[Libav-user] Multithreaded decoding of multi-program transport streams
chisholm at mitre.org
Tue Dec 3 19:13:48 CET 2013
On 12/3/2013 2:36 AM, Antoine Martin wrote:
>> I use multiple encoders, in my case, simultaneously in different
>> threads, and it works fine. I think decoders should work fine too.
>> One thing I do is register a custom locking function via
>> av_lockmgr_register() just in case some bit of internal library code
>> wants to serialize threads through a critical section.
> Can someone please clarify for me when using "av_lockmgr_register" is
> required as I am not 100% sure I understand: on what types of builds
> (win32threads? pthreads? all?) and under what circumstances? (encoders,
> decoders, both? type of streams?)
> Maybe it's always required and we've just not really hit the problem yet
> because we don't use multi-clients often.. (each client gets its own
> encoding thread). And multiple contexts from the same thread works just
I can't give an official answer, but awhile back during development of
my tool, I started seeing warning messages from ffmpeg about
insufficient locking. I looked into the source and I remember there was
a simple check: a counter incremented on entry to a function, and
decremented on exit. If the counter ever went greater than 1, the
warning was generated. It's a simple concurrency check, independent of
thread library. That led me to the av_lockmgr_register() function,
which I used to make the warning go away.
I peeked into the ffmpeg source again, and I see it being used in
avcodec_open2(), as an example. The doxygen docs say that function is
not thread safe, but it appears to have some locking code which could
make it thread-safe as long as you've registered a locking function.
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