[FFmpeg-user] x11grab video stalls every 30 seconds in screencapture

Star Brilliant b13253 at gmail.com
Fri Jun 7 08:47:13 CEST 2013

 > We are using latest ffmpeg, for screencapture in the cloud.
 > We are seeing odd dropped frame stalls in video capture every 30 seconds
 > for about 1-2 seconds in duration.
 > Audio is captured without stalls, and  Audio Video sync is perfectly fine
 > despite stalls.
 > Hoping someone had ideas, this is driving us nuts and there's nothing
 > obvious to explain it.
 > This is the capture setting we are using.
 > ffmpeg -f alsa -ac 2 -i pulse -f x11grab -r 30 -s 1024x768 -i :0.0  
 > libvorbis -ar 44100 -vcodec libx264 -preset ultrafast -gp 0 -threads 0
 > ffg4.mkv
 > This is the a drop frame test case capture  with lossless x264 that shows
 > dropped frames around 25-26 and significant stall errors at 30 and 60
 > seconds
 > We get this trying different options, like keyframes.
 > This is the output  with flashsrv, which shows similar pause around 
23 and
 > 53 seconds, and indicates it's not a libx264 specific issue.
 > flashsrv
 > This is running on an instance with 16 vcpu + 16gb ram, on the Amazon AWS
 >  there is no noticeable background process that we can see at least that
 > would explaing
 > and happens regardless if it's debian, or ubuntu.
 > This is the version of ffmpeg.
 > ffmpeg version 0.11.1 Copyright (c) 2000-2012 the FFmpeg developers
 >   built on Jun 26 2012 12:42:06 with gcc 4.7.1
 >   configuration: --arch=x86_64 --enable-avfilter --enable-vdpau
 > --enable-bzlib --enable-libgsm --enable-libschroedinger --enable-libspeex
 > --enable-libtheora --enable-libvorbis --enable-pthreads --enable-zlib
 > --disable-stripping --enable-gpl --enable-postproc --enable-swscale
 > --enable-x11grab --enable-libdc1394 --enable-shared --disable-static
 > --enable-libx264 --enable-libvpx --enable-libmp3lame --enable-libfaac
 > --enable-nonfree --enable-libopencore-amrwb --enable-libopencore-amrnb
 > --enable-version3

I have been experiencng this problem as well. After it stalls, the video and
audio lost sync. However I noticed that my HDD LED flashes when the video
I think that perhaps Linux kernel is trying to write buffered data to 
HDD, which
caused delay.
I wonder if there is a way to switch to write-through mode.

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