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

[Anton Khirnov]

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.

-- 
Anton Khirnov