[FFmpeg-devel] av_resample_init() optimization

Olivier Guilyardi list
Thu Jan 7 23:46:31 CET 2010

On 01/06/2010 11:43 PM, Michael Niedermayer wrote:
> On Wed, Jan 06, 2010 at 07:34:42PM +0100, Olivier Guilyardi wrote:
>> I'm using libavcodec resampling on an Android ARM (Qualcomm MSM7200A 528 MHz)
>> device, without FPU support. The fact that it's integer-based is really great
>> for performances (this is for a closed source application, but I plan full LGPL
>> compliance with dynamic linking).
>> Nevertheless, there is an overhead upon initialization, when calling:
>> av_resample_init(44100, 48000, 16, 10, 0, 0.8).
>> This comes from av_build_filter() which takes about 6.9 *seconds* to build the
>> polyphase filterbank.
> uhm 7 seconds
>> About 300ms (4%) can be saved by applying resample-xfactor.1.patch to r21036. It
>> caches some constant factor, and prevent bessel() and sqrt() from being called
>> when unnecessary. I'm not sure whether the cached factor affects accuracy.
> dont use kaiser windows if you have no FPU or optimize the code properly
> its surely possibly to do this faster with table and cubic interpolation
> also i think your float/double emulation might be trash because even without
> FPU 7sec appears quite extreem, it just calculates ~16k samples of a function
> on a ~500mhz cpu in 7sec thats 200k cpu cycles for a single value

Well, I tried the cubic type by settings WINDOW_TYPE to 0, and calling
av_resample_init() as above. Initialization was indeed much faster, but it
sounded terrible. Any advice on the parameters?

In regard to floating emulation, I'm using gcc 4.2.1 bundled with the Android
NDK, which reports itself as:

Using built-in specs.
Target: arm-eabi
Configured with:
--target=arm-eabi --host=x86_64-unknown-linux-gnu
--build=x86_64-unknown-linux-gnu --enable-languages=c,c++ --disable-libssp
--enable-threads --disable-nls --disable-libmudflap --disable-libgomp
--disable-libstdc__-v3 --disable-sjlj-exceptions --disable-shared
--with-float=soft --with-fpu=vfp --with-arch=armv5te --enable-target-optspace
--with-abi=aapcs --disable-nls
Thread model: single
gcc version 4.2.1

And resample2.c is compiled with (I don't have full control on these):

-Ibuild/platforms/android-3/arch-arm/usr/include -march=armv5te -mtune=xscale
-msoft-float -fpic -mthumb-interwork -ffunction-sections -funwind-tables
-fstack-protector -fno-short-enums -D__ARM_ARCH_5__ -D__ARM_ARCH_5T__
-D__ARM_ARCH_5E__ -D__ARM_ARCH_5TE__  -O2 -fomit-frame-pointer -fstrict-aliasing
-funswitch-loops -finline-limit=300
-I/home/olivier/dev/android/ar/project/jni/../../lib/ffmpeg/android -DANDROID
-Wall -Drestrict="" -DCONFIG_RESAMPLE_FAST_INIT -O2 -DNDEBUG -g  -c -MMD -MP -MF
-o out/apps/audiorec//objs/avcodec-resample/../libavcodec/resample2.o

Another point: my timing measures are approximative, because the system is busy
doing over things during initialization, but nothing computational intensive
that I know of.


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