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Member "ffmpeg-3.4.2/doc/multithreading.txt" (31 Dec 2017, 3037 Bytes) of package /linux/misc/ffmpeg-3.4.2.tar.xz:


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    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 AV_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().