[opus] Multithreaded encoding?

[Emily Bowman]

I can't think of any good way to multithread the encoder without ruining
efficiency, because so much efficiency is gained from holding state over,
from the good old MP2 days all the way up to now. Running at 12x even on an
RPi makes it very questionable whether to spend *any *engineer resources in
that direction, too. On desktop systems it's closer to 100x for
single-threaded stereo CELT encoding.

You could easily separate decoding/SRC and encapsulating onto their own
threads, but I doubt that would gain you more than 10% or so, since those
are normally very fast operations. Splitting the encoder stages into
individual threads seems like it would be promising, even if the referenced
PDF abandoned the idea due to implementation effort, but you rapidly run
into the problem of memory contention: So much time is wasted moving data
between cores and caches and main memory. While real throughput goes
slightly up, electrical efficiency goes way down.

If fidelity is less important than raw speed, then just chopping the stream
into X chunks and encoding each on its own thread will work; the shorter
the chunks, the more wasted data, but at least it generally works. As jm
says, there may be compatibility issues. You might be able to get around
that with a combination of FEC packets and marking other packets lost,
instead of stitching, but I'm not sure if relying on FEC would even be more
or less compatible than relying on stitching.

As for the direct question, no, there's no drop-in replacement available.
Opusenc could use a general cleanup, so if someone threw some basic
pthreads into that effort it might well be accepted.

Em


On Sun, Mar 29, 2020 at 5:57 PM Jesus Cea <jcea at jcea.es> wrote:

> I am interested in being able to encode a single Opus stream using
> several CPU cores.
>
> I get a raw audio input and "opusenc" can transcode it at 1200% speed
> (Raspberry PI 3B+). It saturates a single CPU core, but the other three
> are idle.
>
> Is out there any project to add multithreading options to "opusenc", or
> something in that line?
>
> Looking around, I have found this:
>
> https://github.com/enzo1982/superfast#superfast-codecs
> https://hydrogenaud.io/index.php?topic=114598.0
> <
> https://github.com/enzo1982/superfast/blob/master/doc/SuperFast%20Codecs.pdf
> >
>
> Is it out there any other multithreaded "opusenc" drop in replacement?.
> Any plan for future "opusenc" improvement in this area?
>
> Thanks.
>
> --
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