Repository navigation
Source snapshot capture blocks the main thread for ~2 s after each script compile in a large project #3222
Description
Activity
Thank you for the detailed report and the profiler measurements. They make the cost very clear.
We are treating this as our top priority and will follow up here as the fix lands.
Thanks again for the report. A first mitigation has been merged into
main(#3278) and will ship in the next package release. This issue stays open, because the capture still runs on the main thread.What the mitigation does:
- Only the changed sources are rewritten. The capture after a compile now keeps the previous build's copies of unchanged sources instead of copying every source of the assembly into a new
Main-<MVID>/directory each time. In our measurements on a large project, the capture went from about 1.5 s to about 1.0 s after a one-line edit in a method body, and from about 5.4 s to about 1.4 s after adding or removing files. - The capture can be turned off. If you do not use hot reload or pause points, disable both
hot-reloadandpause-pointunder Tool Settings inWindow > Unity CLI Loop > Settings. With both disabled, the capture and the hot reload warm-up are skipped after a domain reload, so a compile no longer pays for them. With only one of them disabled the capture still runs, because the other tool reads the snapshot.
What we are working on now is the root fix: moving the capture off the main thread, so that a domain reload no longer waits for it. The goal is to bring the main-thread cost of the capture close to zero. We will follow up here when it lands.
- Only the changed sources are rewritten. The capture after a compile now keeps the previous build's copies of unchanged sources instead of copying every source of the assembly into a new
Summary
After a domain reload that follows a script compile,
HotReloadEditorStartup.CaptureOnFirstUpdateTickrunsHotReloadSourceSnapshotter.CaptureAfterDomainReload()synchronously on the main thread. In my project it takes about 1.9–2.3 s, which is added to every script compile even when hot reload is not used.Environment
Main) has 825 source filesHow to reproduce
ProfilerDriver.profileEditor = true; ProfilerDriver.enabled = true;).uloop compile.EditorApplication.update: HotReloadEditorStartup.<Initialize>g__CaptureOnFirstUpdateTick.Measurement
EditorApplication.update: HotReloadEditorStartup.<Initialize>g__CaptureOnFirstUpdateTickLibrary/UloopHotReload/SourceSnapshot/Main-<MVID>/with 781 files (3.3 MB).uloop compiletakes about 17.5 s (Bee build ~4.9 s, domain reload ~5.1 s), so this capture is a noticeable part of the wait.Suggestion
I understand from #3195 that the first-update hook was kept so a capture still happens early when no request comes, and that the pause point path reads the snapshot without going through the new gate. With that in mind, one of these might be worth considering: