Libav multi-threaded decoding

Question

According to the documentation here , Libav provides the "infrastructure" for multithreaded decoding. But the docs are vague and confusing regarding how multithreaded decoding is implemented. Is it internally supported and just requires setting a flag in the struct, or does the user have to provide his own implementation with the provided functions? I searched a lot but could not find even one example of multithreaded video decoding with libav.

Answer 1

The link you have referred to looks like a description towards codec developers rather than to end-user of FFmpeg libraries using existing codecs.

Multi-threaded support is indeed implemented by framework itself - it requires FFmpeg to be built with thread support (like --enable-pthreads or --enable-w32threads configure options), varies across specific codecs (eg one codec may support multiple threads while others don't) and implement different approaches (decoding multiple frames in parallel or multiple slices within a single frame).

End-user application may configure the number of threads to utilize (via AVCodecContext::thread_count property set before avcodec_open2() ) and threaded mode ( AVCodecContext::thread_type set to FF_THREAD_FRAME or FF_THREAD_SLICE ). Thread pool will be managed by FFmpeg itself, although some answers say that it is also possible using application-provided pool.

Some documents refer that AVCodecContext::thread_count default value set to 0 allows FFmpeg to automatically decide how many threads to use (which will be done based on a number of logical CPUs in system), but I've never tried this (always set this parameter manually). So probably it already does multi-threaded decoding on your system - check CPU load in task manager.

What FFmpeg doesn't do is managing multiple threads for reading packets from a file, decoding different streams in different threads and other similar things which a video player normally does - this is normally implemented by application itself. Although I recall some features have been integrated to FFmpeg simplifying implementation of these routines (like a packets queue).