How to limit AVAssetWriter from using too much memory when encoding a video?

Question

I use AVAssetWriter to generate a video with frame images ( UIImage array). When it is doing the encoding work, memory usage sometimes spikes to more than 700MB. On my iPhone (with 3GB MEM), this doesn't cause OOM issue, but I am afraid it would cause trouble in older iPhones.

Can I somehow limit AVAssetWriter memory usage? I guess internally it may use some queue to cache the frame bytes, if I can specify the size of the cache, then I can set an upper limit of memory. But I didn't find such options/settings in the documentation.

Here is my code:

    let settings:[String:Any] = [
        AVVideoCodecKey: AVVideoCodecH264,
        AVVideoWidthKey: FRAME_SIZE,
        AVVideoHeightKey: FRAME_SIZE
    ]
    let assetWriter = AVAssetWriter(url: self.url!, fileType: AVFileType.mov)        
    let assetWriterInput = AVAssetWriterInput(mediaType: AVMediaType.video, outputSettings: settings)
    self.assetWriter?.add(assetWriterInput)

    let bufferAttributes:[String: Any] = [kCVPixelBufferPixelFormatTypeKey as String: Int(kCVPixelFormatType_32ARGB)]
    let bufferAdapter = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: assetWriterInput, sourcePixelBufferAttributes: bufferAttributes)
    let frameTime = CMTimeMake(1, 30)

    self.assetWriter?.startWriting()
    self.assetWriter?.startSession(atSourceTime: kCMTimeZero)

    let mediaInputQueue = DispatchQueue(label: "mediaInputQueue")

    var i = 0

    assetWriterInput.requestMediaDataWhenReady(on: mediaInputQueue){
        var localframes = frames
        while(true){
            if(localframes.isEmpty){
                break
            }
            if (assetWriterInput.isReadyForMoreMediaData){
                let image = localframes.removeFirst()
                let pixelBuffer = self.newPixelBufferFrom(cgImage: image.cgImage!)

                var time:CMTime
                if i == 0 {
                    time = kCMTimeZero
                } else {
                    time = CMTimeMake(Int64(i), frameTime.timescale)
                }

                bufferAdapter.append(pixelBuffer!, withPresentationTime: time)

                // TODO: how to release memory?
                i += 1
            }
        }
        assetWriterInput.markAsFinished()
        self.assetWriter?.finishWriting(completionHandler: {
                finishedHandler()
        })
    }

Answer 1

Not sure if it will help, but I had a similar problem inside a while loop. You can check this answer: Is the ios iPhone simulator causing memory usage analysis to swell?

You can give it a try, and see if it improves:

let pixelBuffer = autoreleasepool { return self.newPixelBufferFrom(cgImage: image.cgImage!) }