Next.js cold start profiling for the masses

Usage

Add a vercel.json in your Next.js folder (next to package.json) with the following contents:

{
    "builds": [
        {
            "src": "package.json",
            "use": "https://profile-next-cold-starts.vercel.app/builder.tgz"
        }
    ]
}

Downloading the cold starts profiles

After you deploy your application, you can append ?vercel-profile-cpu to an url to download the cpu profile and ?vercel-profile-require to download a profile of allrequire calls (which is usually what causes long cold starts).

For example:

https://my-app.vercel.app/api/auth?vercel-profile-cpu

https://my-app.vercel.app/api/auth?vercel-profile-require

The downloaded .cpuprofile file will end with hot or cold based on wether the function was hot (already booted) or cold.

Analyzing the profiles

After you download the profile, you can use DevTools Timeline Viewer to analyze it or you can also use the VSCode built in visualizer, simply drag and drop it into VSCode.

How It Works

This repository contains a fork for the Vercel builder, the server-launcher.ts file has been modified to add the profiling code.

Example