MTLBuffer allocation + CPU/GPU synchronisation

问题:

I am using a metal performance shader(MPSImageHistogram) to compute something in an MTLBuffer that I grab, perform computations, and then display via MTKView. The MTLBuffer output from the shader is small (~4K bytes). So I am allocating a new MTLBuffer object for every render pass, and there are atleast 30 renders per second for every video frame.

calculation = MPSImageHistogram(device: device, histogramInfo: &histogramInfo)
let bufferLength = calculation.histogramSize(forSourceFormat: MTLPixelFormat.bgra8Unorm)
let buffer = device.makeBuffer(length: bufferLength, options: .storageModeShared)
let commandBuffer = commandQueue?.makeCommandBuffer()

calculation.encode(to: commandBuffer!, sourceTexture: metalTexture!, histogram: buffer!, histogramOffset: 0)
commandBuffer?.commit()

commandBuffer?.addCompletedHandler({ (cmdBuffer) in
    let dataPtr = buffer!.contents().assumingMemoryBound(to: UInt32.self)
    ...
    ...

}

My questions -

1. Is it okay to make a new buffer every time using device.makeBuffer(..), or better to statically allocate few buffers and implement reuse those buffers? If reuse is better, what do we do for synchronizing CPU/GPU data write/read on these buffers?

2. Another unrelated question, is it okay to draw in MTKView the results on a non-main thread? Or MTKView draws must only be in main thread (even though I read Metal is truly multithreaded)?

回答1:

1. Allocations are somewhat expensive, so I'd recommend a reusable buffer scheme. My preferred way to do this is to keep a mutable array (queue) of buffers, enqueuing a buffer when the command buffer that used it completes (or in your case, after you've read back the results on the CPU), and allocating a new buffer when the queue is empty and you need to encode more work. In the steady state, you'll find that this scheme will rarely allocate more than 2-3 buffers total, assuming your frames are completing in a timely fashion. If you need this scheme to be thread-safe, you can protect access to the queue with a mutex (implemented with a dispatch_semaphore).

2. You can use another thread to encode rendering work that draws into a drawable vended by an MTKView, as long as you follow standard multithreading precautions. Remember that while command queues are thread-safe (in the sense that you can create and encode to multiple command buffers from the same queue concurrently), command buffers themselves and encoders are not. I'd advise you to profile the single-threaded case and only introduce the complication of multi-threading if/when absolutely necessary.

回答2:

1. If it is a small amount of data (under 4K) you can use setBytes(): https://developer.apple.com/documentation/metal/mtlcomputecommandencoder/1443159-setbytes

That might be faster/better than allocating a new buffer every frame. You could also use a triple-buffered approach so that successive frames' access to the buffer do not interfere. https://developer.apple.com/library/content/documentation/3DDrawing/Conceptual/MTLBestPracticesGuide/TripleBuffering.html

This tutorial shows how to set up triple buffering for rendering: https://www.raywenderlich.com/146418/metal-tutorial-swift-3-part-3-adding-texture

That's actually like the third part of the tutorial but it is the part that shows the triple-buffering setup, under "Reusing Uniform Buffers".