[FFmpeg-devel] [PATCH] Yield on AVERROR(EAGAIN).

[Måns Rullgård]

Ramiro Polla <ramiro.polla at gmail.com> writes:

> Hi,
>
> Currently FFmpeg eats 100% cpu on devices that return EAGAIN. Attached
> patch makes FFmpeg yield when it gets EAGAIN. In Windows it gets down
> to normal cpu levels, but on Linux this didn't work as I expected, it
> still eats 100% cpu. Any ideas?

It's the wrong approach.  The spec for sched_yield says:

  The sched_yield() function shall force the running thread to
  relinquish the processor until it again becomes the head of its
  thread list.

On some kernels this puts the thread out of action for a full tick,
which is probably what you are seeing on Windows (yes, it has a
different name there).  On a modern Linux system it will merely invoke
the scheduler, which will notice that nothing else wants to run and
put this thread back on the CPU immediately.  Check with "vmstat 1" if
system the call rate shoots up while running this.

The correct solution is to make the devices somehow notify when input
is available so the process can sleep properly while waiting.  On Unix
systems this is typically done with select() on a file descriptor.  On
Windows something like WaitForMultiPleObJectS() is used.  Perhaps that
is something we could create an abstraction for.

> +static inline void ff_yield(void)
> +{
> +#if defined(__MINGW32__)
> +    Sleep(1);
> +#elif HAVE_PTHREADS
> +    sched_yield();
> +#endif
> +}

That conditional on HAVE_PTHREADS looks weird.

-- 
M?ns Rullg?rd
mans at mansr.com