title: Unpredictable return time for currentDrawable
body: <p>I’m writing a view to plot real time data in Metal. I’m drawing the samples using point primitives, and I’m triple buffering both the vertices, and the uniform data. The issue I’m having is that the time it takes for a call to currentDrawable to return seems to be unpredictable. It’s almost as if sometimes there are no drawables ready, and I have to wait a whole frame for one to become available. Usually the time for currentDrawable to return is ~0.07 ms (which is about what I would expect), but other times it’s a full 1/60 s. This causes the whole main thread to block, which is to say the least not very desirable.</p>
  
  <p>I’m seeing this issue on an iPhone 6S Plus and an iPad Air. I have not yet seen this behavior an the Mac (I have a 2016 MPB with an AMD 460 GPU). My guess is that this somehow has to do with the fact that the GPUs in iOS devices are TBDR-based. I don’t think I’m bandwidth constrained, because I get the exact same behavior no matter how many or how few samples I’m drawing.</p>
  
  <p>To illustrate the issue I wrote a minimal example that draws a static sine wave. This is a simplified example as I normally would have memcpy’ed the samples into the current vertexBuffer just like I do with the uniforms. This is why I’m triple buffering the vertex data as well as the uniforms. It’s still enough to illustrate the problem though. Just set this view as your base view in a storyboard, and run. On some runs it works just fine. Other times currentDrawable starts out with return time of 16.67 ms, then after a few seconds jumps to 0.07 ms, then after a while back to 16.67. It seems to jump from 16.67 to 0.07 if you rotate the device for some reason.</p>
  
  <p>MTKView Subclass</p>
  
  <pre><code>import MetalKit
  
  let N = 500
  
  class MetalGraph: MTKView {
      typealias Vertex = Int32
  
      struct Uniforms {
          var offset: UInt32
          var numSamples: UInt32
      }
  
      // Data
      var uniforms = Uniforms(offset: 0, numSamples: UInt32(N))
  
      // Buffers
      var vertexBuffers  = [MTLBuffer]()
      var uniformBuffers = [MTLBuffer]()
      var inflightBufferSemaphore = DispatchSemaphore(value: 3)
      var inflightBufferIndex = 0
  
      // Metal State
      var commandQueue: MTLCommandQueue!
      var pipeline: MTLRenderPipelineState!
  
  
      // Setup
  
      override func awakeFromNib() {
          super.awakeFromNib()
  
          device = MTLCreateSystemDefaultDevice()
          commandQueue = device?.makeCommandQueue()
          colorPixelFormat = .bgra8Unorm
  
          setupPipeline()
          setupBuffers()
      }
  
      func setupPipeline() {
          let library = device?.newDefaultLibrary()
  
          let descriptor = MTLRenderPipelineDescriptor()
          descriptor.colorAttachments[0].pixelFormat = .bgra8Unorm
          descriptor.vertexFunction   = library?.makeFunction(name: "vertexFunction")
          descriptor.fragmentFunction = library?.makeFunction(name: "fragmentFunction")
  
          pipeline = try! device?.makeRenderPipelineState(descriptor: descriptor)
      }
  
      func setupBuffers() {
          // Produces a dummy sine wave with N samples, 2 periods, with a range of [0, 1000]
          let vertices: [Vertex] = (0..&lt;N).map {
              let periods = 2.0
              let scaled = Double($0) / (Double(N)-1) * periods * 2 * .pi
              let value = (sin(scaled) + 1) * 500 // Transform from range [-1, 1] to [0, 1000]
              return Vertex(value)
          }
  
          let vertexBytes  = MemoryLayout&lt;Vertex&gt;.size * vertices.count
          let uniformBytes = MemoryLayout&lt;Uniforms&gt;.size
  
          for _ in 0..&lt;3 {
              vertexBuffers .append(device!.makeBuffer(bytes: vertices,  length: vertexBytes))
              uniformBuffers.append(device!.makeBuffer(bytes: &amp;uniforms, length: uniformBytes))
          }
      }
  
