[FFmpeg-devel] h264_qsv decoder speed

[Chao Liu]

On Wed, Aug 17, 2016 at 4:41 PM, Andy Furniss <adf.lists at gmail.com> wrote:

> Mark Thompson wrote:
>
>> On 17/08/16 20:47, Chao Liu wrote:
>>
>>> Hi there,
>>> I compared h264_qsv decoder from ffmpeg to intel media sdk sample_decode.
>>> There is pretty big speed gap. I wonder whether I did sth. wrong or there
>>> are really some problems with ffmpeg's implementation..
>>>
>>> The test video was captured from a 3MP(2048x1536) camera. The commands I
>>> used:
>>> -  ffmpeg -c:v h264_qsv -async_depth 10 -i test.h264 -c:v rawvideo -f
>>> null
>>> /dev/null
>>> -  sample_decode h264 -i test.h264
>>> Both uses 100% cpu (a full core). ffmpeg got 170FPS. sample_decode got
>>> 370FPS.
>>>
>>> I haven't got time debugging into this. Sending this out to see whether
>>> you
>>> guys might have sth. in mind..
>>>
>>
>> I think in both cases your speed bound must be on something other than
>> the decode, because the hardware goes a lot faster than either of those for
>> me.  Perhaps you are downloading the all of the output frames to normal
>> memory in order to write them to a null device output, and one of the cases
>> is doing that less efficiently somehow?
>>
>
> Only tested with AMD UVD, but unless you use -pix_fmt nv12 you will also
> get cpu load from ffmpeg doing nv12 -> yuv420p conversion.

I tried  ffmpeg -c:v h264_qsv -async_depth 4 -i test.h264 -pix_fmt nv12 -f
null /dev/null
I couldn't tell any difference.

>
>
>
>> Using vaapi on a low-power Haswell mobile chip (i.e. the same Quick Sync
>> hardware that libmfx uses) decodes a single 2048x1536 stream at around
>> 800fps with less than 50% CPU for me.
>>
>> - Mark
>>
>>
>> (My command to compare is:
>>
>> ./ffmpeg_g -vaapi_device /dev/dri/renderD128 -hwaccel vaapi
>> -hwaccel_output_format vaapi -i input.mp4 -an -vf
>> 'format=nv12|vaapi,hwupload' -f null -
>>
>
> Oh nice, I always wondered if there was a way to bench without copy back.
>
>
>
>> The nasty filtering there is contrived to do nothing, even with the
>> inconvenient stream reinitialisation.  I think libmfx might also work
>> somehow with "-c:v h264_qsv -hwaccel qsv", but I'm not sure and I don't
>> have anything to try it on right now.)
>>
>
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