On Sat, Oct 17, 2015 at 9:28 PM, Hendrik Leppkes <h.lepp...@gmail.com> wrote: > On Sat, Oct 17, 2015 at 9:27 PM, Hendrik Leppkes <h.lepp...@gmail.com> wrote: >> --- >> libavcodec/utils.c | 6 ++++++ >> 1 file changed, 6 insertions(+) >> > > Above patch is submitted as an RFC, however I strongly believe its the > only way to keep hwaccels sane in the future. > There are several known problems when a hwaccel is used with frame > threading, at least with DXVA2, ranging from simple image corruption > to crashes in the GPU drivers. > > All the problems essentially come down to two factors: > > (1) while avcodec tries to prevent simultaneous access from different > threads, it cannot control user code. > As a API user you have no chance to avoid simultaneous access to the > hardware surfaces with frame threading, because as soon as avcodec > hands you a decoded surface, > it'll already have started working on the next one. This is a common > source for image corruption, as the decoder may fail to get a > reference frame if its currently > locked by the user code. > > This issue is not really fixable, other than introducing a mutex > around every call that the user code would have to lock as well. > API break and making hwaccel even more complex to use. > > (2) decoders often have had (or still have) trouble initializing the > hwaccel properly with multiple threads, which can result in multiple > threads re-creating the hwaccel > and disrupting decoding, or even crashing in the driver. > > While this is in theory fixable inside the decoders in question, it > can lead to quite a bit of complexity, which should be avoided. > > We should not be offering a mode that is known to be broken and even > crash (even if the crash is not in our code). > On top of that, frame threading with hwaccel does not actually improve > the speed at all, if anything it might be slightly slower, and will > definitely use more GPU memory. > > The only reason this combination even exists is because API users > insisted on it, and it was then added without properly taking the > consequences into account. > > There is an easy solution to keep using frame threads with software > decoding while using a hwaccel otherwise - its used by myself, by mpv > and by Kodi - simply re-open the decoder with the new threading flags. > Its really not a lot of work, and keeps the code sane and the decoding > reliable. >
No further comments? No thoughts? No-one that would object if I were to push the proposed patch? - Hendrik _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