  
  
      // Drawing
  
      func updateUniformBuffers() {
          uniforms.offset = (uniforms.offset + 1) % UInt32(N)
  
          memcpy(
              uniformBuffers[inflightBufferIndex].contents(),
              &amp;uniforms,
              MemoryLayout&lt;Uniforms&gt;.size
          )
      }
  
      override func draw(_ rect: CGRect) {
          _ = inflightBufferSemaphore.wait(timeout: .distantFuture)
  
          updateUniformBuffers()
  
          let start = CACurrentMediaTime()
          guard let drawable = currentDrawable else { return }
          print(String(format: "Grab Drawable: %.3f ms", (CACurrentMediaTime() - start) * 1000))
  
          guard let passDescriptor = currentRenderPassDescriptor else { return }
  
          passDescriptor.colorAttachments[0].loadAction = .clear
          passDescriptor.colorAttachments[0].storeAction = .store
          passDescriptor.colorAttachments[0].clearColor = MTLClearColorMake(0.2, 0.2, 0.2, 1)
  
          let commandBuffer = commandQueue.makeCommandBuffer()
  
          let encoder = commandBuffer.makeRenderCommandEncoder(descriptor: passDescriptor)
          encoder.setRenderPipelineState(pipeline)
          encoder.setVertexBuffer(vertexBuffers[inflightBufferIndex],  offset: 0, at: 0)
          encoder.setVertexBuffer(uniformBuffers[inflightBufferIndex], offset: 0, at: 1)
          encoder.drawPrimitives(type: .point, vertexStart: 0, vertexCount: N)
          encoder.endEncoding()
  
          commandBuffer.addCompletedHandler { _ in
              self.inflightBufferSemaphore.signal()
          }
          commandBuffer.present(drawable)
          commandBuffer.commit()
  
          inflightBufferIndex = (inflightBufferIndex + 1) % 3
      }
  }
  </code></pre>
  
  <p>Shaders</p>
  
  <pre><code>#include &lt;metal_stdlib&gt;
  using namespace metal;
  
  struct VertexIn {
      int32_t value;
  };
  
  struct VertexOut {
      float4 pos [[position]];
      float pointSize [[point_size]];
  };
  
  struct Uniforms {
      uint32_t offset;
      uint32_t numSamples;
  };
  
  vertex VertexOut vertexFunction(device   VertexIn *vertices [[buffer(0)]],
                                  constant Uniforms *uniforms [[buffer(1)]],
                                  uint vid [[vertex_id]])
  {
      // I'm using the vertex index to evenly spread the
      // samples out in the x direction
      float xIndex = float((vid + (uniforms-&gt;numSamples - uniforms-&gt;offset)) % uniforms-&gt;numSamples);
      float x = (float(xIndex) / float(uniforms-&gt;numSamples - 1)) * 2.0f - 1.0f;
  
      // Transforming the values from the range [0, 1000] to [-1, 1]
      float y = (float)vertices[vid].value / 500.0f - 1.0f ;
  
      VertexOut vOut;
      vOut.pos = {x, y, 1, 1};
      vOut.pointSize = 3;
  
      return vOut;
  }
  
  fragment half4 fragmentFunction() {
      return half4(1, 1, 1, 1);
  }
  </code></pre>
  
  <p>Possibly related to this: In all the examples I’ve seen, inflightBufferSemaphore is incremented inside the commandBuffer’s completionHandler, just before the semaphore is signaled (which makes sense to me). When I have that line there I get a weird jittering effect, almost as if the framebuffers are being displayed out of order. Moving this line to the bottom of the draw function fixes the issue, although it doesn’t make a lot of sense to me. I’m not sure if this is related to currentDrawable’s return time being so unpredictable, but I have a feeling these two issues are emerging from the same underlying problem.</p>
  
  <p>Any help would be very much appreciated!</p>