On Sat, Jan 2, 2016 at 1:02 PM, Ronald S. Bultje <rsbul...@gmail.com> wrote: > Hi, > > On Sat, Jan 2, 2016 at 3:23 PM, Ganesh Ajjanagadde <gajja...@mit.edu> wrote: >> >> On Fri, Jan 1, 2016 at 8:07 AM, Ganesh Ajjanagadde <gajja...@mit.edu> >> wrote: >> > Hi all, >> > >> > Motivated by a remark by Ronald: >> > https://ffmpeg.org/pipermail/ffmpeg-devel/2016-January/186200.html, >> > this is a request for comment on disabling compile time tablegen for >> > cbrt if the total cycle count < 200000. Note that cbrt tables are only >> > used in aacdec. >> >> To start some effort towards a more principled understanding of the >> costs of runtime table initialization, I did some benchmarks. >> Note: I am not familiar with avcodec, so I don't know if this reflects >> correctly the static vs dynamic cost. >> file: ~/samples/aac/al04_44.mp4 >> stream_loop: 100 >> number of calls of avcodec_decode_audio4: 35956 >> cost per call (avcodec_decode_audio4): >> 834030 decicycles in decode_audio4, 1 runs, 0 skips >> 556200 decicycles in decode_audio4, 2 runs, 0 skips >> [...] >> 177365 decicycles in decode_audio4, 16384 runs, 0 skips >> 177059 decicycles in decode_audio4, 32768 runs, 0 skips >> decoding cost: 17706*35956 = 636,636,936 cycles >> duration: 832.55 seconds >> cost per second of audio: 764,683 cycles >> cost of table init: 200,000 cycles >> fraction: 0.26 >> >> So in a clip of n seconds duration, the relative overhead of dynamic >> initialization of these cbrt tables is 0.26/n. For a more concrete >> number, say a clip is of 180 seconds duration, then the overhead is >> 0.26/180 = 0.15%. > > > What if I only want to play the first 3 second of 1000 clips by calling > ffmpeg.exe in a shell script? E.g. for fingerprinting. The number of use > cases you cover needs to be more than just playback, ffmpeg can do much more > than just that.
Two remarks: 1. As I said, this was only a start of the discussion; and the general c/t decay holds; constant c should be close to what I obtained. So yes, if you have such a thing, it will be slower. 2. I thought ffmpeg had the ability to handle multiple input files in a single invocation? Thus, someone doing such a thing is IMHO doing it incorrectly. > > Ronald _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
