Merge pull request #1997 from billhollings/fix-Intel-timestampPeriod

Fix VkPhysicalDeviceLimits::timestampPeriod calculations on Intel GPU.

MoltenVK/MoltenVK/GPUObjects/MVKDevice.mm

@@ -1567,16 +1567,24 @@
  return rslt;
 }
 
-// Mark both CPU and GPU timestamps, and if needed, update the timestamp period for this device.
-// On Apple GPUs, the CPU & GPU timestamps are the same, and the timestamp period never changes.
+// If needed, update the timestamp period for this device, using a crude lowpass filter to level out
+// wild temporary changes, particularly during initial queries before much GPU activity has occurred.
+// On Apple GPUs, CPU & GPU timestamps are the same, and timestamp period never changes.
 void MVKPhysicalDevice::updateTimestampsAndPeriod() {
- if (_properties.vendorID == kAppleVendorId) {
- _prevGPUTimestamp = _prevCPUTimestamp = mvkGetElapsedNanoseconds();
- } else {
- MTLTimestamp earlierCPUTs = _prevCPUTimestamp;
- MTLTimestamp earlierGPUTs = _prevGPUTimestamp;
- [_mtlDevice sampleTimestamps: &_prevCPUTimestamp gpuTimestamp: &_prevGPUTimestamp];
- _properties.limits.timestampPeriod = (double)(_prevCPUTimestamp - earlierCPUTs) / (double)(_prevGPUTimestamp - earlierGPUTs);
+ if (_properties.vendorID == kAppleVendorId) { return; }
+
+ MTLTimestamp earlierCPUTs = _prevCPUTimestamp;
+ MTLTimestamp earlierGPUTs = _prevGPUTimestamp;
+ [_mtlDevice sampleTimestamps: &_prevCPUTimestamp gpuTimestamp: &_prevGPUTimestamp];
+ double elapsedCPUNanos = _prevCPUTimestamp - earlierCPUTs;
+ double elapsedGPUTicks = _prevGPUTimestamp - earlierGPUTs;
+ if (elapsedCPUNanos && elapsedGPUTicks) { // Ensure not zero
+ float tsPeriod = elapsedCPUNanos / elapsedGPUTicks;
+
+ // Basic lowpass filter Y = (1 - a)Y + a*X.
+ // The lower a is, the slower Y will change over time.
+ static const float a = 0.05;
+ _properties.limits.timestampPeriod = ((1.0 - a) * _properties.limits.timestampPeriod) + (a * tsPeriod);
  }
 }