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Matt Hamilton
Matt Hamilton

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Stress Testing Choirless Render Pipeline

I've been working on a Call for Code project, called Choirless. I've written a number of posts on here about IBM Cloud Functions, which are used for most of the render pipeline. A few weeks ago we did the "alpha" release of Choirless.

Since then, we've had a number of people contact us about using Choirless. From barbershop choirs to marching bands. At the larger end of the scale, the marching bands have around 100-120 musicians performing at once.

Could Choirless handle that many? Well... we certainly want it to, but out of interest could the 'alpha' version we've put together in the past 8 weeks cope with it? I decided to give it a try!

The rendering pipeline is a series of IBM Cloud Functions, that have triggers on IBM Cloud Object Storage (COS) buckets. So a file gets uploaded from the Choirless front end to the first COS bucket, and then a trigger fires calling a serverless function to process it and put the output in a different bucket. That bucket in turn has triggers on it, and so on. Until we get to the end.

The current pipeline processing stages are:

  1. Convert the raw video file to standard format, reduce size, normalise audio loudness
  2. Calculate the alignment with the reference audio track
  3. Trim the audio/video based on alignment calculation
  4. Stitch all the videos together to produce final video
  5. Take snapshot jpg of video

My colleague, Sean, got out his bagpipes and recorded 6 renditions of a particular piece of music. This was how that came out on Choirless to start with:

So to stress test the render pipeline I was going to need to replicate Sean's videos multiple times. So I create a new cloud function called "wateraftermidnight" (can you guess the reference?). This serverless function counted how many "gremlin" pieces there are in the COS bucket:

    choir_id = args['choir_id']
    song_id = args['song_id']
    num_gremlins = int(args['num_gremlins'])

    bucket = "choirless-videos-converted"
    prefix_key = f"{choir_id}+{song_id}"

    # list all objects in the COS bucket
    contents = cos.list_objects(

    # Just get the "keys" (filenames)
    files = [ x['Key'] for x in contents['Contents'] ]

    # Find the gremlins and non-gremlins
    gremlins = [ x for x in files if 'gremlin' in x ]
    non_gremlins = [ x for x in files if 'gremlin' not in x ]

    # if we have more gremlins than we need, abort
    if len(gremlins) > num_gremlins:
        args["success"] = "true"
        return args

    # pick a random non_gremlins as the source file   
    source_key = random.choice(non_gremlins)
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So by this point we have checked if we have got enough files, if so we abort.

Later on in the code, we take the source part and apply a random delay to the audio of up to 50ms

       i = len(gremlins) + 1
        new_part = f"gremlin-{i}"
        new_filename = f"{choir_id}+{song_id}+{part_id}-{new_part}{file_path.suffix}"
        new_path = file_path.with_name(new_filename)

        delay = random.randint(0,50)

        stream = ffmpeg.input(str(file_path))
        audio = stream.filter_('adelay', delays=[delay, delay]).filter_('asetpts', 'PTS-STARTPTS')
        video =
        out = ffmpeg.output(audio, video, str(new_path))
        stdout, stderr =
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This is so that we simulate pieces being out a sync slightly so that our sync code in our pipeline has some work to do.

At the end, we call ourselves again, so this function will run over and over again until we have enough "gremlins"

url = ""

data = {'blocking': False,
        'choir_id': args['choir_id'],
        'song_id': args['song_id'],
        'num_gremlins': args['num_gremlins'],
        'endpoint': args['endpoint'],
        'auth': args['auth'],

headers = {'X-Require-Whisk-Auth': args['auth']}

response =,
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How did it perform? Well as the number of parts increased, it certainly took longer to process. Eventually we hit the 10 minute max processing time for an IBM Cloud Function:

2020-07-25 17:48:57 9b844c7035a54743844c7035a5574376 blackbox warm  10m0.1s    developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:48:46 deb00965e52a4517b00965e52a451702 blackbox warm  8m16.267s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:48:23 803eebed9a094efbbeebed9a094efb8d blackbox cold  10m0.887s  developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:48:14 9a3f147606e84495bf147606e8b49567 blackbox cold  10m0.738s  developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:47:56 b51ddd379e0841a49ddd379e08e1a43b blackbox warm  10m0.1s    developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:47:37 2360edede1bf4c81a0edede1bfcc81f9 blackbox warm  10m0.1s    developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:47:08 51d0a0c7a58c4a8c90a0c7a58c7a8c97 blackbox cold  10m0.9s    developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:45:56 87e813b70ed0489ba813b70ed0a89b33 blackbox cold  10m0.654s  developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:45:49 cd6ecfbfc80b468aaecfbfc80b968a76 blackbox cold  5m48.593s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
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2020-07-25 17:45:29 0590fda27cb041a490fda27cb061a489 blackbox cold  5m37.71s   success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:44:35 8b00060e460d40f480060e460db0f446 blackbox cold  10m0.588s  developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:42:59 2fdfd1b0f6684d629fd1b0f668bd6255 blackbox cold  5m47.901s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
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2020-07-25 17:42:22 fdaec354afa848faaec354afa828fa6e blackbox warm  7m30.957s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 17:42:13 bdcb0526692047468b052669204746ac blackbox warm  8m22.11s   success         1d0ffa5a-8...a35f028/stitcher:0.0.1
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Originally step 1. above in our pipeline was reducing our input video files down to a max width of 640px. But if we have 120 videos on a screen, then the final videos will each be tiny. So I wrote a script to reduce them all down further to 160px each. And re-ran the experiment:

% ic fn action invoke choirless/wateraftermidnight --param choir_id 001jr0nG1EKeJr2W13rs3ZIZi42mi3fO --param song_id 001jxwlK1NkxRI0Y7T3S1jUSHW2Ndy5D --param num_gremlins 120
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It was still close to the 10m limit, but in managed to get through, with the final run being 7m37s:

2020-07-25 20:33:31 7f8a96c60f544dd28a96c60f540dd217 blackbox cold  7m37.326s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 19:49:31 89326734e14d4dacb26734e14d7dacbf blackbox cold  8m46.551s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 19:49:30 d779c856e14a4836b9c856e14a083602 blackbox cold  10m0.612s  developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
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2020-07-25 19:49:04 f16bbd2d6f754109abbd2d6f75410992 blackbox cold  8m27.366s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 19:49:04 4481728f79b246e281728f79b206e25f blackbox cold  8m39.499s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
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2020-07-25 19:48:54 79a5603cd3744b05a5603cd3742b0599 blackbox warm  9m39.024s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 19:48:52 1d6c6d0a42ff4661ac6d0a42ffa661b7 blackbox cold  7m32.764s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 19:48:52 02b087103f444b56b087103f44fb567f blackbox cold  7m24.082s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 19:48:49 17f4fdea027148f2b4fdea027188f27b blackbox warm  10m0.102s  developer error 1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 19:48:48 9258a88f95864ddd98a88f95860dddda blackbox cold  9m54.567s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
2020-07-25 19:48:46 d38d78db20ee48e38d78db20eea8e31e blackbox cold  7m34.876s  success         1d0ffa5a-8...a35f028/stitcher:0.0.1
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And how did it come out? Pretty amazingly!

I was expecting it to fail due to running out of temp disk space, or file descriptors or memory... but it runs in under 2GB of RAM.

This is very promising. We are already planning a re-write of the rendering pipeline to break it up into stages a bit more. And the part that actually forms the ffmpeg command to run, currently in node.js will be re-written in Python and made more modular so we can add additional functionality in (adjust volume of individual parts, manually tune the panning, etc)

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