[FFmpeg-devel] [RFC] Disable compile-time tablegen for cbrt if total cycle count < 200000
gajjanag at mit.edu
Sat Jan 2 22:08:40 CET 2016
On Sat, Jan 2, 2016 at 1:02 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> On Sat, Jan 2, 2016 at 3:23 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>> On Fri, Jan 1, 2016 at 8:07 AM, Ganesh Ajjanagadde <gajjanag at mit.edu>
>> > 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.
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
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