[FFmpeg-trac] #8533(undetermined:new): complex_filter very slow when stichting together many fragments

[FFmpeg]

#8533: complex_filter very slow when stichting together many fragments
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             Reporter:  stefanreich  |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 Summary of the bug:

 complex_filter turns very slow when splicing a long video into parts and
 re-assembling them using complex_filter.

 The script contains 894 parts which are taken from one input video and
 assembled into a new one. When I run the command as shown, it starts with
 less .01x realtime speed and very slowly goes up to around .1x. It keeps
 climbing, but only very slowly. I noticed ffmpeg also doesn't use all the
 cores anymore (only around 1.5 cores).

 When I take less of those 894 parts, e.g. only 300, ffmpeg is noticeably
 faster. With less than 100 parts, it runs in more than realtime as I
 expect it to.

 How is this explained? I can see no logical reason for this behavior,
 processing speed should stay high even with many fragments (they are not
 getting shorter, there are just fewer of them).

 It really hinders my video editing of large videos using ffmpeg, so I
 think this is an actual bug.

 How to reproduce:
 {{{
 % ffmpeg  -y -i winograd2-amp.mkv -filter_complex_script script.txt -map
 [outv] -map [outa] winograd2-amp-cut.mkv

 with this file: https://botcompany.de/files/1400345/complex-script.txt

 ffmpeg version 3.4.6-0ubuntu0.18.04.1
 built with gcc 7 (Ubuntu 7.3.0-16ubuntu3)
 }}}

 I can provide the input .mkv on request.

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Ticket URL: <https://trac.ffmpeg.org/ticket/8533>
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