Edit File: multithreading.txt
FFmpeg multithreading methods ============================================== FFmpeg provides two methods for multithreading codecs. Slice threading decodes multiple parts of a frame at the same time, using AVCodecContext execute() and execute2(). Frame threading decodes multiple frames at the same time. It accepts N future frames and delays decoded pictures by N-1 frames. The later frames are decoded in separate threads while the user is displaying the current one. Restrictions on clients ============================================== Slice threading - * The client's draw_horiz_band() must be thread-safe according to the comment in avcodec.h. Frame threading - * Restrictions with slice threading also apply. * For best performance, the client should set thread_safe_callbacks if it provides a thread-safe get_buffer() callback. * There is one frame of delay added for every thread beyond the first one. Clients must be able to handle this; the pkt_dts and pkt_pts fields in AVFrame will work as usual. Restrictions on codec implementations ============================================== Slice threading - None except that there must be something worth executing in parallel. Frame threading - * Codecs can only accept entire pictures per packet. * Codecs similar to ffv1, whose streams don't reset across frames, will not work because their bitstreams cannot be decoded in parallel. * The contents of buffers must not be read before ff_thread_await_progress() has been called on them. reget_buffer() and buffer age optimizations no longer work. * The contents of buffers must not be written to after ff_thread_report_progress() has been called on them. This includes draw_edges(). Porting codecs to frame threading ============================================== Find all context variables that are needed by the next frame. Move all code changing them, as well as code calling get_buffer(), up to before the decode process starts. Call ff_thread_finish_setup() afterwards. If some code can't be moved, have update_thread_context() run it in the next thread. If the codec allocates writable tables in its init(), add an init_thread_copy() which re-allocates them for other threads. Add AV_CODEC_CAP_FRAME_THREADS to the codec capabilities. There will be very little speed gain at this point but it should work. If there are inter-frame dependencies, so the codec calls ff_thread_report/await_progress(), set AVCodecInternal.allocate_progress. The frames must then be freed with ff_thread_release_buffer(). Otherwise leave it at zero and decode directly into the user-supplied frames. Call ff_thread_report_progress() after some part of the current picture has decoded. A good place to put this is where draw_horiz_band() is called - add this if it isn't called anywhere, as it's useful too and the implementation is trivial when you're doing this. Note that draw_edges() needs to be called before reporting progress. Before accessing a reference frame or its MVs, call ff_thread_await_progress().