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• avutil/lls : speed up performance of solve_lls

  24 novembre 2015, par Ganesh Ajjanagadde

  avutil/lls : speed up performance of solve_lls
  This is a trivial rewrite of the loops that results in better
  
  prefetching and associated cache efficiency. Essentially, the problem is
  
  that modern prefetching logic is based on finite state Markov memory, a reasonable
  
  assumption that is used elsewhere in CPU’s in for instance branch
  
  predictors.
  Surrounding loops all iterate forward through the array, making the
  
  predictor think of prefetching in the forward direction, but the
  
  intermediate loop is unnecessarily in the backward direction.
  Speedup is nontrivial. Benchmarks obtained by 10^6 iterations within
  
  solve_lls, with START/STOP_TIMER. File is tests/data/fate/flac-16-lpc-cholesky.err.
  
  Hardware : x86-64, Haswell, GNU/Linux.
  new :
  
    17291 decicycles in solve_lls, 2096706 runs,    446 skips
  
    17255 decicycles in solve_lls, 4193657 runs,    647 skips
  
    17231 decicycles in solve_lls, 8384997 runs,   3611 skips
  
    17189 decicycles in solve_lls,16771010 runs,   6206 skips
  
    17132 decicycles in solve_lls,33544757 runs,   9675 skips
  
    17092 decicycles in solve_lls,67092404 runs,  16460 skips
  
    17058 decicycles in solve_lls,134188213 runs,  29515 skips
  old :
  
    18009 decicycles in solve_lls, 2096665 runs,    487 skips
  
    17805 decicycles in solve_lls, 4193320 runs,    984 skips
  
    17779 decicycles in solve_lls, 8386855 runs,   1753 skips
  
    18289 decicycles in solve_lls,16774280 runs,   2936 skips
  
    18158 decicycles in solve_lls,33548104 runs,   6328 skips
  
    18420 decicycles in solve_lls,67091793 runs,  17071 skips
  
    18310 decicycles in solve_lls,134187219 runs,  30509 skips
  Reviewed-by : Michael Niedermayer <michael@niedermayer.cc>
  
  Signed-off-by : Ganesh Ajjanagadde <gajjanagadde@gmail.com>
  

  • [DH] libavutil/lls.c

• avfilter/af_afade : improve accuracy and speed of gain computation

  25 novembre 2015, par Ganesh Ajjanagadde

  avfilter/af_afade : improve accuracy and speed of gain computation
  Gain computation for various curves was being done in a needlessly
  
  inaccurate fashion. Of course these are all subjective curves, but when
  
  a curve is advertised to the user, it should be matched as closely as
  
  possible within the limitations of libm. In particular, the constants
  
  kept here were pretty inaccurate for double precision.
  Speed improvements are mainly due to the avoidance of pow, the most
  
  notorious of the libm functions in terms of performance. To be fair, it
  
  is the GNU libm that is among the worst, but it is not really GNU libm’s fault
  
  since others simply yield a higher error as measured in ULP.
  "Magic" constants are also accordingly documented, since they take at
  
  least a minute of thought for a casual reader.
  Reviewed-by : Paul B Mahol <onemda@gmail.com>
  
  Signed-off-by : Ganesh Ajjanagadde <gajjanagadde@gmail.com>
  

  • [DH] libavfilter/af_afade.c

• How do I play back video on Android with custom playback speed ?

  13 novembre 2015, par guidow

  I want to play back a video in Android, rendering it to a SurfaceTexture or something else that is usable as an OpenGL ES texture, but I need very precise control over the playback speed of the video to synchronize it to a stream of external events. These events will happen at a roughly predictable speed, but the exact speed will depend on many small mechanical details, influenced by many small factors, including the weather (and possibly even the phase of the moon...).

  The android.media.MediaPlayer class provided by android allows rendering to a SurfaceTexture (and the 360Videos app from the Oculus Mobile SDK successfully uses that feature), but unfortunately does not seem to allow altering the playback speed, at least not as far as I could tell from the documentation here.

  I have tried pausing and resuming the playback using pause() and start() respectively, to influence the playback speed, but that leads to extremely choppy and slow playback. My idea here was to make the video have a higher framerate than would ever be needed, and to then manually retard every frame until it actually needs to be shown. From the messages in the log, it looks to me like the MediaPlayer class will release various needed resources on pause and rerequest them on resume, which obviously kills performance if you do that once per frame.

  Another option I am looking at is ffmpeg. This one seems like it will do what i want, it doesn’t do any timings itself, it just decodes frames whenever I tell it to to a buffer, leaving me to use it however I want, whenever I want. The obvious drawback is that ffmpeg, at least on android, doesn’t do hardware decoding and probably won’t be able to decode 4K media in realtime.

  Yet another thing I was looking at was OpenMAX AL. Unfortunately, OpenMax AL is pretty hard to get into. I haven’t found any good beginner’s documentation yet, only some old, maybe outdated, READMEs as well as the interface specification from Khronos. The latter is a very long and cumbersome read, though, and I couldn’t yet even figure out if OpenMAX AL will even allow me to do my own timings...