DispatchQueue.concurrentPerform中AVAudioConverter缓冲区过度保留致内存飙升

我在为音频分析工具实现并行化处理时，遇到批量处理音频文件堆空间占用持续攀升的问题。排查发现，在DispatchQueue.concurrentPerform块内调用AVAudioConverter.convert(to:error:withInputFrom:)时，每次转换相关的大量内存会被保留至所有并发任务结束，而非单轮迭代完成后释放，导致内存占用不断上升，而非保持稳定。替换转换调用为大内存分配或其他计算时，内存能正常释放，说明问题仅与AVAudioConverter的convert调用行为相关。

简化后的测试代码（实际为处理大量不同文件，此处重复转换同一文件）：

let outputFormat = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: 8000, channels: 1, interleaved: false)!

DispatchQueue.global(qos: .userInitiated).async {
    DispatchQueue.concurrentPerform(iterations: 1000) { index in

        let inputFile = try! AVAudioFile(forReading: URL(filePath: "/path/to/some/song.m4a"))

        let inputFormat = inputFile.processingFormat
        let inputFrameCount = AVAudioFrameCount(inputFile.length)
        let inputBuffer = AVAudioPCMBuffer(pcmFormat: inputFormat, frameCapacity: inputFrameCount)!

        try! inputFile.read(into: inputBuffer)

        let sampleRateCoefficient = inputFormat.sampleRate / outputFormat.sampleRate
        let outputFrameCount = UInt32(Double(inputFrameCount) / sampleRateCoefficient)
        let outputBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: outputFrameCount)!

        let converter = AVAudioConverter(from: inputBuffer.format, to: outputFormat)!
        let converterInputBlock: AVAudioConverterInputBlock = { _, outStatus in
            outStatus.pointee = AVAudioConverterInputStatus.haveData
            return inputBuffer
        }

        // 替换为其他繁重计算时内存正常释放，仅调用convert时内存保留至所有任务结束
        converter.convert(
            to: outputBuffer,
            error: nil,
            withInputFrom: converterInputBlock
        )
    }
}

问题根源分析 #

AVAudioConverter的convert(to:error:withInputFrom:)调用可能存在内部引用循环，或是其内部缓存的资源未在当前迭代结束时及时释放。另外，AVAudioConverterInputBlock捕获了inputBuffer，转换器可能在内部持有该block的引用，导致buffer无法被ARC及时回收。

解决方案尝试 #

• 显式打破引用循环：在inputBlock中使用弱引用捕获inputBuffer，避免循环引用导致内存无法释放：

  let converterInputBlock: AVAudioConverterInputBlock = { [weak inputBuffer] _, outStatus in
      guard let buffer = inputBuffer else {
          outStatus.pointee = .endOfStream
          return nil
      }
      outStatus.pointee = .haveData
      return buffer
  }
  

• 手动触发内存回收：将所有相关对象声明为可选类型，在迭代结束前显式置空，并触发自动释放池回收底层Objective-C对象：

  DispatchQueue.concurrentPerform(iterations: 1000) { index in
      var inputFile: AVAudioFile?
      var inputBuffer: AVAudioPCMBuffer?
      var outputBuffer: AVAudioPCMBuffer?
      var converter: AVAudioConverter?
      
      do {
          inputFile = try AVAudioFile(forReading: URL(filePath: "/path/to/some/song.m4a"))
          let inputFormat = inputFile!.processingFormat
          let inputFrameCount = AVAudioFrameCount(inputFile!.length)
          inputBuffer = AVAudioPCMBuffer(pcmFormat: inputFormat, frameCapacity: inputFrameCount)!
          try inputFile!.read(into: inputBuffer!)
          
          let sampleRateCoefficient = inputFormat.sampleRate / outputFormat.sampleRate
          let outputFrameCount = UInt32(Double(inputFrameCount) / sampleRateCoefficient)
          outputBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: outputFrameCount)!
          
          converter = AVAudioConverter(from: inputBuffer!.format, to: outputFormat)!
          let converterInputBlock: AVAudioConverterInputBlock = { _, outStatus in
              outStatus.pointee = .haveData
              return inputBuffer
          }
          
          converter?.convert(
              to: outputBuffer!,
              error: nil,
              withInputFrom: converterInputBlock
          )
      } catch {
          // 处理错误
      }
      
      // 显式置空触发ARC回收
      inputFile = nil
      inputBuffer = nil
      outputBuffer = nil
      converter = nil
      
      // 强制回收底层自动释放对象
      autoreleasepool { }
  }
  

• 限制并发任务数量：concurrentPerform会根据系统核心数创建尽可能多的并发任务，易导致内存过载。改用带信号量的自定义并发队列，限制最大并发数：

  let converterQueue = DispatchQueue(label: "com.audio.converter", attributes: .concurrent, target: .global(qos: .userInitiated))
  let semaphore = DispatchSemaphore(value: 4) // 限制最大4个并发任务
  
  for index in 0..<1000 {
      converterQueue.async {
          semaphore.wait()
          defer { semaphore.signal() }
          
          // 放置原迭代内的音频转换代码
      }
  }
  

• 调整转换器配置：尝试显式设置AVAudioConverter的maximumOutputPacketSize参数，减少不必要的内存缓存，避免资源长期占用。

内容的提问来源于stack exchange，提问作者ibsh