Task 1 of 4

The First Call Is a Lie

The first time you invoke a kernel, gpu.js does far more than run it: it transpiles your JavaScript function to shader code, hands it to the GPU driver to compile and link, allocates buffers — and then runs it. In auto mode there is more still: that first call is also where gpu.js asks the browser for a WebGPU adapter and rebuilds your kernel for it, so the backend swap happens inside the first await too. Every call after it skips straight to the run.

So timing the first call measures the compiler, not your kernel. How wide the gap looks depends on the backend and on what the driver has already cached: on this page the first call typically costs a few times a warm one on WebGPU and ten times or more on WebGL, and on a big kernel with a cold shader cache it is wider still. What never changes is that it happens exactly once — which is why every honest benchmark warms up first and throws that first measurement away.

the first call buys the compiler — time the calls after it

Goal: finish the kernel, then use Date.now() to time the first call and the warmed-up average separately — and log both.

Requirements

Finish the kernel: return Math.sin(x / 100) * 100 where x is this thread's index
Time the first call with Date.now() and log it: first call: N ms
Await 10 more calls in one timed block and log the average: warm call: N ms

Hint 1 — the stopwatch pattern

Snapshot the clock, do the work, subtract:

const t0 = Date.now();
// … the work …
console.log('first call:', Date.now() - t0, 'ms');

Hint 2 — averaging the warm calls

One stopwatch around a loop of 10 calls, then divide:

t0 = Date.now();
for (let i = 0; i < 10; i++) await wave();
console.log('warm call:', (Date.now() - t0) / 10, 'ms');

Same idea elsewhere

Every platform has a version of this pause: CUDA JIT-compiles PTX at first launch (then caches it), WebGPU builds the shader in createComputePipeline, Metal compiles MSL when the pipeline state is created. Benchmarking guides on all of them open with the same rule — discard the first iteration.

All tasks in Measuring Speed Honestly

This page is an interactive exercise — the editor, the GPU runner and your saved progress need JavaScript. The text above is the full brief.