[FFmpeg-devel] [RFC] Disable compile-time tablegen for cbrt if total cycle count < 200000

[Ganesh Ajjanagadde]

On Fri, Jan 1, 2016 at 8:07 AM, Ganesh Ajjanagadde <gajjanag at 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%.