loopfunc	(GstElement *element);

    static void
    gst_my_filter_init (GstMyFilter *filter)
    {
    [..]
      gst_element_set_loopfunc (GST_ELEMENT (filter), gst_my_filter_loopfunc);
    [..]
    }

    static void
    gst_my_filter_loopfunc (GstElement *element)
    {
      GstMyFilter *filter = GST_MY_FILTER (element);
      GstData *data;

      /* acquire data */
      data = gst_pad_pull (filter->sinkpad);

      /* send data */
      gst_pad_push (filter->srcpad, data);
    }

Obviously, this specific example has no single advantage over a chain-based element, so you should never write such elements. However, it's a good introduction to the concept.

## Multi-Input Elements

Elements with multiple sink pads need to take manual control over their input to assure that the input is synchronized. The following example code could (should) be used in an aggregator, i.e. an element that takes input from multiple streams and sends it out intermangled. Not really useful in practice, but a good example, again.

    typedef struct _GstMyFilterInputContext {
      gboolean   eos;
      GstBuffer *lastbuf;
    } GstMyFilterInputContext;

    [..]

    static void
    gst_my_filter_init (GstMyFilter *filter)
    {
      GstElementClass *klass = GST_ELEMENT_GET_CLASS (filter);
      GstMyFilterInputContext *context;

      filter->sinkpad1 = gst_pad_new_from_template (
    	gst_element_class_get_pad_template (klass, "sink"), "sink_1");
      context = g_new0 (GstMyFilterInputContext, 1);
      gst_pad_set_private_data (filter->sinkpad1, context);
    [..]
      filter->sinkpad2 = gst_pad_new_from_template (
    	gst_element_class_get_pad_template (klass, "sink"), "sink_2");
      context = g_new0 (GstMyFilterInputContext, 1);
      gst_pad_set_private_data (filter->sinkpad2, context);
    [..]
      gst_element_set_loopfunc (GST_ELEMENT (filter),
    			    gst_my_filter_loopfunc);
    }

    [..]

    static void
    gst_my_filter_loopfunc (GstElement *element)
    {
      GstMyFilter *filter = GST_MY_FILTER (element);
      GList *padlist;
      GstMyFilterInputContext *first_context = NULL;

      /* Go over each sink pad, update the cache if needed, handle EOS
       * or non-responding streams and see which data we should handle
       * next. */
      for (padlist = gst_element_get_padlist (element);
           padlist != NULL; padlist = g_list_next (padlist)) {
        GstPad *pad = GST_PAD (padlist->data);
        GstMyFilterInputContext *context = gst_pad_get_private_data (pad);

        if (GST_PAD_IS_SRC (pad))
          continue;

        while (GST_PAD_IS_USABLE (pad) &&
               !context->eos && !context->lastbuf) {
          GstData *data = gst_pad_pull (pad);

          if (GST_IS_EVENT (data)) {
            /* We handle events immediately */
            GstEvent *event = GST_EVENT (data);

            switch (GST_EVENT_TYPE (event)) {
              case GST_EVENT_EOS:
                context->eos = TRUE;
                gst_event_unref (event);
                break;
              case GST_EVENT_DISCONTINUOUS:
                g_warning ("HELP! How do I handle this?");
                /* fall-through */
              default:
                gst_pad_event_default (pad, event);
                break;
            }
          } else {
            /* We store the buffer to handle synchronization below */
            context->lastbuf = GST_BUFFER (data);
          }
        }

        /* synchronize streams by always using the earliest buffer */
        if (context->lastbuf) {
          if (!first_context) {
            first_context = context;
          } else {
            if (GST_BUFFER_TIMESTAMP (context->lastbuf) <
    		GST_BUFFER_TIMESTAMP (first_context->lastbuf))
              first_context = context;
          }
        }
      }

      /* If we handle no data at all, we're at the end-of-stream, so
       * we should signal EOS. */
      if (!first_context) {
        gst_pad_push (filter->srcpad, GST_DATA (gst_event_new (GST_EVENT_EOS)));
        gst_element_set_eos (element);
        return;
      }

      /* So we do have data! Let's forward that to our source pad. */
      gst_pad_push (filter->srcpad, GST_DATA (first_context->lastbuf));
      first_context->lastbuf = NULL;
    }

Note that a loop-function is allowed to return. Better yet, a loop function has to return so the scheduler can let other elements run (this is particularly true for the optimal scheduler). Whenever the scheduler feels right, it will call the loop-function of the element again.

## The Bytestream Object

A second type of elements that wants to be loop-based, are the so-called bytestream-elements. Until now, we've only dealt with elements that receive of pull full buffers of a random size from other elements. Often, however, it is wanted to have control over the stream at a byte-level, such as in stream parsers or demuxers. It is possible to manually pull buffers and merge them until a certain size; it is easier, however, to use bytestream, which wraps this behaviour.

Bytestream-using elements are ususally stream parsers or demuxers. For now, we will take a parser as an example. Demuxers require some more magic that will be dealt with later in this guide: . The goal of this parser will be to parse a text-file and to push each line of text as a separate buffer over its source pad.

    static void
    gst_my_filter_loopfunc (GstElement *element)
    {
      GstMyFilter *filter = GST_MY_FILTER (element);
      gint n, num;
      guint8 *data;

      for (n = 0; ; n++) {
        num = gst_bytestream_peek_bytes (filter->bs, &data, n + 1);
        if (num != n + 1) {
          GstEvent *event = NULL;
          guint remaining;

          gst_bytestream_get_status (filter->bs, &remaining, &event);
          if (event) {
            if (GST_EVENT_TYPE (event) == GST_EVENT_EOS)) {
              /* end-of-file */
              gst_pad_push (filter->srcpad, GST_DATA (event));
              gst_element_set_eos (element);

              return;
            }
            gst_event_unref (event);
          }

          /* failed to read - throw error and bail out */
          gst_element_error (element, STREAM, READ, (NULL), (NULL));

          return;
        }

        /* check if the last character is a newline */
        if (data[n] == '\n') {
          GstBuffer *buf = gst_buffer_new_and_alloc (n + 1);

          /* read the line of text without newline - then flush the newline */
          gst_bytestream_peek_data (filter->bs, &data, n);
          memcpy (GST_BUFFER_DATA (buf), data, n);
          GST_BUFFER_DATA (buf)[n] = '\0';
          gst_bytestream_flush_fast (filter->bs, n + 1);
          g_print ("Pushing '%s'\n", GST_BUFFER_DATA (buf));
          gst_pad_push (filter->srcpad, GST_DATA (buf));

          return;
        }
      }
    }

    static void
    gst_my_filter_change_state (GstElement *element)
    {
      GstMyFilter *filter = GST_MY_FILTER (element);

      switch (GST_STATE_TRANSITION (element)) {
        case GST_STATE_READY_TO_PAUSED:
          filter->bs = gst_bytestream_new (filter->sinkpad);
          break;
        case GST_STATE_PAUSED_TO_READY:
          gst_bytestream_destroy (filter->bs);
          break;
        default:
          break;
      }

      if (GST_ELEMENT_CLASS (parent_class)->change_state)
        return GST_ELEMENT_CLASS (parent_class)->change_state (element);

      return GST_STATE_SUCCESS;
    }

In the above example, you'll notice how bytestream handles buffering of data for you. The result is that you can handle the same data multiple times. Event handling in bytestream is currently sort of wacky, but it works quite well. The one big disadvantage of bytestream is that it requires the element to be loop-based. Long-term, we hope to have a chain-based usable version of bytestream, too.

## Adding a second output

WRITEME

## Modifying the test application

WRITEME