[FFmpeg-devel] [RFC] Disable compile-time tablegen for cbrt if total cycle count < 200000
gajjanag at mit.edu
Sat Jan 2 21:23:20 CET 2016
On Fri, Jan 1, 2016 at 8:07 AM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> Hi all,
> Motivated by a remark by Ronald:
> 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.
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
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%.
More information about the ffmpeg-devel