Memory Reuse and Decode Length

[anthumchris]

Would it be helpful to offer APIs that use pre-defined ring buffers to reduce garbage collection and maintain low latency? SharedArrayBuffer (SAB) could also be used for cross-realm/thread processing and browser support is returning.

Additionally, would it be helpful to control the decoder by specifying how many samples/frames to decode per call? We could decode quickly at first for low-latency playback and then gradually increase frame sizes after we have enough decoded data for playback continuity.

For example, consider a streaming audio AudioWorklet where GC is reduced using ring buffers and specifying 128 samples to decode synchronously (relates to #19).

audio-worklet-processer.js

// ring buffer of encoded bytes (set by "onmessage" or SAB from main/worker thread)
inputBuffer = new ArrayBuffer(...)

// ring buffers for decoded stereo 2.5s PCM @ 48,000hz
outLeft =  new ArrayBuffer(Float32Array.BYTES_PER_ELEMENT * 48000 * 2.5) // 469K
outRight = new ArrayBuffer(Float32Array.BYTES_PER_ELEMENT * 48000 * 2.5) // 469K

// decoded PCM samples
samplesLeft =  new Float32Array(outLeft)   // 120,000 samples
samplesRight = new Float32Array(outRight)  // 120,000 samples

// new stereo decoder (could also be on Worker/main thread via SAB)
decoder = new AudioDecoder({
  srcBuffer: inputBuffer,
  outputBuffers: [outLeft, outRight]
})

// buffer read/write index values
inStart, inEnd, outStart, outEnd

// return values after decode() call
totalSrcBytesUsed, totalSamplesDecoded

// AudioWorkletProcessor.process - processes 128 frames per quantum
process(inputs_NOT_USED, outputs) {
  // specify the max samples to decode (could also be called on Worker/main thread)
  { totalSrcBytesUsed, totalSamplesDecoded } = decoder.decode({ maxToDecode: 128 })

  // update src & output buffers read/write indexes
  ...

  // output decoded [samplesLeft, samplesRight] to @outputs
 ...
}

[padenot]

The discussion in #39 is related. In general Bring You Own Buffer (often refered to as BYOB) are favored these days when designing new specs.

[chcunningham]

We've had the BYOB request from WASM folks as well, and we're keen to do something here. Unfortunately, using a SAB for output creates security concerns because the decoder may internally reference a decoded frame for sometime while it continues to decode later frames. Apps could manipulate the SAB during this period and cause crashes.

We're involved in a cross team (WASM, WebGPU, ...) discussion for memory re-use / reducing copies. The WebCodecs position is here: WICG/reducing-memory-copies#1

I'll go ahead and close and continue tracking in that repo.