[Bug 747270] New: avfvideosrc: BUFFER_QUEUE_SIZE too small

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=747270

            Bug ID: 747270
           Summary: avfvideosrc: BUFFER_QUEUE_SIZE too small
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: All
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-bad
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: ilya.konstantinov at gmail.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

In avfvideosrc.m, we have:

  #define BUFFER_QUEUE_SIZE     2

This buffer queue is used in a producer-consumer fashion between the AV capture
callbacks (on AVFoundation's thread) and the element (on the main thread).

With such a small queue, we were constantly:

 a) losing performance due to the main thread going to sleep constantly

 b) periodically dropping frames in this code:

  if ([bufQueue count] == BUFFER_QUEUE_SIZE)
    [bufQueue removeLastObject];

Increasing the queue size to e.g. 32 improves performance significantly. On
iPhone 5s, improvement of ~7% is seen in the following pipeline:

  avfvideosrc device-index=0 ! video/x-raw,width=1280,height=720,framerate=30/1
! vtenc_h264 bitrate=400 ! fakesink

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