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1 FFmpeg multithreading methods
4 FFmpeg provides two methods for multithreading codecs.
6 Slice threading decodes multiple parts of a frame at the same time, using
7 AVCodecContext execute() and execute2().
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.
14 Restrictions on clients
17 Slice threading -
18 * The client's draw_horiz_band() must be thread-safe according to the comment
19 in avcodec.h.
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.
29 Restrictions on codec implementations
32 Slice threading -
33 None except that there must be something worth executing in parallel.
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.
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().
45 Porting codecs to frame threading
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
54 If the codec allocates writable tables in its init(), add an init_thread_copy()
55 which re-allocates them for other threads.
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.
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.
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.
70 Before accessing a reference frame or its MVs, call ff_thread_await_progress().