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timing-mre.html
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>Measuring Peak Memory Bandwidth</title>
</head>
<body>
<script type="module">
// begin TimingHelper code
function assert(cond, msg = "") {
if (!cond) {
throw new Error(msg);
}
}
class TimingHelper {
#canTimestamp;
#device;
#querySet;
#resolveBuffer;
#resultBuffer;
#resultBuffers = [];
// state can be 'free', 'need resolve', 'wait for result'
#state = "free";
constructor(device) {
this.#device = device;
this.#canTimestamp = device.features.has("timestamp-query");
if (this.#canTimestamp) {
this.#querySet = device.createQuerySet({
type: "timestamp",
count: 2,
});
this.#resolveBuffer = device.createBuffer({
size: this.#querySet.count * 8,
usage: GPUBufferUsage.QUERY_RESOLVE | GPUBufferUsage.COPY_SRC,
});
}
}
#beginTimestampPass(encoder, fnName, descriptor) {
if (this.#canTimestamp) {
assert(this.#state === "free", "state not free");
this.#state = "need resolve";
const pass = encoder[fnName]({
...descriptor,
...{
timestampWrites: {
querySet: this.#querySet,
beginningOfPassWriteIndex: 0,
endOfPassWriteIndex: 1,
},
},
});
const resolve = () => this.#resolveTiming(encoder);
pass.end = (function (origFn) {
return function () {
origFn.call(this);
resolve();
};
})(pass.end);
return pass;
} else {
return encoder[fnName](descriptor);
}
}
beginRenderPass(encoder, descriptor = {}) {
return this.#beginTimestampPass(
encoder,
"beginRenderPass",
descriptor
);
}
beginComputePass(encoder, descriptor = {}) {
return this.#beginTimestampPass(
encoder,
"beginComputePass",
descriptor
);
}
#resolveTiming(encoder) {
if (!this.#canTimestamp) {
return;
}
assert(
this.#state === "need resolve",
"must call addTimestampToPass"
);
this.#state = "wait for result";
this.#resultBuffer =
this.#resultBuffers.pop() ||
this.#device.createBuffer({
size: this.#resolveBuffer.size,
usage: GPUBufferUsage.COPY_DST | GPUBufferUsage.MAP_READ,
});
encoder.resolveQuerySet(
this.#querySet,
0,
this.#querySet.count,
this.#resolveBuffer,
0
);
encoder.copyBufferToBuffer(
this.#resolveBuffer,
0,
this.#resultBuffer,
0,
this.#resultBuffer.size
);
}
async getResult() {
if (!this.#canTimestamp) {
return 0;
}
assert(this.#state === "wait for result", "must call resolveTiming");
this.#state = "free";
const resultBuffer = this.#resultBuffer;
await resultBuffer.mapAsync(GPUMapMode.READ);
const times = new BigInt64Array(resultBuffer.getMappedRange());
console.log(
"Returned timestamps in result buffer:",
Number(times[1]),
Number(times[0])
);
const duration = Number(times[1] - times[0]);
resultBuffer.unmap();
this.#resultBuffers.push(resultBuffer);
return duration;
}
}
// end TimingHelper code
const adapter = await navigator.gpu?.requestAdapter();
const canTimestamp = adapter.features.has("timestamp-query");
const device = await adapter?.requestDevice({
requiredFeatures: [...(canTimestamp ? ["timestamp-query"] : [])],
});
if (!device) {
fail("Fatal error: Device does not support WebGPU.");
}
if (!canTimestamp) {
fail(
'Fatal error: Device does not support WebGPU timestamp query (`adapter.features.has("timestamp-query")` is false).'
