ffmpeg/doc/multithreading.txt

   1 FFmpeg multithreading methods
   2 ==============================================
   3
   4 FFmpeg provides two methods for multithreading codecs.
   5
   6 Slice threading decodes multiple parts of a frame at the same time, using
   7 AVCodecContext execute() and execute2().
   8
   9 Frame threading decodes multiple frames at the same time.
  10 It accepts N future frames and delays decoded pictures by N-1 frames.
  11 The later frames are decoded in separate threads while the user is
  12 displaying the current one.
  13
  14 Restrictions on clients
  15 ==============================================
  16
  17 Slice threading -
  18 * The client's draw_horiz_band() must be thread-safe according to the comment
  19   in avcodec.h.
  20
  21 Frame threading -
  22 * Restrictions with slice threading also apply.
  23 * For best performance, the client should set thread_safe_callbacks if it
  24   provides a thread-safe get_buffer() callback.
  25 * There is one frame of delay added for every thread beyond the first one.
  26   Clients must be able to handle this; the pkt_dts and pkt_pts fields in
  27   AVFrame will work as usual.
  28
  29 Restrictions on codec implementations
  30 ==============================================
  31
  32 Slice threading -
  33  None except that there must be something worth executing in parallel.
  34
  35 Frame threading -
  36 * Codecs can only accept entire pictures per packet.
  37 * Codecs similar to ffv1, whose streams don't reset across frames,
  38   will not work because their bitstreams cannot be decoded in parallel.
  39
  40 * The contents of buffers must not be read before ff_thread_await_progress()
  41   has been called on them. reget_buffer() and buffer age optimizations no longer work.
  42 * The contents of buffers must not be written to after ff_thread_report_progress()
  43   has been called on them. This includes draw_edges().
  44
  45 Porting codecs to frame threading
  46 ==============================================
  47
  48 Find all context variables that are needed by the next frame. Move all
  49 code changing them, as well as code calling get_buffer(), up to before
  50 the decode process starts. Call ff_thread_finish_setup() afterwards. If
  51 some code can't be moved, have update_thread_context() run it in the next
  52 thread.
  53
  54 If the codec allocates writable tables in its init(), add an init_thread_copy()
  55 which re-allocates them for other threads.
  56
  57 Add CODEC_CAP_FRAME_THREADS to the codec capabilities. There will be very little
  58 speed gain at this point but it should work.
  59
  60 If there are inter-frame dependencies, so the codec calls
  61 ff_thread_report/await_progress(), set AVCodecInternal.allocate_progress. The
  62 frames must then be freed with ff_thread_release_buffer().
  63 Otherwise leave it at zero and decode directly into the user-supplied frames.
  64
  65 Call ff_thread_report_progress() after some part of the current picture has decoded.
  66 A good place to put this is where draw_horiz_band() is called - add this if it isn't
  67 called anywhere, as it's useful too and the implementation is trivial when you're
  68 doing this. Note that draw_edges() needs to be called before reporting progress.
  69
  70 Before accessing a reference frame or its MVs, call ff_thread_await_progress().