1Libav multithreading methods 2============================================== 3 4Libav provides two methods for multithreading codecs. 5 6Slice threading decodes multiple parts of a frame at the same time, using 7AVCodecContext execute() and execute2(). 8 9Frame threading decodes multiple frames at the same time. 10It accepts N future frames and delays decoded pictures by N-1 frames. 11The later frames are decoded in separate threads while the user is 12displaying the current one. 13 14Restrictions on clients 15============================================== 16 17Slice threading - 18* The client's draw_horiz_band() must be thread-safe according to the comment 19 in avcodec.h. 20 21Frame 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 29Restrictions on codec implementations 30============================================== 31 32Slice threading - 33 None except that there must be something worth executing in parallel. 34 35Frame 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 45Porting codecs to frame threading 46============================================== 47 48Find all context variables that are needed by the next frame. Move all 49code changing them, as well as code calling get_buffer(), up to before 50the decode process starts. Call ff_thread_finish_setup() afterwards. If 51some code can't be moved, have update_thread_context() run it in the next 52thread. 53 54If the codec allocates writable tables in its init(), add an init_thread_copy() 55which re-allocates them for other threads. 56 57Add CODEC_CAP_FRAME_THREADS to the codec capabilities. There will be very little 58speed gain at this point but it should work. 59 60Call ff_thread_report_progress() after some part of the current picture has decoded. 61A good place to put this is where draw_horiz_band() is called - add this if it isn't 62called anywhere, as it's useful too and the implementation is trivial when you're 63doing this. Note that draw_edges() needs to be called before reporting progress. 64 65Before accessing a reference frame or its MVs, call ff_thread_await_progress(). 66