[FFmpeg-devel] [PATCH 05/10] lavc/libopenh264enc: prompt slice number changing according to cpus

[Fu, Linjie]

> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> Anton Khirnov
> Sent: Saturday, April 11, 2020 16:38
> To: FFmpeg development discussions and patches <ffmpeg-
> devel at ffmpeg.org>
> Subject: Re: [FFmpeg-devel] [PATCH 05/10] lavc/libopenh264enc: prompt
> slice number changing according to cpus
> 
> Quoting Fu, Linjie (2020-04-10 15:49:30)
> > > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> > > Anton Khirnov
> > > Sent: Friday, April 10, 2020 18:23
> > > To: FFmpeg development discussions and patches <ffmpeg-
> > > devel at ffmpeg.org>
> > > Subject: Re: [FFmpeg-devel] [PATCH 05/10] lavc/libopenh264enc: prompt
> > > slice number changing according to cpus
> > >
> > > Quoting Linjie Fu (2020-04-06 13:14:48)
> > > > Libopenh264enc would set the slice according to the number of cpu
> cores
> > > > if uiSliceNum equals to 0 (auto) in SM_FIXEDSLCNUM_SLICE mode.
> > > >
> > > > Prompt a warning for user to catch this.
> > > >
> > > > Signed-off-by: Linjie Fu <linjie.fu at intel.com>
> > > > ---
> > > >  libavcodec/libopenh264enc.c | 3 +++
> > > >  1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/libavcodec/libopenh264enc.c b/libavcodec/libopenh264enc.c
> > > > index dab8244..01a85fb 100644
> > > > --- a/libavcodec/libopenh264enc.c
> > > > +++ b/libavcodec/libopenh264enc.c
> > > > @@ -237,6 +237,9 @@ FF_ENABLE_DEPRECATION_WARNINGS
> > > >      param.sSpatialLayers[0].sSliceCfg.uiSliceMode               = s-
> >slice_mode;
> > > >      param.sSpatialLayers[0].sSliceCfg.sSliceArgument.uiSliceNum =
> avctx-
> > > >slices;
> > > >  #endif
> > > > +    if (avctx->slices == 0 && s->slice_mode == SM_FIXEDSLCNUM_SLICE)
> > > > +        av_log(avctx, AV_LOG_WARNING, "Auto slice number, "
> > > > +               "default to use the number of CPU cores: %d\n",
> av_cpu_count());
> > >
> > > Generally makes sense, but I'd avoid the call to av_cpu_count() since we
> > > don't know what method precisely will libopenh264 use to set the slice
> > > count. So IMO just say something like "slice count will be set
> > > automatically".
> >
> > There is a statement in the API definition[1] for uiSliceNum which says:
> >         when uiSliceNum=0 means auto design it with cpu core number.
> > So IMHO it seems clear enough if I got it correctly.
> 
> My concern is that different methods of computing the core count might
> return different results in certain cases. E.g. if the code is running
> under some resource management system that restricts it to a subset of
> total cores on the system. So we should not assume that av_cpu_count()
> will always necessarily return the same number that libopenh264 uses.

Indeed, thanks for the elaborations.

- Linjie