[FFmpeg-devel] Tee improvement - discussion

[Marton Balint]

On Thu, 19 May 2016, Nicolas George wrote:

> Le tridi 23 floréal, an CCXXIV, Jan Sebechlebsky a écrit :
>> My current idea is to create queue for each output (as Marton suggested) and
>> process each output in separate thread. I was also considering using just
>> single queue, but since the AVPackets are referenced counted, the memory
>> overhead is negligible and using multiple queues will simplify the code.
>> Apart from getting advantage of non-blocking processing with multiple slave
>> muxers, error handling will also be improved.
> <snip>
>> Another question is what to do when some of the queues becomes full,
>> discussed options were so far:
>>     - Block write_packet call until the queue frees - this might be useful
>> when producer is faster than consumer, and we don't want to drop any packets
>> when recording to file.
>>     - Drop some yet unprocessed packets (until next keyframe, or free some
>> portion of queue) to free the queue - this might be useful for network
>> outputs.
>
> I must say, I am not very happy with the direction this project takes.
>
> Non-blocking muxers (and demuxers, and protocols) is a white whale for our
> API: we really really want it, but it huge and very hard to catch.
>
> It is something that needs to be implemented globally with a very careful
> design. Definitely not something that can be added in a corner of an obscure
> muxer. We already went down that path twice, with the thread for the UDP
> protocol and with running demuxer in threads in FFmpeg. Both did lead to
> endless complications: bugs, inconsistencies, portability trouble.

What caused these complications? Do you have some references?

>
> If you want to work on that (in order to bring these enhancements to the tee
> muxer, fitting the topic of the GSoC project), you probably can. But I
> expect this to be very hard. It has been rejected as a proposal for a GSoC
> project in the past on the basis that it is probably too hard for a random
> student. Also, I am not sure that Marton, your official mentor, would want
> such a huge shift in the project's goals.

The only way this can work if someone steps up and helps both designing 
the generic API and co-mentoring Jan.

If that not happens, I would like to stick to the original plan. We can 
only hope that we will learn something valuable which can be used 
later when somebody takes the time and effort to actually redesign the 
API.

Regards,
Marton