);
}
const workgroupSize = 64;
const memsrcSize = 2 ** 24;
const timingHelper = new TimingHelper(device);
const workgroupCount = memsrcSize / workgroupSize;
const dispatchGeometry = [workgroupCount, 1];
while (
dispatchGeometry[0] > adapter.limits.maxComputeWorkgroupsPerDimension
) {
dispatchGeometry[0] /= 2;
dispatchGeometry[1] *= 2;
}
console.log(`workgroup count: ${workgroupCount}
workgroup size: ${workgroupSize}
maxComputeWGPerDim: ${adapter.limits.maxComputeWorkgroupsPerDimension}
dispatchGeometry: ${dispatchGeometry}`);
const memsrc = new Uint32Array(memsrcSize);
for (let i = 0; i < memsrc.length; i++) {
memsrc[i] = i;
}
const memcpyModule = device.createShaderModule({
label: "copy large chunk of memory from memSrc to memDest",
code: /* wgsl */ `
/* output */
@group(0) @binding(0) var<storage, read_write> memDest: array<u32>;
/* input */
@group(0) @binding(1) var<storage, read> memSrc: array<u32>;
@compute @workgroup_size(${workgroupSize}) fn memcpyKernel(
@builtin(global_invocation_id) id: vec3u,
@builtin(num_workgroups) nwg: vec3u,
@builtin(workgroup_id) wgid: vec3u) {
let i = id.y * nwg.x * ${workgroupSize} + id.x;
memDest[i] = memSrc[i] + 1;
}
`,
});
const memcpyPipeline = device.createComputePipeline({
label: "memcpy compute pipeline",
layout: "auto",
compute: {
module: memcpyModule,
},
});
// create buffers on the GPU to hold data
// read-only inputs:
const memsrcBuffer = device.createBuffer({
label: "memory source buffer",
size: memsrc.byteLength,
usage: GPUBufferUsage.STORAGE | GPUBufferUsage.COPY_DST,
});
device.queue.writeBuffer(memsrcBuffer, 0, memsrc);
const memdestBuffer = device.createBuffer({
label: "memory destination buffer",
size: memsrc.byteLength,
usage: GPUBufferUsage.STORAGE | GPUBufferUsage.COPY_SRC,
});
const mappableMemdstBuffer = device.createBuffer({
label: "mappable memory destination buffer",
size: memsrc.byteLength,
usage: GPUBufferUsage.MAP_READ | GPUBufferUsage.COPY_DST,
});
/** Set up bindGroups per compute kernel to tell the shader which buffers to use */
const memcpyBindGroup = device.createBindGroup({
label: "bindGroup for memcpy kernel",
layout: memcpyPipeline.getBindGroupLayout(0),
entries: [
{ binding: 0, resource: { buffer: memdestBuffer } },
{ binding: 1, resource: { buffer: memsrcBuffer } },
],
});
const encoder = device.createCommandEncoder({
label: "memcpy encoder",
});
const memcpyPass = timingHelper.beginComputePass(encoder, {
label: "memcpy compute pass",
});
memcpyPass.setPipeline(memcpyPipeline);
memcpyPass.setBindGroup(0, memcpyBindGroup);
// TODO handle not evenly divisible by wgSize
memcpyPass.dispatchWorkgroups(...dispatchGeometry);
memcpyPass.end();
// Encode a command to copy the results to a mappable buffer.
// this is (from, to)
encoder.copyBufferToBuffer(
memdestBuffer,
0,
mappableMemdstBuffer,
0,
mappableMemdstBuffer.size
);
// Finish encoding and submit the commands
const command_buffer = encoder.finish();
device.queue.submit([command_buffer]);
// kbr@ suggested the following line, which had no effect on M3 [23 Oct 2024]
// await device.queue.onSubmittedWorkDone();
// Read the results
await mappableMemdstBuffer.mapAsync(GPUMapMode.READ);
const memdest = new Uint32Array(
mappableMemdstBuffer.getMappedRange().slice()
);
mappableMemdstBuffer.unmap();
let errors = 0;
for (let i = 0; i < memdest.length; i++) {
if (memsrc[i] + 1 != memdest[i]) {
if (errors < 5) {
console.log(
`Error ${errors}: i=${i}, src=${memsrc[i]}, dest=${memdest[i]}`
);
}
errors++;
}
}
if (errors > 0) {
console.log(`Memdest size: ${memdest.length} | Errors: ${errors}`);
} else {
console.log(`Memdest size: ${memdest.length} | No errors!`);
}
timingHelper.getResult().then((ns) => {
let bytesTransferred = 2 * memdest.byteLength;
console.log(
`Timing result: ${ns} ns; transferred ${bytesTransferred} bytes; bandwidth = ${
bytesTransferred / ns
} GB/s`
);
});
function fail(msg) {
// eslint-disable-next-line no-alert
alert(msg);
}
</script>
</body>
</html>