[FFmpeg-devel] [PATCH] avcodec: only warn about hwaccel with frame threads

[Ronald S. Bultje]

Hi,

On Sat, Jan 23, 2016 at 8:45 AM, Hendrik Leppkes <h.leppkes at gmail.com>
wrote:

> On Sat, Jan 23, 2016 at 2:38 PM, Ronald S. Bultje <rsbultje at gmail.com>
> wrote:
> > Hi,
> >
> > On Sat, Jan 23, 2016 at 8:28 AM, wm4 <nfxjfg at googlemail.com> wrote:
> >
> >> On Sat, 23 Jan 2016 13:50:32 +0100
> >> Michael Niedermayer <michael at niedermayer.cc> wrote:
> >>
> >> > On Sat, Jan 23, 2016 at 11:42:54AM +0100, Hendrik Leppkes wrote:
> >> > > On Sat, Jan 23, 2016 at 10:12 AM, Andreas Cadhalpun
> >> > > <andreas.cadhalpun at googlemail.com> wrote:
> >> > > > VLC uses hwaccel with frame threads and it works fine, but
> returning
> >> > > > an error here made it fail.
> >> > > >
> >> > > > This regression was introduced in commit 31741ae.
> >> > > >
> >> > >
> >> > > I'm still opposed to this, and so is everyone else that commented on
> >> the issue.
> >> >
> >> > I have no oppinion on the patch itself but i think
> >> > if MT+HWaccel works in some case(s) and is faster then there should be
> >> > a way to enable that. We have optional "fast" decoding support too
> >> > that skips some checks and could crash or uses simpler dequantization
> >> > or our skip loopfilter support, ...
> >>
> >> Whether it's faster is an entirely different question.
> >>
> >> The problem is that we don't want to enable hwaccel with MT because
> >> it's much pain for little gain, but then if fallback to software
> >> happens, decoding will remain single-threaded.
> >>
> >> Possible solutions:
> >> - somehow allow changing the number of frame threading threads during
> >>   fallback
> >> - add a wrapper codec, which behaves like the old one, but reopens the
> >>   codec behind the scenes in order to change the number of threads on
> >>   fallback
> >> - making sure hwaccels actually work with MT (complicated => not worth
> >>   the trouble)
> >
> >
> > Why don't we ignore threading (set active_thread_type to NONE) if hwaccel
> > is enabled? Doesn't that fix everything in one single place?
> >
>
> Threading is already setup and running when the hwaccel gets
> activated, and its not designed to go in and out of threading mode at
> will.


Ok, so we can change how threading works. A fairly simple solution may be
to have a hwaccel flag that says whether it's threadsafe (whitelist style),
and if that's not set and hwaccel is enabled and active, have either
hwaccel or threading code "throttle" the number of iterating threads to 1
(i.e. you have N running threads, but N-1 idle and only 1 is ever accessed
while throttle-mode is active). This way, software fallback still works.

This way, it's all centralized in one location. You could even enable
threading for these hwaccels that really do support it if it really does
help and is threadsafe.

Ronald