feat(cli): build --profile

[ArnaudBarre]

This will allow to profile vite build to detect performance issues in plugins only applied at build time.

Usage example:

Note: There is a small regression on the initial build message in Vite 4

[patak-dev]

Thanks for the PR @ArnaudBarre, I think this feature makes now a lot more sense for build than its current state for dev.

Looks like it is currently only measuring the config and buildStart hooks for plugins and the HTTP server start during dev. At one point it was also measuring pre-bundling of dependencies but that was now moved after the server start. Maybe we should stop the profiler during dev once every statically imported module has been processed, deps prebundled, and the server is idle. We have that signal now. But this could be discussed in another PR.

[ArnaudBarre]

I though of that and was thinking of a dumb "setTimeout 10_000" but that not ideal. What is this signal?

Or maybe we should only write on server exit so that people can profile HMR?

[patak-dev]

The signal is currently tied to the optimizer here. We use it during dev to delay sending to the browser optimized dependencies that may be invalidated if there is a missing dependency. We could extract the onCrawlEnd signal out of the optimizer and use it to have a new onServerIdle (after crawl end + the optimizer was re-run if needed). This would measure the real work done by the server to get the full static module graph to the browser. Another way is to let the browser send us a message through websockets when has loaded, maybe that is even more reliable.

But interesting idea about profiling HMR. Maybe by default having the profiler stop when the server exits is more useful. Another idea is to have something like #9673 and a shortcut to start and stop the profiler at any point during the server run. Or maybe having Vite output several profiles, like:

• vite-profile:start.cpuprofile
• vite-profile:hmr-1.cpuprofile
• vite-profile:hmr-2.cpuprofile

[ArnaudBarre]

Thanks pour the pointers. I will update the PR this evening.

Interesting idea for using the current shortcuts PR pour profiling HMR. Do you want to go forward with this PR? I did a review. I like some of the possibilities, even if it could mean a yet another cli shortcut GitHub label in the future 😁

[patak-dev]

Thanks for reviewing #9673 @ArnaudBarre. I think we shouldn't block this PR waiting for it. Let's address Blue comment to stop at finally in both dev and build, and we can merge this one. We can then work the details of #9673 without additional shortcuts, it would be good to get it in Vite 4. And once that one is merged, we can propose additional shortcuts (like start/stop profiling). I hope we don't need that new label though 😅

[ArnaudBarre]

I added also preview because that's cheap

I looked at the onCrawlEnd but it would be too hacky to put it there I think.

Maybe exposing something like:

server.onInitialLoadEnded(cb: () => void)

Not the best name, but I would like both something that is not tied to the optimiser and only call once.