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membw.html
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>Measuring Peak Memory Bandwidth</title>
</head>
<body>
<script
type="text/javascript"
src="./webgpufundamentals-timing.js"
></script>
<script type="module">
// for uniform handling
import {
makeShaderDataDefinitions,
makeStructuredView,
} from "https://greggman.github.io/webgpu-utils/dist/1.x/webgpu-utils.module.js";
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.");
}
const timingHelper = new TimingHelper(device);
const WORKGROUP_SIZE = 1; // assumes 1D workgroups for now
const memsrcSize = 2 ** 24; // (1M elements)
const itemsPerWorkgroup = memsrcSize / WORKGROUP_SIZE;
const dispatchGeometry = [itemsPerWorkgroup, 1];
while (
dispatchGeometry[0] > adapter.limits.maxComputeWorkgroupsPerDimension
) {
dispatchGeometry[0] /= 2;
dispatchGeometry[1] *= 2;
}
console.log(`itemsPerWorkgroup: ${itemsPerWorkgroup}
workgroup size: ${WORKGROUP_SIZE}
maxComputeWGPerDim: ${adapter.limits.maxComputeWorkgroupsPerDimension}
dispatchGeometry: ${dispatchGeometry}`);
const memsrc = new Uint32Array(memsrcSize);
for (let i = 0; i < memsrc.length; i++) {
memsrc[i] = i;
}
const memcpyModule = device.createShaderModule({
label: "copy large chunk of memory from memSrc to memDest",
code: /* wgsl */ `
/* output */
@group(0) @binding(0) var<storage, read_write> memDest: array<u32>;
/* input */
@group(0) @binding(1) var<storage, read> memSrc: array<u32>;
@compute @workgroup_size(${WORKGROUP_SIZE}) fn memcpyKernel(
@builtin(global_invocation_id) id: vec3u,
@builtin(num_workgroups) nwg: vec3u,
@builtin(workgroup_id) wgid: vec3u) {
let i = id.y * nwg.x * ${WORKGROUP_SIZE} + 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 WORKGROUP_SIZE
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]);
// Read the results
await mappableMemdstBuffer.mapAsync(GPUMapMode.READ);
const memdest = new Uint32Array(
mappableMemdstBuffer.getMappedRange().slice()
);
mappableMemdstBuffer.unmap();
console.log(`Memdest size: ${memdest.length}`);
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(`Errors: ${errors}`);
} else {
console.log("No errors!");
}
timingHelper.getResult().then((ns) => {
console.log("Timing result", ns);
let bytesTransferred = 2 * memdest.byteLength;
console.log(`Transferred ${bytesTransferred} bytes`);
console.log(`Bandwidth = ${bytesTransferred / ns} GB/s`);
});
function fail(msg) {
// eslint-disable-next-line no-alert
alert(msg);
}
</script>
</body>
</html>