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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(). |
71 |