Add timestamp query example

sample/timestampQuery/PerfCounter.ts

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+// A minimalistic perf timer class that computes mean + stddev online
+export default class PerfCounter {
+  sampleCount: number;
+  accumulated: number;
+  accumulatedSq: number;
+
+  constructor() {
+    this.sampleCount = 0;
+    this.accumulated = 0;
+    this.accumulatedSq = 0;
+  }
+
+  addSample(value: number) {
+    this.sampleCount += 1;
+    this.accumulated += value;
+    this.accumulatedSq += value * value;
+  }
+
+  getAverage(): number {
+    return this.sampleCount === 0 ? 0 : this.accumulated / this.sampleCount;
+  }
+
+  getStddev(): number {
+    if (this.sampleCount === 0) return 0;
+    const avg = this.getAverage();
+    const variance = this.accumulatedSq / this.sampleCount - avg * avg;
+    return Math.sqrt(Math.max(0.0, variance));
+  }
+}

sample/timestampQuery/TimestampQueryManager.ts

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+// Regroups all timestamp-related operations and resources.
+export default class TimestampQueryManager {
+  // The device may not support timestamp queries, on which case this whole
+  // class does nothing.
+  timestampSupported: boolean
+
+  // Number of timestamp counters
+  timestampCount: number
+
+  // The query objects. This is meant to be used in a ComputePassDescriptor's
+  // or RenderPassDescriptor's 'timestampWrites' field.
+  timestampQuerySet: GPUQuerySet
+
+  // A buffer where to store query results
+  timestampBuffer: GPUBuffer
+
+  // A buffer to map this result back to CPU
+  timestampMapBuffer: GPUBuffer
+
+  // State used to avoid firing concurrent readback of timestamp values
+  hasOngoingTimestampReadback: boolean
+
+  // Device must have the "timestamp-query" feature
+  constructor(device: GPUDevice, timestampCount: number) {
+    this.timestampSupported = device.features.has("timestamp-query");
+    if (!this.timestampSupported) return;
+
+    this.timestampCount = timestampCount;
+
+    // Create timestamp queries
+    this.timestampQuerySet = device.createQuerySet({
+      type: "timestamp",
+      count: timestampCount, // begin and end
+    });
+
+    // Create a buffer where to store the result of GPU queries
+    const timestampByteSize = 8; // timestamps are uint64
+    const timestampBufferSize = timestampCount * timestampByteSize;
+    this.timestampBuffer = device.createBuffer({
+      size: timestampBufferSize,
+      usage: GPUBufferUsage.COPY_SRC | GPUBufferUsage.QUERY_RESOLVE,
+    });
+
+    // Create a buffer to map the result back to the CPU
+    this.timestampMapBuffer = device.createBuffer({
+      size: timestampBufferSize,
+      usage: GPUBufferUsage.COPY_DST | GPUBufferUsage.MAP_READ,
+    });
+
+    this.hasOngoingTimestampReadback = false;
+  }
+
+  // Resolve all timestamp queries and copy the result into the map buffer
+  resolveAll(commandEncoder: GPUCommandEncoder) {
+    if (!this.timestampSupported) return;
+
+    // After the end of the measured render pass, we resolve queries into a
+    // dedicated buffer.
+    commandEncoder.resolveQuerySet(
+      this.timestampQuerySet,
+      0 /* firstQuery */,
+      this.timestampCount /* queryCount */,
+      this.timestampBuffer,
+      0, /* destinationOffset */
+    );
+
+    if (!this.hasOngoingTimestampReadback) {
+      // Copy values to the mapped buffer
+      commandEncoder.copyBufferToBuffer(
+        this.timestampBuffer, 0,
+        this.timestampMapBuffer, 0,
+        this.timestampBuffer.size,
+      );
+    }
+  }
+
+  // Once resolved, we can read back the value of timestamps
+  readAsync(onTimestampReadBack: (timestamps: BigUint64Array) => void): void {
+    if (!this.timestampSupported) return;
+    if (this.hasOngoingTimestampReadback) return;
+
+    this.hasOngoingTimestampReadback = true;
+
+    const buffer = this.timestampMapBuffer;
+    void buffer.mapAsync(GPUMapMode.READ)
+      .then(() => {
+        const rawData = buffer.getMappedRange();
+        const timestamps = new BigUint64Array(rawData);
+
+        onTimestampReadBack(timestamps);
+
+        buffer.unmap();
+        this.hasOngoingTimestampReadback = false;
+      });
+  }
+}

sample/timestampQuery/index.html

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+<!DOCTYPE html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
+    <title>webgpu-samples: timestampQuery</title>
+    <style>
+      :root {
+        color-scheme: light dark;
+      }
+      html, body {
+        margin: 0;      /* remove default margin */
+        height: 100%;   /* make body fill the browser window */
+        display: flex;
+        place-content: center center;
+      }
+      canvas {
+        width: 600px;
+        height: 600px;
+        max-width: 100%;
+        display: block;
+      }
+    </style>
+    <script defer src="main.js" type="module"></script>
+    <script defer type="module" src="../../js/iframe-helper.js"></script>
+  </head>
+  <body>
+    <canvas></canvas>
+  </body>
+</html>