Re: [Mlt-devel] [PATCH mlt] Clear audio and video context to NULL

[Dan D. <da...@de...>]

On Tue, Oct 11, 2011 at 2:16 PM, Dan Dennedy <da...@de...> wrote:
> On Tue, Oct 11, 2011 at 2:05 PM, Mikko Rapeli <mik...@ik...> wrote:
>> Previewed clip without proxy and then enabled proxy back on in kdenlive.
>> I think context can be NULL after return from pthread_mutex_lock() at any
>> time and that would need to be checked everywhere.
>
> It should definitely not be the case that context can be NULL anywhere
> anytime. There is somehow inconsistent state introduced. Once again,
> the crash stems from KThumb.

Mikko, I think my mailbox is missing the latest posts on this thread!
Nonetheless, I did some analysis and here are some findings:

- The frames produced by avformat do indeed add a reference to the
avformat "guts." So, the idea of an "old" frame does not apply as the
reference prevents the producer's guts from being flushed from the
cache.
- Kdenlive does not call mlt_service_cache_set_size(). Rather, I added
it to mlt_multitrack to provide a universal solution. However, that
size does not take into account additional usage of avformat producers
outside of a multitrack composition.
- If VDPAU is enabled (mlt ./configure --avformat-vdpau required),
then the producer restricts itself to 5 active instances due to
limited GPU resources.

It *might* be possible to overrun the cache size if you are using
VDPAU playing a timeline that has 4 clips (including music) active at
the same point in time on the timeline and 2 KThumb threads are
running. Likewise, without VDPAU, and on a 8-core, you could overrun
the avformat cache size with multiple clips mixing and thumbnailing.
More likely there is some race condition. I will do some more of my
own testing with Kdenlive. Have to admit I have not been using it much
lately due to various projects and tasks.