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• Writing multithreaded video and audio packets with FFmpeg

  27 février 2017, par Robert Jones

  I couldn’t find any information on the way av_interleaved_write_frame deals with video and audio packets.

  I have multiple audio and video packets coming from 2 threads. Each thread calls a write_video_frame or write_audio_frame, locks a mutex, initialize an AVPacket and writes data to an .avi file.

  Initialization of AVCodecContext and AVFOrmatContext is ok.

  — Edit 1 —

  Audio and video are coming from an external source (microphone and camera) and are captured as raw data without any compression (even for video).
  I use h264 to encode video and no compression for Audio (PCM).

  Audio captured is : 16bits, 44100khz, stereo

  Video captured is 25FPS

  Question :

  1) Is it a problem if I write multiple video packets at once (let’s say 25 packets/sec) and just one audio packet/sec.

  Answer : Apparently not, the function av_interleaved_write_frame should be able to manage that kind of data as soon as pts and dts is well managed

  This means I call av_interleaved_write_frame 25 times for video writing and just 1 for audio writing per second. Could this be a problem ? If it is how can I deal with this scenario ?

  2) How can I manage pts and dts in this case ? It seems to be a problem in my application since I cannot correctly render the .avi file. Can I use real time stamps for both video and audio ?

  Answer : The best thing to do here is to use the timestamp given when capturing audio / video as pts and dts for this kind of application. So these are not exactly real time stamps (from wall clock) but media capture timestamps.

  Thank you for your precious advices.

• Camera Rendering Buffers and Stutters When Processing Large Video Files with FFmpeg

  20 avril 2023, par TIANYU HU

  When rendering a real-time camera, I use ffmpeg to process a large video file(like 4G or even larger) at the same time. However, I noticed that the video frames are buffering and stuttering, indicated they are probably competing for limited system resources, like cpu, memory or I/O bandwidth etc.

  


  I‘ve tried a lot of experiments to figure out the root cause. Firstly I limit the cpu usage of ffmpeg to 25%, but sadly it’s not getting better.

  


  Then I suspect that ffmpeg would read the large video file from disk to page cache in memory as much as it can before processing, and the generated files are going to be written back from page cache to disk. The RAM of our computer is 8G, apparently it needs to swap in and swap out pages between memory and disk. This process is costly for the CPU, and other processes are likely to trigger an interrupt named “Page Fault” when they access pages that are not actually loaded into memory. If the time taken for “page fault” is too long, the program may lag.

  


  Lastly I configure the system parameters related to “write back dirty pages to disk”, such as vm.dirty_writeback_centisecs and vm.dirty_background_ratio, to try to write back the dirty (Disk I/O) more frequently or infrequently. But I’m not quite sure what would happen if I modify these parameters.

  


  Expection :
The requirement can be summarized as “real-time video rendering has higher priority, and the low rate of large file processing is accepted”, are there any possible solutions of this issue from your perspective ? Thanks in advance.

  


• Copying .avi file to a server while it is growing

  16 mars 2017, par DoubleSDouleU

  I am designing a streaming system using VLC media player.

  Luckily, VLC has .avi file format output because I need real-time video slicing while it is recording and avi can do that.

  Anyways, I wonder if it is possible that copying a dynamically growing avi file from local to server.

  I have tried some bash commands (rsync, scp, etc.), but it failed to copy.