FFmpeg/doc/multithreading.txt
Michael Niedermayer 444e9874a7 Merge commit 'def97856de6021965db86c25a732d78689bd6bb0'
* commit 'def97856de6021965db86c25a732d78689bd6bb0':
  lavc: AV-prefix all codec capabilities

Conflicts:
	cmdutils.c
	ffmpeg.c
	ffplay.c
	libavcodec/8svx.c
	libavcodec/aacenc.c
	libavcodec/ac3dec.c
	libavcodec/adpcm.c
	libavcodec/alac.c
	libavcodec/atrac3plusdec.c
	libavcodec/bink.c
	libavcodec/dnxhddec.c
	libavcodec/dvdec.c
	libavcodec/dvenc.c
	libavcodec/ffv1dec.c
	libavcodec/ffv1enc.c
	libavcodec/fic.c
	libavcodec/flacdec.c
	libavcodec/flacenc.c
	libavcodec/flvdec.c
	libavcodec/fraps.c
	libavcodec/frwu.c
	libavcodec/gifdec.c
	libavcodec/h261dec.c
	libavcodec/hevc.c
	libavcodec/iff.c
	libavcodec/imc.c
	libavcodec/libopenjpegdec.c
	libavcodec/libvo-aacenc.c
	libavcodec/libvorbisenc.c
	libavcodec/libvpxdec.c
	libavcodec/libvpxenc.c
	libavcodec/libx264.c
	libavcodec/mjpegbdec.c
	libavcodec/mjpegdec.c
	libavcodec/mpegaudiodec_float.c
	libavcodec/msmpeg4dec.c
	libavcodec/mxpegdec.c
	libavcodec/nvenc_h264.c
	libavcodec/nvenc_hevc.c
	libavcodec/pngdec.c
	libavcodec/qpeg.c
	libavcodec/ra288.c
	libavcodec/rv10.c
	libavcodec/s302m.c
	libavcodec/sp5xdec.c
	libavcodec/takdec.c
	libavcodec/tiff.c
	libavcodec/tta.c
	libavcodec/utils.c
	libavcodec/v210dec.c
	libavcodec/vp6.c
	libavcodec/vp9.c
	libavcodec/wavpack.c
	libavcodec/yop.c

Merged-by: Michael Niedermayer <michael@niedermayer.cc>
2015-07-27 22:50:18 +02:00

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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().