[FFmpeg-devel] ffmpeg stuck in transcoding H264 using QSV

Chao Liu yijinliu at gmail.com
Tue Jul 26 09:53:35 EEST 2016

On Mon, Jul 25, 2016 at 11:08 PM, Chao Liu <yijinliu at gmail.com> wrote:

> On Mon, Jul 25, 2016 at 11:04 PM, Chao Liu <yijinliu at gmail.com> wrote:
>> Hi,
>> (Ivan, I am sending this mail to you directly because you said you are
>> the maintainer of QSV related code in a recent mail. Apologize if this is
>> inappropriate.. )
>> I am trying to use h264_qsv to transcode a MP4 file. This is my command:
>> ffmpeg -c:v h264_qsv -i input.mp4 -look_ahead 0 -c:v h264_qsv -b:v 800k
>> -maxrate 1600k -preset fast output.mp4
>> It always stucks in this loop
>> <https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/qsvenc.c#L970>.
>> It works if I change the command to
>> ffmpeg -i input.mp4 -look_ahead 0 -c:v h264_qsv -b:v 800k -maxrate 1600k
>> -preset fast output.mp4
>> (not using qsv for decoding).
>> Looks like the decoding and encoding were contending for the resources
>> and there is a deadlock or sth. like that..
>> I tried to debug it a bit by comparing ffmpeg code with intel media SDK.
>> There is sth. I don't understand. Not sure whether it's related..
>> In ffmpeg, we decode the frame in a loop
>> <https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/qsvdec.c#L357>,
>> it sleeps and retries as long as MFXVideoDECODE_DecodeFrameAsync returns
>> busy.
>> In intel media SDK sample_decode, it calls SyncOperation when
>> MFXVideoDECODE_DecodeFrameAsync returns busy.
> Sorry, forgot to paste the link to the code
> https://github.com/Intel-Media-SDK/samples/blob/master/samples/sample_decode/src/pipeline_decode.cpp#L1631

>> Could this be the cause?
I think this is red herring. I could repro the deadlock w/o qsv decoding,
just the chance is pretty low.
And looks like this is a known issue: https://trac.ffmpeg.org/ticket/4832

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