id,summary,reporter,owner,description,type,status,priority,component,version,resolution,keywords,cc,blockedby,blocking,reproduced,analyzed
2122,Commit 75d900d makes skipping in the input while using swresample very slow,eelco,michael,"In commit 75d900d52edb281eb4ab4989800e75abe7555937 the memory allocation method used for allocating memory for audio switched to using av_mallocz (from av_malloc). I’m curious to learn why this change was made, because it makes skipping in a file (using -ss before the input) several orders of magnitude slower.

Version 1.0:
{{{
% time ./ffmpeg -ss 2 -i ~/Downloads/example.mkv -t 1 -vcodec copy -f matroska -map 0:0 -acodec ac3 -async 1 -map 0:1 -y /dev/null
ffmpeg version 1.0 Copyright (c) 2000-2012 the FFmpeg developers
  built on Dec 28 2012 14:35:11 with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
...
frame=   74 fps=7.2 q=-1.0 Lsize=       0kB time=00:00:01.01 bitrate=   0.0kbits/s    
video:28kB audio:56kB subtitle:0 global headers:0kB muxing overhead -100.000000%
       10.32 real         3.52 user         6.74 sys
}}}

Version 1.0 with commit 75d900d reverted:
{{{
% time ./ffmpeg -ss 2 -i ~/Downloads/example.mkv -t 1 -vcodec copy -f matroska -map 0:0 -acodec ac3 -async 1 -map 0:1 -y /dev/null
ffmpeg version 1.0 Copyright (c) 2000-2012 the FFmpeg developers
  built on Dec 28 2012 14:35:11 with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
...
frame=   74 fps=0.0 q=-1.0 Lsize=       0kB time=00:00:01.01 bitrate=   0.0kbits/s    
video:28kB audio:56kB subtitle:0 global headers:0kB muxing overhead -100.000000%
        0.46 real         0.05 user         0.35 sys
}}}

(Note that it wasn’t a clean build, so the ‘built on’ banner didn’t change.)

I also tried testing with an -ss value of 100, but killed it after a about 5 minutes. With the commit reverted it takes about a second.

We found the cause of the problem by profiling ffmpeg and saw that all time was being spent in av_mallocz. I haven’t looked into it any further, but I suspect that the av_mallocz call exposes a bug that previously didn’t cause any problem.

I also checked the 1.1 release, the problem is still there.",defect,closed,important,swresample,git-master,fixed,regression,,,,1,0