[FFmpeg-devel] [PATCH 3/3] avformat: add youtube-dl based demuxer
nfxjfg at googlemail.com
Wed Apr 8 22:25:01 CEST 2015
On Wed, 8 Apr 2015 22:16:22 +0200
Gilles Chanteperdrix <gilles.chanteperdrix at xenomai.org> wrote:
> On Wed, Apr 08, 2015 at 09:59:17PM +0200, Nicolas George wrote:
> > Le nonidi 19 germinal, an CCXXIII, Gilles Chanteperdrix a écrit :
> > > As I explained, I did it this way for portability. system is ANSI
> > > not POSIX, redirections work with windows shell too, and av_tempfile
> > > is implemented by ffmpeg, so I expect it to be portable.
> > Implementing pipe reading for other systems looks like a better time
> > investment.
> > > Well, actually, it looks not so hard to me.
> > Every person who got it wrong thought that too.
> > > One argument is an URL so, we could url encode it before passing it to
> > > youtube-dl
> > No, you can not, because it is probably already URL-encoded.
> > > Well, actually, vfork() + exec() is better than fork() + exec(),
> > > both performance wise and because it works on machines without an
> > > MMU, and the implementation of posix_spawn() in glibc is too
> > > conservative and uses fork() + exec() even in situations where
> > > vfork() could be used, see:
> > > https://sourceware.org/bugzilla/show_bug.cgi?id=10354
> > >
> > > So, better use vfork() + exec() (that is what system does for
> > > instance). I did not use it simply because I thought ffmpeg could
> > > have to be compiled on machines without the POSIX interface.
> > That was arguments for 20 years ago. Nowadays, vfork() was left to bitrot
> > for compatibility reasons while fork() was insanely optimized. And in the
> > meantime, threads were invented, with their own set of problems when it
> > comes to spawning a child process.
> That is what the doc says, but the doc is wrong. Measurements show
> that fork() has a detrimental effect, not as much at the moment of
> the fork, this is true, this instant is short, but later: all
> private writable mappings in the parent process are made read-only
> and need to take a fault and go to kernel-space to re-change the MMU
> configuration. You can make a simple experiment, map a large private
> anonymous area (say 512 MB), and measure the time it takes to write
> one byte on each page before and after a fork. At least the
> difference was huge last time I tested.
> And again, on a platform without MMU, you have vfork(), not fork().
> So, when you just need to run exec, it is stupid
So you would rather use a function that has been marked as
"obsolescent" in POSIX instead of posix_spawn(), only because a single
implementation (glibc) somehow messed up?
> > So the short of it: use posix_spawn().
> > The way it is implemented is at best not your concern, at worst premature
> > optimization.
> > > I did not use it simply because I thought ffmpeg could have to be compiled
> > > on machines without the POSIX interface.
> > FFmpeg requires POSIX. posix_spawn() is a POSIX option, yet, so it requires
> > a configure check. But apart from that, let someone who cares implement for
> > other systems.
> > Anyway:
> > > > Then I suggest you just endeavour to write a pseudo-demuxer for the output
> > > > of "youtube-dl -J". No need of a shell or an external command.
> > Just do that, no need for any fancy system call for that, and it would serve
> > as a base for a more fully automatic version anyway later.
> I do not follow you here. youtube-dl -J has a uselessly verbose
> output, much harder to parse than youtube-dl -g which gives you just
> one URL. But you still need to run youtube-dl. So, I do not see the
This is just retarded. Do you evne know how much more information -J
gives you? We don't need this in libavformat, and we ESPECIALLY don't
need a half-assed implementation of this in libavformat.
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