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• Multiple nVidia GPU transcoding (NOT computing) bottleneck

  11 décembre 2018, par Daniel Cantarin

  I’m doing some nVidia multi-GPU testing. However, this tests are not on the computing field but transcoding, using nvenc/nvdec.

  I have a setup with 3 Quadro GPUs (max 4 on this motherboard), and running some transcoding jobs using ffmpeg.

  Thing is, up to 8 jobs everything is fine, with tolerable GPU and CPU stats.
  But when reaching the 9th job, the nVidia cards metrics start to fall down. Whats troublesome is this : it doesn’t matter the job distribution. That is, if I send 8 jobs to GPU0, and 1 job to GPU1, is the same as 4-5, or 4-4-1, or 0-4-5, etc.

  What I see beggining on the 9th job is :

  1. CPU gets to about 60% usage (30% up to 8th jobs) and doesn’t go up much after adding more jobs.
  2. DECODING metrics falls from about 75% (when single card has 8 jobs) to about 20%.
  3. Every card behaves the same when this problem starts, no matter how many jobs they have.

  And the last strange thing I see : when that problem happens, and I kill all the jobs, the cards keep working for a while (sometimes even minutes).

  All this points to some bottleneck somewhere on the motherboard. Maybe the PCIe bus, maybe some CPU subsystem, I’m not sure. It also points to some buffering happening somewhere. I’m using the usual popular tools to see high-level metrics and curves (top/htop, nvidia-smi, nvtop, etc).

  My question : does anybody knows some common bottlenecks regarding multi-GPU setups that could lead to a problem like this ?

  Any tip would be nice.

  Thanks in advance.

• Crop MP3 to first 30 seconds

  11 octobre 2020, par Cheekysoft

  Original Question

  



  I want to be able to generate a new (fully valid) MP3 file from an existing MP3 file to be used as a preview — try-before-you-buy style. The new file should only contain the first n seconds of the track.

  



  Now, I know I could just "chop the stream" at n seconds (calculating from the bitrate and header size) when delivering the file, but this is a bit dirty and a real PITA on a VBR track. I'd like to be able to generate a proper MP3 file.

  



  Anyone any ideas ?

  



  Answers

  



  Both mp3split and ffmpeg are both good solutions. I chose ffmpeg as it is commonly installed on linux servers and is also easily available for windows. Here's some more good command line parameters for generating previews with ffmpeg

  



  

  • -t <seconds></seconds> chop after specified number of seconds

  


  • -y force file overwrite

  


  • -ab <bitrate></bitrate> set bitrate e.g. -ab 96k

  


  • -ar <rate hz="hz"></rate> set sampling rate e.g. -ar 22050 for 22.05kHz

  


  • -map_meta_data <outfile>:<infile></infile></outfile> copy track metadata from infile to outfile

  


  



  instead of setting -ab and -ar, you can copy the original track settings, as Tim Farley suggests, with :

  



  

  • -acodec copy

  


  


• FFMPEG zoompan for continuous zoom in/zoom out from start duration

  10 juillet 2020, par Nikhil Solanki

  I am creating continues zoomin/zoomout effect for input image using this command :

  


  ffmpeg -i combine.mp4 -i image1.jpg -filter_complex "[0]split=2[color][alpha]; 
[color]crop=iw/2:ih:0:0[color]; [alpha]crop=iw/2:ih:iw/2:ih[alpha]; [colo][alpha]alphamerge[v1];
[1]scale=540*2:960*2, setsar=1, zoompan=z='if(lte(zoom,1.0),1.02,max(1.001,zoom-0.0015))':d=25*0.25:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)':s=540x960[v2]; 
[v2]zoompan=z='if(gte(zoom,1.1),1.0,min(zoom+0.0015,1.1))':d=25*0.25:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)':s=540x960[v2];
[v2]curves=vintage, format=yuv444p[v2];
[v2][v1] overlay=1" output_video.mp4 -y


  


  This command will continues zoomin and zoomout input image1.jpg for 1 second and stop after 1 second its OK. But problem is I want perform zoomin zoomout effect after 5 second of video. Video duration is 20s. So, how can I perform zoompan after some duration.